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  • Articles  (980)
  • American Institute of Physics (AIP)  (980)
  • Low Temperature Physics  (980)
  • 810
  • 1
    Publication Date: 2016-07-30
    Description: We studied the optical absorption and luminescence of agate (SiO 2 ), topaz (Al 2 [SiO 4 ](F,OH) 2 ), beryl (Be 3 Al 2 Si 6 O 18 ), and prehnite (Ca 2 Al(AlSi 3 O 10 )(OH) 2 ) doped with different concentrations of transition metal ions and exposed to fast neutron irradiation. The exchange interaction between the impurity ions and the defects arising under neutron irradiation causes additional absorption as well as bands' broadening in the crystals. These experimental results allow us to suggest the method for obtaining new radiation-defect induced jewellery colors of minerals due to neutron irradiation.
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  • 2
    Publication Date: 2016-07-30
    Description: We analyzed carefully the experimental kinetics of the low-temperature diffusion-controlled F, H center recombination in a series of irradiated alkali halides and extracted the migration energies and pre-exponential parameters for the hole H centers. The migration energy for the complementary electronic F centers in NaCl was obtained from the colloid formation kinetics observed above room temperature. The obtained parameters were compared with data available from the literature.
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  • 3
    Publication Date: 2016-07-30
    Description: The effect of low-temperature uniaxial deformation on the self-trapping-limited mean free path of excitons in a KI–Tl crystal was revealed from x-ray luminescence spectra. The analysis of the dependence of the intensity ratio of the Tl-center emission (2.85 eV) and the luminescence of self-trapped excitons (π-component; 3.3 eV) on the extent of low-temperature deformation showed that in the KI–Tl crystal (3 × 10 −3 mol. %) the self-trapping-limited mean free path of excitons is comparable with the distance between Tl atoms (20–27) a under a deformation ε = 2%. As the compression increases to ε ≥ 2%–5%, the mean free path drops to (27-5.35) a . The results of modeling based on the continuum approximation showed that with increasing temperature and the degree of low-temperature deformation the height of the potential barrier for the exciton self-trapping drops, which is consistent with the reduction of the mean free path of excitons in the KI–Tl crystal.
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  • 4
    Publication Date: 2016-07-30
    Description: Free volume and pore size distribution size in functional micro and macro-micro-modified Cu 0.4 Co 0.4 Ni 0.4 Mn 1.8 O 4 ceramics are characterized by positron annihilation lifetime spectroscopy in comparison with Hg-porosimetry and scanning electron microscopy technique. Positron annihilation results are interpreted in terms of model implication positron trapping and ortho-positronium decaying. It is shown that free volume of positron traps are the same type for macro and micro modified Cu 0.4 Co 0.4 Ni 0.4 Mn 1.8 O 4 ceramics. Classic Tao-Eldrup model in spherical approximation is used to calculation of the size of nanopores smaller than 2 nm using the ortho-positronium lifetime.
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  • 5
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    American Institute of Physics (AIP)
    Publication Date: 2016-07-30
    Description: In order to predict optical properties of insulating materials under intensive laser excitation, we generalized methods of quantum electrodynamics, allowing us to simulate excitation of electrons and holes, interacting with each other and acoustic phonons. The prototypical model considers a two-band dielectric material characterized by the dispersion relations for electron and hole states. We developed a universal description of excited electrons, holes and acoustic phonons within joint quantum kinetics formalism. Illustrative solutions for the quasiparticle birth-annihilation operators, applicable at short laser pulses at 0 K, are obtained by the transition from the macroscopic description to the quantum field formalism.
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  • 6
    Publication Date: 2016-07-30
    Description: Monoclinic antiferromagnetic NiWO 4 was studied by far-infrared (30–600 cm −1 ) absorption spectroscopy in the temperature range of 5–300 K using the synchrotron radiation from SOLEIL source. Two isomorphous CoWO 4 and ZnWO 4 tungstates were investigated for comparison. The phonon contributions in the far-infrared range of tungstates were interpreted using the first-principles spin-polarized linear combination of atomic orbital calculations. No contributions from magnetic excitations were found in NiWO 4 and CoWO 4 below their Neel temperatures down to 5 K.
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  • 7
    Publication Date: 2016-07-30
    Description: The creation spectrum of stable F centres (being part of F-H pairs of Frenkel defects) by synchrotron radiation of 7–40 eV has been measured for highly pure NaCl single crystals at 12 K using a highly sensitive luminescent method. It is shown that the efficiency of F centre creation in a closely packed NaCl is low at the decay of anion or cation excitons (7.8–8.4 and 33.4 eV, respectively) or at the recombination of relaxed conduction electrons and valence holes. Only the recombination of nonrelaxed (hot) electrons with holes provides the energy exceeding threshold value E FD , which is sufficient for the creation of Frenkel defects at low temperature.
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  • 8
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    American Institute of Physics (AIP)
    Publication Date: 2016-07-30
    Description: An unambiguous attribution of the absorption spectra to definite paramagnetic centres identified by the EPR techniques in the most cases is problematic. This problem may be solved by applying of a direct measurement techniques—the EPR detected via the magnetic circular dichroism, or briefly MCD–EPR. The present survey reports on the advantages and disadvantages applying the MCD–EPR techniques to simple and complex paramagnetic centres in crystals as well as glasses and glass-ceramics.
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  • 9
    Publication Date: 2016-07-30
    Description: Luminescence properties of SiO 2 in different structural states are compared. Similar comparison is made for GeO 2 . Rutile and α-quartz structures as well as glassy state of these materials are considered. Main results are that for α-quartz crystals the luminescence of self-trapped exciton is the general phenomenon that is absent in the crystal with rutile structure. In rutile structured SiO 2 (stishovite) and GeO 2 (argutite) the main luminescence is due to a host material defect existing in as-received (as-grown) samples. The defect luminescence possesses specific two bands, one of which has a slow decay (for SiO 2 in the blue and for GeO 2 , in green range) and another, a fast ultraviolet (UV) band (4.75 eV in SiO 2 and at 3 eV in GeO 2 ). In silica and germania glasses, the luminescence of self-trapped exciton coexists with defect luminescence. The latter also contains two bands: one in the visible range and another in the UV range. The defect luminescence of glasses was studied in details during last 60–70 years and is ascribed to oxygen deficient defects. Analogous defect luminescence in the corresponding pure nonirradiated crystals with α-quartz structure is absent. Only irradiation of a α-quartz crystal by energetic electron beam, γ-rays and neutrons provides defect luminescence analogous to glasses and crystals with rutile structure. Therefore, in glassy state the structure containing tetrahedron motifs is responsible for existence of self-trapped excitons and defects in octahedral motifs are responsible for oxygen deficient defects.
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  • 10
    Publication Date: 2016-07-30
    Description: The ground state properties of cubic scandium trifluoride (ScF 3 ) perovskite were studied using first-principles calculations. The electronic structure of ScF 3 was determined by linear combination of atomic orbital (LCAO) and plane wave projector augmented-wave (PAW) methods using modified hybrid exchange-correlation functionals within the density functional theory (DFT). The comprehensive comparison of the results obtained by two methods is presented. Both methods allowed us to reproduce the lattice constant found experimentally in ScF 3 at low temperatures and to predict its electronic structure in good agreement with known experimental valence-band photoelectron and F 1 s x-ray absorption spectra.
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  • 11
    Publication Date: 2016-07-30
    Description: Photoluminescence and excitation spectra of microcrystalline and nanocrystalline nickel tungstate (NiWO 4 ) were measured using UV-VUV synchrotron radiation source. The origin of the bands is interpreted using comparative analysis with isostructural ZnWO 4 tungstate and based on the results of recent first-principles band structure calculations. The influence of the local atomic structure relaxation and of Ni 2+ intra-ion d–d transitions on the photoluminescence band intensity are discussed.
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  • 12
    Publication Date: 2016-07-30
    Description: In this paper a novel method for synthesis of LaInO 3 :Er 3+ is reported and upconversion luminescence properties of the synthesized material at different temperatures (9–300 K) are studied. The samples were prepared by co-precipitation and subsequent heat treatment of lanthanum, indium and erbium hydroxides. It is shown that the excitation at 980 nm leads to a strong green upconversion luminescence in the material. At the concentrations above 0.1 mol. % of Er 3+ the energy transfer upconversion mechanism of the luminescence becomes evident. Further increase of Er 3+ content in the material leads to higher red-to-green upconversion luminescence intensity ratio. The mechanisms responsible for the observed variation are discussed.
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  • 13
    Publication Date: 2016-07-30
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  • 14
    Publication Date: 2016-07-30
    Description: Possible proximity effects in gases of cold, multiply charged atoms are discussed. Here we deal with rarefied gases with densities n d of multiply charged ( Z ≫ 1) atoms at low temperatures in the well-known Thomas-Fermi (TF) approximation, which can be used to evaluate the statistical properties of single atoms. In order to retain the advantages of the TF formalism, which is successful for symmetric problems, the external boundary conditions accounting for the finiteness of the density of atoms (donors), n d ≠ 0, are also symmetrized (using a spherical Wigner-Seitz cell) and formulated in a standard way that conserves the total charge within the cell. The model shows that at zero temperature in a rarefied gas of multiply charged atoms there is an effective long-range interaction E proxi ( n d ), the sign of which depends on the properties of the outer shells of individual atoms. The long-range character of the interaction E proxi is evaluated by comparing it with the properties of the well-known London dispersive attraction E Lond ( n d ) 〈 0, which is regarded as a long-range interaction in gases. For the noble gases argon, krypton, and xenon E proxi 〉0 and for the alkali and alkaline-earth elements E proxi 〈 0. At finite temperatures, TF statistics manifests a new, anomalously large proximity effect, which reflects the tendency of electrons localized at Coulomb centers to escape into the continuum spectrum. The properties of thermal decay are interesting in themselves as they determine the important phenomenon of dissociation of neutral complexes into charged fragments. This phenomenon appears consistently in the TF theory through the temperature dependence of the different versions of E proxi . The anomaly in the thermal proximity effect shows up in the following way: for T ≠ 0 there is no equilibrium solution of TS statistics for single multiply charged atoms in a vacuum when the effect is present. Instability is suppressed in a Wigner-Seitz model under the assumption that there are no electron fluxes through the outer boundary R 3 ∝ n −1 d of a Wigner-Seitz cell. E proxi corresponds to the definition of the correlation energy in a gas of interacting particles. This review is written so as to enable comparison of the results of the TF formalism with the standard assumptions of the correlation theory for classical plasmas. The classic example from work on weak solutions (including charged solutions)—the use of semi-impermeable membranes for studies of osmotic pressure—is highly appropriate for problems involving E proxi . Here we are speaking of one or more sharp boundaries formed by the ionic component of a many-particle problem. These may be a metal-vacuum boundary in a standard Casimir cell in a study of the vacuum properties in the 2 l gap between conducting media of different kinds or different layered systems (quantum wells) in semiconductors, etc. As the mobile part of the equilibrium near a sharp boundary, electrons can (should) escape beyond the confines of the ion core into a gap 2 l with a probability that depends, among other factors, on the properties of E proxi for the electron cloud inside the conducting walls of the Casimir cell (quantum well). The analog of the Casimir sandwich in semiconductors is the widely used multilayer heterostructures referred to as quantum wells of width 2 l with sides made of suitable doped materials, which ensure statistical equilibrium exchange of electrons between the layers of the multilayer structure. The thermal component of the proximity effects in semiconducting quantum wells provides an idea of many features of the dissociation process in doped semiconductors. In particular, a positive E proxi 〉 0 (relative to the bottom of the conduction band) indicates that TF donors with a finite density n d ≠ 0 form a degenerate, semiconducting state in the semiconductor. At zero temperature, there is a finite density of free carriers which increases with a power-law dependence on T .
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  • 15
    Publication Date: 2016-07-30
    Description: The radiation properties and the electronic structure of hybrid composites based on suspension polystyrene (PS) and nanocrystals of BaZrO 3 (BZO) ( d 〈 50 nm) have been studied using luminescent spectroscopy and x-ray analysis. A strong cathodoluminescence (CL) in BZO-nanocrystals is observed in temperature range 80–293 K. It is modified in BZO-PS composites: both the low- and a high-energy bands (near 4 eV) appear, together with a significant reduction in the CL intensity. A decrease of the lattice parameter a for BZO phase in the composite and the modification of CL spectra indicate for changes in the nanocrystalline structure induced by the polymer.
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  • 16
    Publication Date: 2016-07-30
    Description: Fresh and aged melt-grown or gas-phase grown CdI 2 crystals are studied by means of low-temperature photoluminescence spectroscopy. Noticeable transformations of emission spectra are observed after long-term aging. The formation of nanostructures containing cadmium oxide and cadmium hydroxide as well as the changes in local surrounding of iodine atoms and the possible growth of polytypic modifications of CdI 2 are taken into account when considering the diversity of optical spectra.
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  • 17
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    American Institute of Physics (AIP)
    Publication Date: 2016-07-08
    Description: Analytical calculations of the potential barrier hindering rotation of the hydrogen molecules in the molecular field of neighboring molecules are performed for molecular solid hydrogen. The calculations are made for the four-sublattice Pca 2 1 lattice which minimizes the electrostatic energy of classical quadrupoles on an hcp lattice.
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  • 18
    Publication Date: 2016-07-08
    Description: The model describing the effect of anharmonicity on the spin-crossover properties of Fe(II) complex is proposed. It is shown that anharmonicity can be one of the important factors controlling the magnetic transitions of the low-spin high-spin type.
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  • 19
    Publication Date: 2016-07-08
    Description: We report Raman light scattering in the phase separated superconducting single crystal Rb 0.77 Fe 1.61 Se 2 with T c = 32 K over a wide temperature region 3–500 K. The observed phonon lines from the majority vacancy ordered Rb 2 Fe 4 Se 5 (245) antiferromagnetic phase with T N = 525 K demonstrate modest anomalies in the frequency, intensity and halfwidth at the superconductive phase transition. We identify phonon lines from the minority compressed Rb δ Fe 2 Se 2 (122) conductive phase. The superconducting gap with d x 2 − y 2 symmetry has been detected in our spectra. In the range 0–600 cm −1 we observe a weak but highly polarized B 1 g -type background which becomes well-structured upon cooling. A possible magnetic or multiorbital origin of this background is discussed. We argue that the phase separation in M 0.8+ x Fe 1 . 6+y Se 2 is of pure magnetic origin. It occurs below the Néel temperature when the magnetic moment of iron reaches a critical value. We state that there is a spacer between the majority 245 and minority 122 phases. Using ab initio spin-polarized band structure calculations we demonstrate that the compressed vacancy ordered Rb 2 Fe 4 Se 5 phase can be conductive and therefore may serve as a protective interface spacer between the purely metallic Rb δ Fe 2 Se 2 phase and the insulating Rb 2 Fe 4 Se 5 phase providing percolative Josephson-junction like superconductivity all throughout of Rb 0.8+ x Fe 1.6+ y Se 2 . Our lattice dynamics calculations show significant differences in the phonon spectra of the conductive and insulating Rb 2 Fe 4 Se 5 phases.
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  • 20
    Publication Date: 2016-07-08
    Description: The relative elongation ε of samples of high purity (99.9999 mol. % with respect to nonhydrogenic impurities) parahydrogen ( p -H 2 , ∼0.2% o -H 2 ) with different amounts of the stable hydrogen isotope deuterium is measured as a function of applied stress σ at temperatures of 1.8–4.2 K. The samples were subjected to uniaxial tension by stepwise loading. The ratio [D]/[H] of the number [D] of deuterium atoms to the number [H] of p -H 2 hydrogen atoms ranged from 0.0055 ± 0.0005 at. % up to 0.07 at. %. For deuterium enriched p- H 2 , the easy slip dislocation stage vanished from the σ(ε) curves and there was a significant reduction in the total relative elongation of the samples, as well as a substantial increase in the hardening coefficient d σ/ d ε. Deformation of samples of p- H 2 with deuterium contents higher than the natural amount produces an unusual change in their shape owing to the appearance of a rotational component of the low-temperature plastic mass transfer.
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  • 21
    Publication Date: 2016-07-08
    Description: Raman spectrum of single-crystal SmFe 3 (BO 3 ) 4 was studied in the frequency range from 3 to 1500 cm −1 at temperatures 10–300 K. All the A 1 and E phonon modes predicted by the group theory for a given symmetry of the crystal were observed. The magnitudes of splitting between the LO and TO components of polar E phonons were determined. It was found that under the transition to a magnetically ordered phase, the behavior of the intensity of the line corresponding to the A 1 vibrational mode is anomalous. It was shown that at low temperatures the spectrum of two-magnon excitations has a complex shape and is observed with both nondiagonal and diagonal components of the scattering tensor. This complex shape reflects the features in the density of states of the magnetic branches. An estimate of the magnon energy E m at the Brillouin zone boundary gave ∼47 cm −1 . The structure of the ground multiplet 6 H 5/2 of a Sm +3 ion in paramagnetic and antiferromagnetic states as well as the effect of the magnetic phase transition on it were studied. Electron-phonon interaction for the electronic excitation at 225 cm −1 was revealed.
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  • 22
    Publication Date: 2016-07-08
    Description: Two series of nanosized cobalt spinel ferrites CoFe 2 O 4 are synthesized from metal salts using high-energy ball milling with the addition of NaCl as a growth agent (series CFO-NaCl), and without (CFO Series). The particle properties are characterized using atomic force microscopy, as well as magnetic and calorimetric measurements. It is shown that the average sizes of the nanoparticles were ∼5.6 and ∼10.3 nm for CFO and CFO-NaCl series, respectively. We performed magnetostatic measurements and determined the parameters that are required to analyze the magnetic state and remagnetization processes of the nanoparticles. It is shown that the blocking temperature is ≈160 K for CFO samples and ≈300 K for the CFO-NaCl series. It was concluded that at 293 K the CFO series particles exhibit a superparamagnetic state, whereas the CFO-NaCl series are in the blocked state. The specific loss power that is scattered by the synthesized nanoparticle ensembles placed in an alternating magnetic field, is measured experimentally and theoretically assessed. The nature of the processes that determine the thermal characteristics of the nanoparticles is analyzed.
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  • 23
    Publication Date: 2016-07-08
    Description: Amplification of acoustic in-plane phonons due to an external temperature gradient (∇ T ) in single-layer graphene (SLG) was studied theoretically. The threshold temperature gradient ( ∇ T ) 0 g and the threshold voltage ( V T ) 0 g in SLG were evaluated. For T = 77   K , the calculated value for ( ∇ T ) 0 g = 746.8     K / cm and ( V T ) 0 g = 6.6   mV . The calculation was done in the hypersound regime. Further, the dependence of the normalized amplification ( Γ / Γ 0 ) on the frequency ω q and ∇ T / T were evaluated numerically and presented graphically. The calculated threshold temperature gradient ( V T ) 0 g for SLG was higher than that obtained for homogeneous semiconductors ( n -InSb) ( ∇ T ) 0 hom ≈ 10 3   K / cm , superlattices ( ∇ T ) 0 S L ≈ 384   K / cm , and cylindrical quantum wire ( ∇ T ) 0 c q w ≈ 10 2   K / cm . This makes SLG a much better material for thermoelectric phonon amplification.
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  • 24
    Publication Date: 2016-07-08
    Description: We found that the coupled system of Josephson junctions under external electromagnetic radiation demonstrates a cascade of parametric instabilities. These instabilities appear along the IV characteristics within bias current intervals corresponding to Shapiro step subharmonics and lead to charging in the superconducting layers. The amplitudes of the charge oscillations increase with increasing external radiation power. We demonstrate the existence of longitudinal plasma waves at the corresponding bias current values. An essential advantage of the parametric instabilities in the case of subharmonics is the lower amplitude of radiation that is needed for the creation of the longitudinal plasma wave. This fact gives a unique possibility to create and control longitudinal plasma waves in layered superconductors. We propose a novel experiment for studying parametric instabilities and the charging of superconducting layers based on the simultaneous variation of the bias current and radiation amplitude.
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  • 25
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    American Institute of Physics (AIP)
    Publication Date: 2016-07-08
    Description: The Coulombic effect on the stability range of the photo-excited electron gas on liquid helium is shown to favor formation of domains of different densities. Domains appear to eliminate or greatly reduce regions with negative conductivity. An analysis of the density domain structure allows explaining remarkable observations reported recently for the photo-excited electron gas.
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  • 26
    Publication Date: 2016-07-08
    Description: We present thoroughly analyzed experimental results that demonstrate the anomalous manifestation of the exciton self-trapping effect, which is already well-known in bulk crystals, in ordered molecular nanoclusters called J -aggregates. Weakly-coupled one-dimensional (1D) molecular chains are the main structural feature of J -aggregates, wherein the electron excitations are manifested as 1D Frenkel excitons. According to the continuum theory of Rashba-Toyozawa, J -aggregates can have only self-trapped excitons, because 1D excitons must adhere to barrier-free self-trapping at any exciton-phonon coupling constant g = ε LR /2β, wherein ε LR is the lattice relaxation energy, and 2β is the half-width of the exciton band. In contrast, very often only the luminescence of free, mobile excitons would manifest in experiments involving J -aggregates. Using the Urbach rule in order to analyze the low-frequency region of the low-temperature exciton absorption spectra has shown that J -aggregates can have both a weak ( g 〈 1) and a strong ( g 〉 1) exciton-phonon coupling. Moreover, it is experimentally demonstrated that under certain conditions, the J -aggregate excited state can have both free and self-trapped excitons, i.e., we establish the existence of a self-trapping barrier for 1D Frenkel excitons. We demonstrate and analyze the reasons behind the anomalous existence of both free and self-trapped excitons in J -aggregates, and demonstrate how exciton-self trapping efficiency can be managed in J -aggregates by varying the values of g , which is fundamentally impossible in bulk crystals. We discuss how the exciton-self trapping phenomenon can be used as an alternate interpretation of the wide band emission of some J -aggregates, which has thus far been explained by the strongly localized exciton model.
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  • 27
    Publication Date: 2016-07-08
    Description: We present results of theoretical and experimental studies of the electronic structure and magnetic properties of RFe 4 Al 8 , RMn 4 Al 8 , and RCr 4 Al 8 compounds with nonmagnetic elements R = Sc, Y, La, and Lu. The electron spectrum and field induced magnetic moment, as well as their dependences on the unit cell volume, are calculated for the paramagnetic phase of the RT 4 Al 8 systems. The calculations are supplemented by measurements of the magnetic susceptibility of representative RT 4 Al 8 compounds as a function of temperature and hydrostatic pressure.
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  • 28
    Publication Date: 2016-07-08
    Description: Negative magnetoresistance of InSb whiskers with different impurity concentrations 4.4 × 10 16 –7.16 × 10 17 cm −3 was studied in longitudinal magnetic field 0–14 T in the temperature range 4.2–77 K. The negative magnetoresistance reaches about 50% for InSb whiskers with impurity concentration in the vicinity to the metal–insulator transition. The negative magnetoresistance decreases to 35 and 25% for crystals with Sn concentration from the metal and dielectric side of the transition. The longitudinal magnetoresistance twice crosses the axis of the magnetic field induction for the lightly doped crystals. The behavior of the negative magnetoresistance could be explained by the existence of classical size effect, in particular boundary scattering in the subsurface whisker layer.
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  • 29
    Publication Date: 2016-04-07
    Description: Anomalously strong change of ferromagnetic ordering parameters upon a small variation of aluminum content was revealed in low-temperature experimental studies of electrical resistivity and galvanomagnetic properties of iron-vanadium-aluminum magnetic alloys with the compositions near the stoichiometric Fe 2 VAl. By comparing the temperature and magnetic field dependences of the electrical resistivity and Hall effect in Fe 2.1 V 0.91 Al 0.99 and Fe 2.05 V 0.91 Al 1.04 alloys, it was shown that a small increase of aluminum content leads to doubling of the Curie temperature and a sharp change in the temperature dependences of the magnetoresistance and saturation of the spontaneous magnetization.
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  • 30
    Publication Date: 2016-04-07
    Description: Specific heat C M ( T ) of polycrystalline Dy 0.6 Y 0.4 Rh 4 B 4 and Dy 0.6 Y 0.4 Rh 3.85 Ru 0.15 B 4 was studied in the temperature range of 0.5–9 K and magnetic fields 0–10 kOe for the first time. It was found that the λ-anomaly in the specific heat exists at T c ≈ 6 K for Dy 0.6 Y 0.4 Rh 4 B 4 and at T c ≈ 6.6 K for Dy 0.6 Y 0.4 Rh 3.85 Ru 0.15 B 4. It is suppressed in a magnetic field and shifted to lower temperatures. Partial substitution of Rh by Ru enhances superconductivity, presumably, due to stronger inner magnetism of the dysprosium sublattice in Dy 0.6 Y 0.4 Rh 4 B 4 as compared with Dy 0.6 Y 0.4 Rh 3.85 Ru 0.15 B 4 . Furthermore, it was observed that the molar heat capacity C M ( T ) of Dy 0.6 Y 0.4 Rh 3.85 Ru 0.15 B 4 increases with decreasing temperature for T 〈 4 K. In Dy 0.6 Y 0.4 Rh 4 B 4 , an increase in C M ( T ) with decreasing temperature is accompanied by the appearance of a maximum at T max = 1.5 K, which might be a manifestation of the magnetic phase transition in the dysprosium subsystem at this temperature.
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  • 31
    Publication Date: 2016-04-07
    Description: The thermoluminescence spectra of impurity-helium condensates (IHC) submerged in superfluid helium have been observed for the first time. Thermoluminescence of impurity-helium condensates submerged in superfluid helium is explained by neutralization reactions occurring in impurity nanoclusters. Optical spectra of excited products of neutralization reactions between nitrogen cations and thermoactivated electrons were rather different from the spectra observed at higher temperatures, when the luminescence due to nitrogen atom recombination dominates. New results on current detection during the IHC destruction are presented. Two different mechanisms of nanocluster charging are proposed to describe the phenomena observed during preparation and warm-up of IHC samples in bulk superfluid helium, and destruction of IHC samples out of liquid helium.
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  • 32
    Publication Date: 2016-04-07
    Description: X-ray diffractometry is used to study samples of type PM-A group B polyimide (Kapton H) subjected to uniaxial tension at room temperature and cooling to liquid nitrogen and helium temperatures. An asymmetry in the halo of the diffraction pattern from the amorphous sample is observed as a result of deformation and cooling of the samples. Deformation and cooling are found to have different effects on the intensity distribution. Thus, deformation produces “stretched” regions, while cooling produces “compressed” regions. An analysis of the diffraction patterns shows that uniaxial tension leads to partial ordering of the polyimide molecules in a sample along the direction of the applied load. The observed changes in the structure during cooling of films may indicate that mutual ordering of some of the molecules relative to one another is taking place.
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  • 33
    Publication Date: 2016-04-07
    Description: Integrated phosphorescence spectra of meta-bromobenzophenone crystals were measured in the temperature range from 1.6 to 297 K. The spectra were found to contain two series of monomeric bands associated with the stretching mode of the C=O carbonyl at all temperatures. Above 70 K in the red spectral region, a broad structureless band of unknown nature was observed, the center of gravity of which was shifted to red with increasing temperature. The above phenomena and others anomalies can be due to the structural properties of both the molecule and the crystal.
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  • 34
    Publication Date: 2016-04-07
    Description: The energy spectrum of a quasi-two-dimensional electron gas in an in-plane magnetic field is studied using the perturbation theory and quasiclassical approach in the presence of the Rashba and Dresselhaus spin-orbit coupling. The existence of the intersection of energy sublevels in electron spectrum is demonstrated. The reciprocal mass tensor of electrons is analyzed. The heat capacity of the degenerate electron gas is examined, and its relations with the key features of the spectrum are shown.
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  • 35
    Publication Date: 2016-04-07
    Description: We discuss and analyze the dependence spectra of the transmission coefficient T on the quasiparticle energy E of one variety of graphene-based Fibonacci superlattices (SL). The SL is built from armchair graphene nanoribbons (GNR), and the quasi-periodicity is produced by metal-like (MGNR) and semiconductor (SCGNR) ribbons, placed along the lattice growth axis in accordance with the Fibonacci sequence, which are used as individual SL elements. It is shown that the difference in the values of quantized transverse quasi-momentum of electrons in MGNR and SCGNR is enough to form an effective quasi-periodic modulation in the examined structure (no additional factors required), and the optimal nanoribbon width range for this purpose is determined. We also analyzed the dependence of the spectral properties of the test structure on the geometric parameters of the superlattice, and the external electrostatic potential. We paid particular attention to the fact that each Fibonacci generation had a Dirac superlattice band gap. The results of the study can be useful in the determination of optimal parameters for graphene-based nanoelectronic devices.
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  • 36
    Publication Date: 2016-04-07
    Description: Quasistable laminar flow of He II at a temperature of 140 mK is studied experimentally. The liquid flow was excited by a vibrating quartz tuning fork with a resonance frequency of about 24 kHz. It was found that for velocities of the tuning fork oscillations from 0.046 to 0.16 m/s, the He II flow can be both quasistable laminar and turbulent. Transitions between these flow regimes were observed. When the velocity of the tuning fork oscillations increases more rapidly, the velocity at which the quasistable flow becomes unstable and undergoes a transition to a turbulent flow is higher. Mechanisms for the dissipation of the energy of the oscillating tines of the tuning fork in the quasistable laminar flow regime are analyzed. It is found that there is an additional mechanism for dissipation of the energy of the oscillating tuning fork beyond internal friction in the quartz. This mechanism is associated with mutual friction owing to scattering of thermal excitations of He II on quantized vortices and leads to a cubic dependence of the exciting force on the fluid velocity.
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  • 37
    Publication Date: 2016-04-07
    Description: We study the dynamics of a qubit-resonator system, when the resonator is driven by two signals. The interaction of the qubit with the high-amplitude driving we consider in terms of the qubit dressed states. Interaction of the dressed qubit with the second probing signal can essentially change the amplitude of this signal. We calculate the transmission amplitude of the probe signal through the resonator as a function of the qubit's energy and the driving frequency detuning. The regions of increase and attenuation of the transmitted signal are calculated and demonstrated graphically. We present the influence of the signal parameters on the value of the amplification, and discuss the values of the qubit-resonator system parameters for an optimal amplification and attenuation of the weak probe signal.
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  • 38
    Publication Date: 2016-04-07
    Description: The previously derived equations for the components of the order parameter (OP) of dense superfluid neutron matter (SNM) with anisotropic spin-triplet p-wave pairing and with taking into account the effects of magnetic field and finite temperatures are reduced to the single equation for the one-component OP in the limit of zero magnetic field. Here this equation is solved analytically for arbitrary parametrization of the effective Skyrme interaction in neutron matter and as the main results the energy gap (in the energy spectrum of neutrons in SNM) is obtained as nonlinear function of temperature T and density n in two limiting cases: for low temperatures near T = 0 and in the vicinity of phase transition temperature T c 0 ( n ) for dense neutron matter from normal to superfluid state. These solutions for the energy gap are specified for generalized BSk21 and BSk24 parametrizations of the Skyrme forces (with additional terms dependent on density n ) and figures are plotted on the interval 0.1 n 0 〈 n 〈 2.0 n 0 , where n 0 = 0.17 fm −3 is nuclear density.
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  • 39
    Publication Date: 2016-01-01
    Description: An analysis of known experimental literature data on the temperature dependence of magnetic susceptibility of beryllium. It is shown that this dependence can be explained if we take into account that beryllium has an electron topological transition of 3½ kind near the Fermi level.
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  • 40
    Publication Date: 2016-01-01
    Description: The electronic structure and optical properties of the SmNi 5– x Cu x ( x = 0, 1, 2) compounds are studied. The band spectra of the studied intermetallics were calculated with LDA + U + SO method supplementing the local density approximation with a correction for strong electron interaction on the shell of the rare-earth element. Optical properties were studied by ellipsometry method in the wide wavelength range. It was found that the substitution of copper for nickel leads to local changes in the optical conductivity spectra. Both the spectroscopic measurements and theoretical calculations demonstrate the presence of a broad absorption band around 4 eV associated with the Cu 3 d → Ni 3 d electron transitions and increasing with the grown of copper content. The experimental dispersion curves of optical conductivity in the interband absorption region were interpreted using the results of the calculations.
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  • 41
    Publication Date: 2016-01-01
    Description: Magnetoresistance Δρ( H , T ) of several heavy-fermion compounds, CeAl 2 , CeAl 3 and CeCu 6 , substitutional solid solutions with quantum critical behavior CeCu 6– x Au x ( x = 0.1, 0.2) and alloys with magnetic ground state Ce(Al 1– x M x ) 2 (M = Co, Ni, x ≤ 0.8) was studied in a wide range of temperatures (1.8–40 K) in magnetic fields up to 80 kOe. It was shown that a consistent interpretation of the field dependences of the resistance for both non-magnetic and magnetically ordered cerium-based intermetallic compounds with strong electron correlations can be achieved within the framework of an approach that accounts for scattering of charge carriers by localized magnetic moments in a metal matrix. Within this approach, three different components of the magnetoresistance of cerium intermetallic compounds were identified: the negative Brillouin contribution proportional to the local magnetization ( − Δ ρ / ρ ∼ M loc 2 ), the alternating linear contribution ( Δ ρ / ρ ∼ H ) and the magnetic component, saturating in magnetic fields below 15 kOe. In the framework of the Yosida model for the cerium alloys under study, estimates of the local magnetic susceptibility χ loc ( H , T 0 ) were obtained from the magnetoresistance data. Numerical differentiation of the magnetoresistance with respect to the magnetic field and analysis of the obtained d ( Δ ρ / ρ ) / d H = f ( H , T ) dependences allowed us to reconstruct the H–T magnetic phase diagrams of the strongly correlated electron systems under study as well as to examine the effects of spin polarization and renormalization of the electronic states on charge transport both in the regime of quantum critical behavior and in the magnetically ordered state.
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  • 42
    Publication Date: 2016-01-01
    Description: We consider the field dependence of magnetization in the paramagnetic phase of manganese monosilicde, MnSi, which is characterized by an anomalously large effective magnetic moment μ* = 5.3μ B , and a small saturation magnetization M 0 = 0.3μ B /Mn. It follows from the conducted analysis, that neither the theory of band magnetism, nor the cluster approach can explain the experimental data, but an adequate description is possible within the framework of the spin-polaron model, in which the spin-polaron is a quasi-bonded state of the band electron and localized magnetic moment of Mn. It is found that the inclusion of specific interaction for a simple ferrimagnet-like configuration that characterizes the spin-polaron, allows us to explain the experimental data, wherein the observed values of μ* are not associated with large localized magnetic moments in the sample volume. The possible methods of experimental verification of the spin-polaron model are analyzed, including experimentum crucis . The obtained results show that the paramagnetic phase of MnSi should be regarded as a phase in which its physical properties are determined by the magnetic inhomogeneities on a nanometer spatial scale.
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  • 43
    Publication Date: 2016-01-01
    Description: Exchange bias phenomenon, evident of antiferromagnetic–ferromagnetic phase segregation state, has been observed in (Nd 1− x Y x ) 2/3 Ca 1/3 MnO 3 ( x = 0, 0.1) compounds at low temperatures. A contribution to the total magnetization of the compounds due to the ferromagnetic phase has been evaluated. It has been found that yttrium doping leads to the growth of the ferromagnetic phase fraction. The ferromagnetic phase in the doped compound has a lower coercivity H c and more rectangular form of the hysteresis loop. The values of the exchange bias field H EB and coercivity are found to be strongly dependent on the cooling magnetic field H cool . In sufficiently high magnetic fields, H cool 〉 5 kOe, H EB in the doped compound is about twice as low as in the parent compound. This difference is attributed to a lower exchange interaction and higher saturation magnetization of the ferromagnetic phase in (Nd 0.9 Y 0.1 ) 2/3 Ca 1/3 MnO 3 .
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  • 44
    Publication Date: 2016-01-01
    Description: The construction and operating principle of a pendulum magnetometer for measuring the magnetic susceptibility of solids under high gas pressures are described. This device is distinctive in having the pendulum mounted directly in the high pressure chamber. Experimental plots of the susceptibility of the compound V 4 S 9 Br 4 as a function of pressure up to 2 kbar at temperatures of 60–300 K are presented as an example of the use of the magnetometer.
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  • 45
    Publication Date: 2016-01-01
    Description: A study of the magnetocaloric effect for Tm 2 Fe 16 , Tm 2 Fe 17 , Tm 2 Fe 18 , Tm 2 Fe 19 , Tm 2 Fe 17− x Mn x ( x ≤ 1.5), Ce 2 Fe 17− x Mn x H y ( x 〈 2, y ≤ 3), Lu 2 Fe 17− x Mn x ( x ≤ 2) systems at magnetic ordering temperatures T C , T N and a ferro-antiferromagnet phase transition Θ T , from magnetization measurements. In alloys with ground ferromagnetic and high-temperature antiferromagnetic states, the change in the magnetic entropy peak −Δ S M increases as the magnetic phase transition temperatures Θ T and T N converge with varying compound composition. Extrapolating the difference T N − Θ T to zero for the initial and doped alloys allows us to determine the maximum possible value of −Δ S M in such systems and the composition of the corresponding alloy. A superposition of the −Δ S M (T) maxima at Θ T and T N for compounds with two magnetic phase transitions creates much higher values of relative cooling power in comparison to compounds with only one magnetic phase transition at T C .
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  • 46
    Publication Date: 2016-01-01
    Description: The electronic structure of (Ti,Mn)O 2 diluted magnetic semiconductors was investigated theoretically from first principles using the fully relativistic Dirac linear muffin-tin orbital band structure method. The electronic structure was obtained with the local spin-density approximation taking into account strong Coulomb correlations in the frame of the LSDA + U approximation. The x-ray absorption spectra and x-ray magnetic circular dichroism spectra at the Mn and Ti L 2,3 and O K edges were investigated theoretically from first principles. The origin of the XMCD spectra in these compounds was examined. The calculated results are compared with available experimental data.
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  • 47
    Publication Date: 2016-04-07
    Description: F-I-S tunnel junctions of Co 2 CrAl-I-Pb and Co 2 Cr 0.6 Fe 0.4 Al-I-Pb based on films of semimetal ferromagnetic Heusler alloys Co 2 CrAl (Curie temperature T C ≈ 334 K) have been fabricated and the features of spin-polarized current in them studied. A theoretical model of spin blocking of the tunnel current shows that the degree of spin polarization of the conduction electrons in quasi-single crystal films of Co 2 Cr 0.6 Fe 0.4 Al and Co 2 CrAl with B 2- and L 2 1 -type order is 0.91–0.97.
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  • 48
    Publication Date: 2016-04-07
    Description: We report an experimental investigation of the electric response of superfluid helium. Our results confirm the presence of electric potential that appears at the relative oscillatory motion of normal fluid and superfluid components in helium generated by the heater. The resonance of the electric potential was observed in the first four harmonics. A suggested method for the detection of the electric response allows the required resonance peak to be distinguished from spurious signals. Our results are in qualitative agreement with the data published by previous researchers. The reasons for the discrepancy in the measured values of the potential difference are discussed.
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  • 49
    Publication Date: 2016-04-07
    Description: A continuation of the theoretical study of the intrinsic properties of dislocation and crowdion structural defects in 2D crystals [V. D. Natsik and S. N. Smirnov, Fiz. Nizk. Temp. 40 , 1366 (2014) and V. D. Natsik and S. N. Smirnov, Fiz. Nizk. Temp. 41 , 271 (2015)]. The atomic lattice model of conservative (glide) and non-conservative (climb) defect movement is discussed in detail. It is shown that given a continuum description of the 2D crystal, an individual defect can be examined as a point carrier of plastic deformation, its value being determined by the topological charge, which is compliant with the crystal geometry defect parameters. It is found that the strain rate depends on the rate at which the defect center moves, as well as its topological charge. The elastic forces acting on the dislocation and crowdion centers in the field of applied mechanical stresses, and the forces of elastic interaction between defects, are calculated in terms of the linear theory of elasticity of a 2D crystal. The non-linear effect pertaining to the interaction between defects and bending deformation of the crystalline membrane, which is specific to 2D crystals, is also discussed.
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  • 50
    Publication Date: 2016-06-02
    Description: The lattice dynamics of compressed atomic cryocrystals are based on ab initio quantum-mechanical theories of deformable and polarizable atoms (Tolpygo model), while taking into account the many-body interaction. The parameters of the three-particle interaction and deformation of the atomic electron shells, which are calculated in terms of the overlap integrals of atomic orbitals and their derivatives, have the same order of magnitude thus demonstrating that they must be considered in tandem. Accounting for the deformation effects of the electron shells in the dipole approximation when calculating phonon frequencies leads to a “softening” of the longitudinal modes at points L and X , for an entire series of Ne-Xe crystals, and of the transverse modes in the directions Σ and Λ for Xe, under high compression. It is shown that it impossible to adequately reproduce the observed deviation from the Cauchi relation δ( p ) for compressed atomic cryocrystals, without accounting for the deformation of electron shells of atoms in a quadrupole approximation. The inputs from a three-particle and quadrupole interaction for Ne, Kr, and Xe crystals are mutually compensated, which provides a weak dependence on pressure for δ( p ). We found a good agreement between the calculated phonon frequencies, Birch and Fuchs elastic moduli, the deviation from the Cauchi relation for the total number of Ne-Xe crystals in a wide range of pressures, and existing experiments.
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  • 51
    Publication Date: 2016-06-02
    Description: We present a theory of spin dynamics caused by spin-orbit coupling for two-dimensional gases of cold atoms and other quasiparticles with pseudospin 1/2 moving in orbital gauge fields. Our approach is based on the gauge transformation in the form of a SU(2) rotation gauging out the spin-orbit coupling. As a result, the analysis of the spin dynamics is reduced to calculation of the density-related susceptibility of the system without spin-orbit coupling at the wavevector determined by the spin-rotation length. This approach allows one to treat the spin dynamics in terms of the linear response theory for bosonic and fermionic ensembles. We study different regimes of irreversible spin relaxation and coherent spin dynamics in these systems. For bosonic gases the effects of low temperature are crucial due to accumulation of particles in the small-momentum subspace even if the Bose–Einstein condensation does not occur due to the system low dimensionality.
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  • 52
    Publication Date: 2016-06-02
    Description: We develop a numerical model for a defect-containing square lattice of microcavities with embedded ultracold atomic clusters (quantum dots). It is assumed that certain fractions of quantum dots and cavities are absent, which leads to transformation of polariton spectrum of the overall structure. The dispersion relations for polaritonic modes are derived as functions of defect concentrations and on this basis the band gap, the effective masses of lower and upper dispersion branch polaritons as well as their densities of states are evaluated.
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  • 53
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    American Institute of Physics (AIP)
    Publication Date: 2016-06-02
    Description: We present a review of the investigations realized in the last decades of the phenomenon of the Bose-Einstein condensation (BEC) in the system of two-dimensional cavity polaritons in semiconductor nanostructures. The conditions at which the excitons interacting with cavity photons form new type of quasiparticles named as polaritons are described. Since polaritons can form in a microcavity a weakly interacting Bose gas, similarly to the exciton gas in semiconductors, the microcavity exciton-polariton BEC emerged in the last decades as a new direction of the exciton BEC in solids, promising for practical applications. The high interest in BEC of exciton-polaritons in semiconductor microcavities is related to the ultra-low threshold lasing which has been demonstrated, in particular, for an electrically injected polariton laser based on bulk GaN microcavity diode working at room temperature.
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  • 54
    Publication Date: 2016-06-02
    Description: Calculations on a microscopic level are used to explain the experimentally observed negative linear thermal expansion along some directions in a number of crystalline compounds with complicated lattices and anisotropic interactions between atoms. Anomalies in the temperature dependence of the coefficient of linear thermal expansion are analyzed in layered crystals made up of monatomic layers (graphite and graphene nanofilms) and multilayer “sandwiches” (transition metal dichalcogenides), in multilayered crystal structures such as high-temperature superconductors where the anisotropy of the interatomic interactions is not conserved in the long-range order, and in graphene nanotubes. The theoretical calculations are compared with data from x-ray, neutron diffraction, and dilatometric measurements.
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  • 55
    Publication Date: 2016-06-02
    Description: Ab initio calculations of structural, electron, and magnetic properties of “armchair” carbon nanotubes (NT) encapsulated by a “zigzag” chain of Fe atoms Fe 2 @( n , n ) m ( m = 1, 2; n = 4, 5, 6, 7, 8, 9), are performed within the framework of the density functional theory. It is shown that optimizing the structure along the NT axis can significantly impact the binding energy of the NT and the Fe atom chain. It follows from the calculations that Fe 2 @(5,5) is the most stable of all the investigated encapsulated nanotubes. A two-fold decrease in the concentration of Fe in an encapsulated NT converts the system from exothermic to endothermic (Fe 2 @(5,5) m ) and vice versa (Fe 2 @(6,6) m )). For large radii of an encapsulated NT (〉4.13 Å) the binding energy of the NT and the Fe atom chain goes to zero, and the magnetic moments of the Fe atoms and the deviation of the Fe atoms from the NT axis go toward analogous values of the free “zigzag” Fe atom chain.
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  • 56
    Publication Date: 2016-06-02
    Description: By the use of the Rayleigh method we have calculated the angular dependence of the reflectivity and the efficiencies of several other diffracted orders when the periodically corrugated surface of an isotropic elastic medium is illuminated by a volume acoustic wave of shear horizontal polarization. These dependencies display the signatures of Rayleigh and Wood anomalies, usually associated with the diffraction of light from a metallic grating. The Rayleigh anomalies occur at angles of incidence at which a diffracted order appears or disappears; the Wood anomalies here are caused by the excitation of the shear horizontal surface acoustic waves supported by the periodically corrugated surface of an isotropic elastic medium. The dispersion curves of these waves in both the nonradiative and radiative regions of the frequency-wavenumber plane are calculated, and used in predicting the angles of incidence at which the Wood anomalies are expected to occur.
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  • 57
    Publication Date: 2016-06-02
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  • 58
    Publication Date: 2016-06-02
    Description: The paper presents an analysis of the properties of bulk superconducting magnesium diboride-based materials obtained by heating at high quasi-hydrostatic pressures (1–2 GPa), hot pressing (30 MPa), spark plasma sintering (16–96 MPa) and loose powder sintering. It is shown that the optimization of impurity distribution in MgB 2 can be achieved by varying the synthesis conditions and introducing dopants. In particular, polycrystalline MgB 2 materials synthesized at 2 GPa and containing a high amount of impurity oxygen demonstrates high critical current densities (10 6 and 10 3 A/cm 2 at 20 K in magnetic fields of 1 and 8.5 T, respectively). It is found that the oxygen impurities are mainly localized in nanolayers or nanoinclusions, homogeneously distributed in the matrix. They act as pinning centers, while the MgB 2 matrix also contains small amounts of dissolved oxygen. Impurity or intentionally added carbon entering the magnesium diboride structure leads to an increase in the critical magnetic fields up to B c 2 (22 K) = 15 T and B irr (18.5 K) = 15 T. The results of ab initio calculations of the electronic structure and stability of the magnesium diboride compounds with partial oxygen or carbon substitution for boron show that it is energetically favorable for carbon to distribute homogeneously in MgB 2 structure, while oxygen atoms replace boron pairwise in neighboring positions or form zigzag chains.
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  • 59
    Publication Date: 2016-06-02
    Description: Thin film heterostructures composed of superconducting electrodes (molybdenum rhenium alloy) and a nanoscale silicon layer doped with tungsten, have been designed and experimentally studied. The current-voltage characteristics of junctions exhibiting local maxima of the current against the background of abrupt current increases for the first time, were measured in the voltage range of −800 to 800 mV, at temperatures of 4.2–8 K. The positions of these singularities, which are symmetrical with respect to zero voltage, varied from sample to sample within the range of 40–300 mV. With increasing temperature, they became blurred and completely vanished with the disappearance of superconductivity in the electrodes. The nature of the observed singularities is associated with the properties of electron tunneling through the impurity states localized in the semiconducting barrier. The use of a superconducting electrode enhances the interaction of the localized electron with the conduction electrons thanks to the root divergence in the density of electron states of a superconductor.
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  • 60
    Publication Date: 2016-06-02
    Description: Superconductor digital electronics using Josephson junctions as ultrafast switches and magnetic-flux encoding of information was proposed over 30 years ago as a sub-terahertz clock frequency alternative to semiconductor electronics based on complementary metal-oxide-semiconductor (CMOS) transistors. Recently, interest in developing superconductor electronics has been renewed due to a search for energy saving solutions in applications related to high-performance computing. The current state of superconductor electronics and fabrication processes are reviewed in order to evaluate whether this electronics is scalable to a very large scale integration (VLSI) required to achieve computation complexities comparable to CMOS processors. A fully planarized process at MIT Lincoln Laboratory, perhaps the most advanced process developed so far for superconductor electronics, is used as an example. The process has nine superconducting layers: eight Nb wiring layers with the minimum feature size of 350 nm, and a thin superconducting layer for making compact high-kinetic-inductance bias inductors. All circuit layers are fully planarized using chemical mechanical planarization (CMP) of SiO 2 interlayer dielectric. The physical limitations imposed on the circuit density by Josephson junctions, circuit inductors, shunt and bias resistors, etc., are discussed. Energy dissipation in superconducting circuits is also reviewed in order to estimate whether this technology, which requires cryogenic refrigeration, can be energy efficient. Fabrication process development required for increasing the density of superconductor digital circuits by a factor of ten and achieving densities above 10 7 Josephson junctions per cm 2 is described.
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  • 61
    Publication Date: 2016-06-02
    Description: Bosonic lasers represent a new generation of coherent light sources. In contrast to conventional, fermionic, lasers they do not require inversion of electronic population and do not rely on the stimulated emission of radiation. Bosonic lasers are based on the spontaneous emission of light by condensates of bosonic quasiparticles. The first realization of bosonic lasers has been reported in semiconductor microcavities where bosonic condensates of exciton-polaritons first studied several decades ago by K. B. Tolpygo can be formed under optical or electronic pumping. In this paper we overview the recent progress in the research area of polaritonics, address the perspective of realization of polariton devices: from bosonic cascade lasers to spin transistors and switches.
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  • 62
    Publication Date: 2016-06-02
    Description: Experiments relating to studies of the coherence of Bose condensates of dipolar excitons in GaAs/AlGaAs heterostructures with a wide, single quantum well and a Schottky gate are analyzed. Dipolar excitons were excited by light in an annular trap formed along the perimeter of a window in a metal gate with an applied electric voltage. A dual-beam interference technique involving interference combination of the amplitudes of the luminescence light field, together with subsequent analysis of first order correlators, is used to study the temporal (longitudinal) and spatial (transverse) coherence of the exciton condensates. It is found that the transverse coherence length of an exciton condensate is considerably longer than its thermal De Broglie wavelength. Experimental studies of the luminescence intensity correlator also confirm the coherence of the exciton Bose condensate.
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  • 63
    Publication Date: 2016-09-03
    Description: We examine the establishment of stationary non-equilibrium states when a flow of heat is turned on in superfluid solutions with a sufficiently high (9.8%) concentration of 3 He. We study the influence of possible relaxation mechanisms, focusing on the Kapitza jump in particular, on the process of establishing a constant temperature gradient. We found the thermal diffusivity, thermal conductivity and the Kapitza coefficients by comparing the theoretical calculations against experimental data. It is shown that it is necessary to include the Kapitza jump in order to perform a quantitative description of the experimental data.
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  • 64
    Publication Date: 2016-09-03
    Description: In a recent paper, it is shown that the thermal boundary Kapitza resistance between a solid and superfluid helium is explained by resonant scattering of phonons from surface roughness heights, as described in the Adamenko and Fuks (AF) model. We reexamine the original experiments of thermal transfer between a solid (platinum and copper) and superfluid helium conducted by Kapitza in 1940. In particular, we analyze his experimental results for the different surface treatments of the solid in light of the AF model. Time scales for diffuse scattering of phonons at the interface are estimated. Also the role of a layer of varnish on a copper surface is reinterpreted.
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  • 65
    Publication Date: 2016-09-03
    Description: There are no explanations for the high-frequency component of the emission from the pulsar in the Crab nebula, but it may be a manifestation of instability in nonlinear reflection from the star's surface. Radiation from relativistic positrons flying from the magnetosphere to the star and accelerated by the electric field of the polar gap is reflected. The instability involves stimulated scattering on surface waves.
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  • 66
    Publication Date: 2016-01-30
    Description: The sorption of 4 He by graphene oxide powders thermally reduced at T = 200, 300, 500, 700, 900 °C has been investigated in the interval 1.5–290 K. The measured dependence of the quantity of sorbed helium upon the reduction temperature shows up as a nonmonotonic curve. The highest quantities of helium were sorbed by the samples reduced at T = 300 and 900 °C. It is assumed that the thermal reduction of graphite oxide by heating it to 300 °C causes evaporation of the water intercalated in the spacings of the carbon layers, this results in exfoliation of the graphene planes, which enhances the sorptive capacity. Heating the samples to 900 °C generates numerous defects in the carbon planes, as a result, the interlayer spacings become accessible for sorption, which enhances the sorptive capacity.
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  • 67
    Publication Date: 2016-01-30
    Description: The thermal conductivity of solid thiophene at equilibrium vapor pressure between 2 K 〈 T 〈 170 K, has been measured in a sequence of incommensurate metastable orientationally disordered phases II, II 1 , II 2 , and II 2 g with different degrees of orientational ordering of the molecules. It is found that in phase states II, II 1 and II 2 with dynamic orientational disorder of the molecules, the thermal conductivity does not depend on the temperature. It is shown that the temperature dependence of the thermal conductivity κ( T ) of orientational glass V g and II 2 g (incommensurate) does not have any of the anomalies that are typical for amorphous materials and glasses. The temperature dependence κ( T ) of the incommensurate state of orientational glass II 2 g is bell-shaped, which is typical for the thermal conductivity of crystals with long-range orientational order. In the II 2 g state, as temperature drops from T g to almost 10 K, the thermal conductivity increases according to κ( T ) = A / T + B , where the first term describes the input of the propagating phonons, wherein the average length of their mean free path is greater than half of the phonon wavelength. The B term is associated with the input of localized short-wave, or “diffuse” vibrational modes. At low temperatures T ≤ 7 K, κ( T ) ∝ T 3 is observed with increasing temperatures, which corresponds to the boundary scattering of phonons.
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  • 68
    Publication Date: 2016-01-30
    Description: Nonlinear conductivity of point contacts (PCs) on the base of FeSe single crystals has been investigated. Measured dV / dI dependencies demonstrate the prevailing contribution to the PC conductivity caused by the degraded surface. Superconducting (SC) feature in dV / dI like a sharp zero-bias minimum develops for relatively low ohmic PCs, where the deep areas of FeSe are involved. Analysis of dV / dI has shown that the origin of the zero-bias minimum is connected with the Maxwell part of the PC resistance, what masks energy dependent spectral peculiarities. Even so, we have found the specific features in dV / dI —the sharp side maxima, which may have connection to the SC gap, since their position follows the BCS temperature dependence. Exploring the dV / dI spectra of the rare occurrence with Andreev-like structure, the two gaps with Δ = 2.5 and 3.5 meV were identified.
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  • 69
    facet.materialart.
    Unknown
    American Institute of Physics (AIP)
    Publication Date: 2016-01-30
    Description: The results of experimental and theoretical studies of the thermodynamic properties of new polymeric phases of nitrogen at ultra-high pressures, as well as computer modeling of the structure and properties of polymeric nitrogen based on the potential models of interaction of atoms and molecules are reviewed. The location of the phase transition lines for the transitions between the molecular and polymeric crystals, molecular and polymeric fluids as well as the position of the melting line for the polymeric nitrogen in the phase diagram are discussed.
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  • 70
    Publication Date: 2016-01-30
    Description: The electronic structure and a number of thermodynamic characteristics of the ternary R Rh 4 B 4 ( R = Y, Lu) systems of superconductors in the normal phase are calculated from first principles. The electronic states and interactions responsible for the superconducting and magnetic properties of these systems are analyzed. It is found that the Fermi level in the compounds YRh 4 B 4 and LuRh 4 B 4 is immediately adjacent to a peak in the density of electronic states and about 1 eV above a pseudogap in the electronic spectrum. The existence of a number of groups of quasi-degenerate electronic states with low effective masses near the Fermi level is revealed. These states may be the cause of a significant diamagnetic contribution of the conduction electrons to the magnetic susceptibility and may be responsible for the strong temperature dependence of the susceptibility in the rhodium borides.
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  • 71
    Publication Date: 2016-01-30
    Description: The drag coefficient characterizing the dissipation of the energy of oscillating tuning forks immersed in liquid helium is studied experimentally. The experiments are done at temperatures from 0.1 to 3.5 K, a range that covers both hydrodynamic flow and the ballistic transport of thermal excitations in superfluid helium below 0.6 K. It is found that a frequency dependence of the drag coefficient exists in the hydrodynamic limit, where the main dissipation mechanism is viscous friction of the liquid against the surface of the oscillating object at temperatures above 0.7 K. In this case, the drag coefficient is proportional to the square root of the oscillation frequency and its temperature dependence in He II is determined by the corresponding relationships between the density of the normal component and the viscosity of the liquid. At lower temperatures, there is no frequency dependence of the drag coefficient and the magnitude of the dissipative losses is determined only by the temperature dependence of the density of the normal component. At the same time, over the entire range of temperatures studied here, the magnitude of the dissipative losses depends on the geometrical dimensions of the oscillating object.
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  • 72
    Publication Date: 2016-01-30
    Description: This is a discussion of the details of the current-voltage characteristics of a 2D Corbino disk with a uniform density of mobile carriers in the quantum Hall effect plateau. It is shown that diffusion may be one of the reasons for the variety of observed current-voltage characteristics. The anomalous role of diffusion in the formation of the current-voltage characteristics of Corbino disks is determined by the dimensionality of the problem, as well as by the “quality” of the density of states of the magnetized 2D conducting system. The diffusion hypothesis is applied to some specific results and provides an explanation of the difference between the current-voltage characteristics shown in Figs. 1 and 2.
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  • 73
    facet.materialart.
    Unknown
    American Institute of Physics (AIP)
    Publication Date: 2016-01-30
    Description: Twokink excitations in the spiral structures of magnets and multiferroics are found and analyzed within the framework for the sine-Gordon model. It is shown that the movement and interaction of the kinks is accompanied by macroscopic translations of the spiral structure. The ways of observing and exciting kinks in the external magnetic field are discussed.
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  • 74
    Publication Date: 2016-01-30
    Description: It is found that magnetic hysteresis in sintered magnets (Nd 0.62 Dy 0.33 Sm 0.05 ) 16.2 (Fe 0.77 Co 0.23 ) 78.1 B 5.7 for the temperature range T  = 150–350 K, is caused by a mechanism associated with the displacement of domain walls, which ceases at temperatures below 150 K. In addition, the formation of magnetic hysteresis in this temperature range is affected by the mechanism involving the nucleation of the reverse magnetization phase, which is observed at temperatures down to 36 K.
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  • 75
    Publication Date: 2016-01-30
    Description: An approach is proposed to obtain an effective Hamiltonian of a harmonically trapped Bose-system. Such a Hamiltonian is quadratic in the creation–annihilation operators and certain approximations allow to simplify higher (three and four operator) products to the required form. After the Hamiltonian diagonalization, the expression for the excitation spectrum is obtained containing in particular temperature-dependent corrections. Numerical calculations are made for a one-dimensional system. Some prospects towards the extension of the suggested approach to study binary bosonic mixtures are briefly discussed.
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  • 76
    Publication Date: 2016-01-30
    Description: Fractional quantum Hall quasiparticles are famous for having fractional electric charge. Recent experiments report that the quasiparticle's effective electric charge determined through tunneling current noise measurements can depend on the system parameters such as temperature or bias voltage. Several works proposed to understand this as a signature for edge theory properties changing with energy scale. I consider two of such experiments and show that in one of them the apparent dependence of the electric charge on a system parameter is likely to be an artefact of experimental data analysis. Conversely, in the second experiment the dependence cannot be explained in such a way.
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  • 77
    Publication Date: 2016-01-30
    Description: It is shown that quantum oscillations of the Bloch point in the domain walls of cylindrical magnetic domains occur at low temperatures. The conditions for this phenomenon are established.
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  • 78
    Publication Date: 2016-02-27
    Description: Weak localization effects and the interactions of charge carriers are studied in two Si 0.7 Ge 0.3 /Si 0.2 Ge 0.8 /Si 0.7 Ge 0.3 p -type heterostructures, where one or two quantum levels are filled, respectively. A weak localization effect for two-dimensional charge carriers is found to occur in weak magnetic fields when the spin-orbital and inelastic scattering times are close, which is indicative of splitting of the spin states under the influence of a perturbing potential related to the formation of a two-dimensional potential well (Rashba mechanism). In higher magnetic fields when one quantum level is occupied, interaction effects appear that are caused by Coulomb interactions with a scatterer. When the two quantum levels are occupied, the dominant mechanism is scattering on Friedel oscillations of the charge carrier density induced by an impurity electric field. In all regions, the quantum corrections are in good agreement with modern theoretical predictions.
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  • 79
    facet.materialart.
    Unknown
    American Institute of Physics (AIP)
    Publication Date: 2016-02-27
    Description: A model of a one-dimensional nonideal gas in an external transverse force field is used to interpret the experimentally observed thermodynamic properties of xenon deposited in grooves on the surface of carbon nanobundles. A nonideal gas model with pairwise interactions is not entirely adequate for describing dense adsorbates (at low temperatures), but makes it easy to account for the exchange of particles between the 1D adsorbate and the 3D atmosphere, which is an important factor at intermediate (on the order of 35 K for xenon) and, especially, high (∼100 K) temperatures. In this paper, we examine a 1D real gas taking only the one-dimensional Lennard-Jones interaction into account, but under exact equilibrium with respect to the number of particles between the 1D adsorbate and the 3D atmosphere of the measurement cell. The low-temperature branch of the specific heat is fitted independently by an elastic chain model so as to obtain the best agreement between theory and experiment over the widest possible region, beginning at zero temperature. The gas approximation sets in after temperatures for which the phonon specific heat of the chain essentially transforms to a one-dimensional equipartition law. Here the basic parameters of both models can be chosen so that the heat capacity C ( T ) of the chain transforms essentially continuously into the corresponding curve for the gas approximation. Thus, it can be expected that an adequate interpretation of the real temperature dependences of the specific heat of low-dimensionality atomic adsorbates can be obtained through a reasonable combination of the phonon and gas approximations. The main parameters of the gas approximation (such as the desorption energy) obtained by fitting the theory to experiments on the specific heat of xenon correlate well with published data.
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  • 80
    Publication Date: 2016-02-27
    Description: Neutron reflectometry offers a unique opportunity for the direct observation of nanostratification in 3 He- 4 He mixtures in the ultra-low temperature limit. Unfortunately the results of recent experiments could not be well-modelled on account of a seemingly anomalous variation of reflectivity with momentum transfer. We now hypothesize that this effect is attributable to an optical distortion caused by the liquid's meniscus near the container wall. The validity of this idea is tested and confirmed through a subsidiary experiment on a D 2 O sample, showing that the meniscus can significantly distort results if the beam size in the horizontal plane is comparable with, or bigger than, the diameter of the container. The meniscus problem can be eliminated if the beam size is substantially smaller than the diameter of the container, such that reflection takes place only from the flat region of the liquid surface thus excluding the meniscus tails. Practical measures for minimizing the meniscus distortion effect are discussed.
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  • 81
    facet.materialart.
    Unknown
    American Institute of Physics (AIP)
    Publication Date: 2016-02-27
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  • 82
    Publication Date: 2016-02-27
    Description: We review the results of nuclear magnetic resonance studies of the molecular dynamics of the quantum gases HD and CH 4 adsorbed in the cages of microporous structures. Measurements of the variation of the nuclear spin-lattice and nuclear spin-spin relaxation times with temperature provide detailed information about the translational and rotational dynamics of the adsorbed molecules over a wide temperature range.
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  • 83
    Publication Date: 2016-02-27
    Description: Sorption and desorption of 4 He by a mesoporous silicate material MCM-41 was studied in the temperature range of 1.5–290 K. It was shown that for T = 25–290 K the thermal activation mechanism is dominant in the sorption kinetics of 4 He atoms by an MCM-41 sample. Its activation energy was estimated as E a ≈ 164.8 K. For T = 12–23 K, the diffusion of 4 He atoms in the MCM-41 was practically independent of temperature, which typically occurs when the tunnelling mechanism of diffusion dominates over the thermally activated one. A change in the mobility of 4 He atoms in MCM-41 channels was observed at T = 6–12 K, which may be indicative of the formation upon cooling (or decay upon heating) of a 4 He monolayer and subsequent multilayers on the inner surfaces of the channels. Below 6 K, the diffusion coefficients of 4 He are only weakly temperature dependent, which may be attributed to the behavior of quantum 4 He liquid in the MCM-41 channels covered with several layers of 4 He atoms.
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  • 84
    Publication Date: 2016-02-27
    Description: The heat capacity at constant pressure C P of 1D-chains of methane molecules adsorbed in the grooves on the outer surface of the bundles of closed single-walled nanotubes was measured in the temperature range from 2 to 60 K for the first time. The behavior of the temperature dependence of C P below 12 K indicates the presence of a Schottky-type anomaly originated from the tunneling between the lowest energy levels of the rotational spectra of the A , T , and E nuclear-spin species of methane molecules. The feature observed in the vicinity of 14 K is presumably caused by an orientational phase transition, in which the nature of the rotational motion of the molecules changes from libration to hindered rotation. It was found that the rotational heat capacity in the temperature range of 30–40 K is close to that of freely rotating methane molecules. An increase in the derivative dC P ( T )/ dT above 40 K and the feature in the C P ( T ) near 52 K are due to the decay of 1D chains of CH 4 .
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  • 85
    Publication Date: 2016-02-27
    Description: We propose a new approach to analyzing the cathodoluminescence spectra of free argon nanoclusters, forming in a supersonic jet flowing into vacuum. Based on this approach, we conduct an analysis of the intensities of the luminescence bands of neutral and charged excimer complexes (Ar 2 )* and (Ar + 4 )*, measured for clusters with an average size ranging from 500 to 8900 atoms per cluster, and a diameter of 32–87 Å. It is shown that the concentration of the substance condensed into clusters, which determines the integrated intensity of the bands, is proportional to the logarithm of the average size of the clusters in the jet. An analysis of the normalized intensities of the (Ar 2 )* and (Ar + 4 )* bands for crystalline clusters with an fcc structure allowed us to establish that the luminescence of neutral (Ar 2 )* molecules comes from within the volume of the cluster, while the charged complexes (Ar + 4 )* emit from the subsurface layer. We highlighted an area of cluster dimensions at which the jet is dominated by quasi-crystalline clusters with an icosahedral structure, and it is shown that the transition from icosahedral clusters to fcc structures occurs when the average size of the cluster in the jet is N ¯ = (1000–1800) atoms/cluster.
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  • 86
    Publication Date: 2016-02-27
    Description: Phonon and electron spectra of metallic bigraphene are analyzed in the presence of step-edge crystal imperfection. Different geometries of step-edge are considered. The dynamic planar stability of the considered structure is proved for temperatures above the ambient. The number of phonon states is shown to grow near the K-point of the first Brillouin zone, compared to pristine graphene. It is found, that this type of defects causes substantially nonuniform distribution of electron states and the pronounced increase in the number of states with energies close to Fermi energy can be expected in electron spectrum of the graphene-based compounds. The performed calculations are in good agreement with inelastic neutron, x-ray and Raman measurements.
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  • 87
    Publication Date: 2016-02-27
    Description: Enhancement of infrared absorption (SEIRA) of adsorbed biological molecules by a nanostructured metal surface is one of the main routes to increasing the sensitivity of modern optical biosensors. The FTIR absorption spectra of thin films of the RNA base uracil deposited on low-temperature substrate ( T = 6 K) with nanoscale silver structures were investigated in the spectral range 2700–600 cm –1 . It was shown that the intensity of the absorption bands corresponding to νCO stretching vibrations (range 1800–1600 cm –1 ) of uracil (Ur) thin films increases 3–4 fold. For multilayer films, the influence of the nanostructures on the vibrational spectra weakens, and for the film layers more than 15 nm away from the surface, the enhancement is essentially absent. The energies and the vibrational spectra of the complexes of uracil monomers and dimers with 20-atom tetrahedral silver nanoclusters were calculated by the quantum-mechanical method DFT/B3LYP. The most stable complexes have the coordination bond between the top of the silver tetrahedron and the oxygen of the carbonyl group C4O. It was found that the formation of such complexes significantly (3–5 fold) enhances the intensity of the νC4O stretching vibration of uracil, while the intensities of the βNH, βCH and ring bending vibrations do not increase significantly.
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  • 88
    Publication Date: 2016-02-27
    Description: X-ray studies of polycrystalline and pressure compacted barium manganite powder Ba 6 Mn 24 O 48 were conducted at room and low temperatures. It was found that the compacting process that leads to the accumulation of macro- and microstress in the sample exerts a substantial amount of influence on the diffraction pattern. We obtained data for the temperature dependence of the lattice parameters a and c , as well as the linear thermal expansion coefficient in the basal plane of the tetragonal phase at temperatures 20–100 K. In the region of the magnetic phase transition temperature 45 K, we found an abnormal change in lattice parameter a and negative values for the linear thermal expansion coefficients α a . However, at the same time there were no singularities along the c ( T ) dependence. We suggest that one possible reason for this temperature behavior of the structural and thermal characteristics of barium manganite, is the significant anisotropy of the crystal lattice.
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  • 89
    Publication Date: 2016-02-27
    Description: Low temperature (20–230 K) spectral-luminescence studies were conducted on C 60 fullerite saturated with carbon monoxide in a physisorption regime. Substantial changes in the photoluminescence characteristics of C 60 –CO solutions with different impurity concentrations were found already for short intercalation times. Strong dependence of the CO solubility on the saturation temperature was revealed by analyzing the contribution of “deep X-traps” to the luminescence. Furthermore, it was found that filling of the octahedral voids by CO molecules occurs with a lower gradient of the impurity distribution into the bulk C 60 crystals as compared with N 2 . The temperature dependences of the integral emission intensity for the samples with different concentrations of carbon monoxide were studied. For the first time, using the spectral-luminescence method, it was revealed that CO molecules, in contrast to H 2 and N 2 , exhibit a significant effect on the formation of the orientational glass and the rotational dynamics of C 60 molecules. Within the model of the transfer of electronic excitation in C 60 crystals, the effect of polar CO molecules on the reorientation of C 60 molecules and the change in the nature of their rotation in concentrated C 60 –CO solutions, leading to the observed strong shift of the temperatures of orientational T c and glass T g transitions to lower temperatures accompanied by “blurring” of the transition boundaries, were explained.
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  • 90
    Publication Date: 2016-02-27
    Description: An x-ray diffraction study of how sorption of CO gas at a pressure of 30 atm in the temperature range of 150–600 °C influences the structural characteristics of polycrystalline and single crystal fullerite C 60 . The sorption kinetics are studied by constructing a dependence of the lattice parameter on the time it takes for fullerite to be saturated by CO molecules. At temperatures T sorb 〉 300 °C there is an observed dissociation of carbon monoxide, accompanied by the precipitation of carbon powder and the chemical interaction of atomic oxygen with C 60 and CO molecules, and possibly with the carbon condensate. These processes have a strong influence on the structural characteristics of fullerite, thus creating, in part, a nonmonotonic dependence of the parameter and lattice matrix volume on the impurity saturation temperature. The concentrations of solid solutions C 60 (CO) x poly- and single crystal samples are determined in the physisorption range for two modes (150 and 250 °C). It is found that the CO impurity has a linear effect on the lattice parameter and the temperature of the orientational transition of fullerite C 60 .
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  • 91
    Publication Date: 2016-02-27
    Description: A new method of statistically describing the dislocation component of low-temperature internal friction in nanostructured materials. The latter term is applied to ultrafine polycrystals with high density intragranular and grain boundary dislocations, the lines of which are divided into nanometer-scale fragments (dislocation relaxants) with random geometrical and energy parameters. The resonant interaction of elastic vibrations and thermally activated excitations of such relaxators are examined, and the maxima caused by these relaxators along the internal friction temperature dependence are analyzed. It is shown that at low temperatures, the shape of the peaks and the region of their localization along the temperature axis are largely determined by the dispersion of the relaxator activation energy. An algorithm is written in order to analyze the internal friction recorded in the experiment, allowing us to identify the physical model corresponding to the relaxator, and obtain empirical estimates of its parameters.
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  • 92
    Publication Date: 2016-02-27
    Description: A highly sensitive torsional vibration technique is used to study the magnetic properties of fullerite C 60 (99.98%) at temperatures of 77–300 K in dynamic and static experiments. Vibrational energy absorption peaks associated with phase transitions and realignment of the magnetic structure of the fullerite are detected at T  = 152, 195, 230, and 260 K. Relaxation magnetic processes in fullerite C 60 at room temperature are studied. “Spontaneous” rotation of a motionless sample of fullerite freely suspended on an elastic filament is observed when external longitudinal or transverse magnetic fields are switched on. The direction of the “spontaneous” rotation changes with time. It is proposed that these phenomena are related to relaxation processes in the rotational subsystem of C 60 molecular rotators, as well as to magnetic flux trapped in the fullerite and weakly damped eddy currents induced in the sample by the applied field.
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  • 93
    Publication Date: 2016-05-03
    Description: The lattice parameters of monoclinic ortho-bromobenzophenone (2-BrBP) were determined using powder X-ray diffraction, in the temperature range of 90–300 K. It is found that ortho-bromobenzophenone has small linear expansion coefficients (about 10 −5 K −1 ) and is characterized by weak anisotropy. There were no phase transitions in the studied temperature range.
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  • 94
    Publication Date: 2016-05-03
    Description: The effective thermal conductivity of samples of cryocrystal nanocomposite obtained from argon and SiO 2 nanopowder was determined in the temperature interval 2–35 K using the steady-state method. The thermal conductivity of crystalline argon with nanoparticles of amorphous silica oxide embedded in its structure shows a weak dependence on particle linear dimension in the interval 5–42 nm. The temperature dependence of the thermal conductivity of the nanocomposites can be well approximated by taking into account only the two mechanisms of heat carrier scattering: phonon-phonon interaction in U-processes and scattering of phonons by dislocations.
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  • 95
    Publication Date: 2016-05-03
    Description: A study of electron tunneling from quasi-two-dimensional (surface) states with spin-orbit interaction into bulk-mode states, within the framework of a model of an infinitely thin inhomogeneous tunnel magnetic barrier between two conductors. We analyze how the scattering of quasi-two-dimensional electrons on a single magnetic defect affects the tunneling current in this system. We also obtain an analytical expression for the conductance of the tunnel point-contact, as a function of its distance from the defect. It is shown that analyzing local magnetization oscillations around the defect using spin-polarized scanning tunneling microscopy allows us to determine the spin-orbit interaction constant.
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  • 96
    Publication Date: 2016-05-03
    Description: Electron transport through Majorana nanowire with strongly asymmetric couplings to normal metal leads is considered. In three terminal geometry (electrically grounded nanowire) it is shown that the presence of unbiased electrode restores zero-bias anomaly even for strong Majorana energy splitting. For effectively two-terminal geometry we show that electrical current through asymmetric Majorana junction is qualitatively different from the analogous current through a resonant (Breit-Wigner) level.
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  • 97
    Publication Date: 2016-05-03
    Description: A study of how the total magnetization of a Ni-Fe heterostructure evolves during the relaxation of non-equilibrium, inhomogeneous, magnetization distributions toward an equilibrium state. These distributions occur within the framework of a superdiffusive mechanism of ultrafast demagnetization, after exposure to a femtosecond laser pulse. We account for relativistic (local) and exchange (non-local) relaxation mechanisms. This question is all the more interesting because exchange relaxation conserves the total magnetization of the sample. For sufficiently smooth distributions (typical dimensions are about several tens of nanometers), the evolution of the total magnetization is determined by different rates of relativistic relaxation in Ni and Fe. However, for sufficiently inhomogeneous magnetization distributions (with a scale of several nanometers, which is realized in the experiment), non-local relaxation is manifested by the fact that the spin current transfers magnetization from the Fe layer to Ni. As such, the difference in the rates of relativistic relaxation in Ni and Fe is expressed to a lesser extent. It is shown that for experimentally realized parameters of magnetic distribution in magnetic heterostructures, the variation of total magnetization decreases by more than two times, due to the spin current.
    Print ISSN: 1063-777X
    Electronic ISSN: 1090-6517
    Topics: Physics
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  • 98
    Publication Date: 2016-05-03
    Description: The magnetoelectric effect in single-crystal LiCoPO 4 ( T N = 21.8 K) is studied in strong pulsed magnetic fields which destroy the antiferromagnetic structure of the crystal spin ordering. The electric polarization along the crystallographic a axis induced by a magnetic field H || b is measured. New features of the electric polarization in a magnetic field are found, including a spike in the polarization near the field for the first magnetic transition at H 1 = 123 kOe, a recovery of the electric polarization at the second magnetic transition H 2 = 210 kOe, and a gradual reduction to zero on approaching the third transition at H 3 = 263 kOe. Various possible magnetic structures in the high field phases are examined which are consistent with their magnetization and electric polarization. The observed linear dependence of the polarization on field strength in the initial antiferromagnetic phase and the vanishing of the polarization in the first high-field phase are in good agreement with previous studies of the magnetoelectric effect in LiCoPO 4 [Wiegelmann et al. , Ferroelectrics 161 , 147 (1994); H. Wiegelman, Ph.D. thesis (University of Konstanz, Konstanz, 1995)].
    Print ISSN: 1063-777X
    Electronic ISSN: 1090-6517
    Topics: Physics
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  • 99
    Publication Date: 2016-05-03
    Description: The elastic and magnetic characteristics of single-crystal Nd 0.9 Dy 0.1 Fe 3 (BO 3 ) 4 are studied at low temperatures in zero magnetic field and in external fields H || C 3 . The temperature dependences of the acoustic mode velocities and the magnetic susceptibility manifest a transition of the magnetic subsystem into a magnetically ordered state and two successive, spontaneous spin-reorientation phase transitions. The possibility of a spontaneous transition into an incommensurate (spiral) magnetic phase in the crystal is discussed. It is shown that an external magnetic field directed along the trigonal axis of the crystal induces a sequence of spin-reorientation phase transitions. An H-T phase diagram ( H || C 3 ) is constructed for this compound.
    Print ISSN: 1063-777X
    Electronic ISSN: 1090-6517
    Topics: Physics
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  • 100
    Publication Date: 2016-05-03
    Description: The point-contact (microcontact) spectra (second derivatives of the current-voltage characteristics) of As/As point homocontacts are measured at liquid helium temperatures. Inversion of the sign of the point-contact spectrum is observed as a result of the destruction of electron localization in the arsenic contacts owing to electron-phonon interactions. The point-contact spectrum contains two major peaks at energies of 10 and 25 meV. The boundary of the single-phonon part of the spectrum corresponds to 34 meV. This agrees with available data on the density of phonon states. Assuming that the inverted point-contact spectrum reflects features of the electro-phonon interaction spectral function, the mean-square frequency of the phonons is calculated and the Debye temperature is estimated.
    Print ISSN: 1063-777X
    Electronic ISSN: 1090-6517
    Topics: Physics
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