ALBERT

Description: The microscopic mechanism of magnetization and polarization owing to the strong spin-orbital coupling of the electron gas in multiferroics is examined. Explicit analytic expressions are obtained for the magnetization and polarization of the electron gas in multiferroics.

Description: The electronic structure and a number of thermodynamic characteristics of layered magnetic superconductors in the systems RNi 2 B 2 C, RFe 4 Al 8 , and FeSe in the normal state are calculated from first principles. The electronic states and the interactions responsible for the electronic, structural, and magnetic properties of these systems are analyzed.

Description: The tunneling current-voltage characteristics, and the temperature and field dependences of the specific heat and static magnetic susceptibility of planar CdB x F 2− x / p -CdF 2 –QW/CdB x F 2− x sandwich structures produced on the surface of n -CdF 2 crystals are studied in order to establish the superconducting properties of the CdB x F 2− x δ-barriers which delimit the ultranarrow p -type CdF 2 quantum well. A comparative analysis of the current and conduction voltage characteristics below and above the superconducting transition temperature shows that the supercurrent quantization and the size quantization of the holes in the p -CdF 2 quantum well are related. Measurements of the field and temperature dependences of the static magnetic susceptibility of these sandwich structures reveal de Haas-van Alphen (dHvA) oscillations at high temperatures and low magnetic fields. The temperature dependences of the amplitudes of the dHvA oscillations give a low value for the effective mass of the two-dimensional holes, so that the strong field condition μ B ≫ 1 is met at high temperatures. A periodic change in the frequency of the dHvA oscillations accompanied by a diamagnetic response with rising temperature is detected for the first time; it exhibits synchronous temperature oscillations in the density and effective mass of the two-dimensional holes owing to the mesoscopic properties of the δ-barriers. These results are explained in terms of the formation of quantum states of a Bose condensate, which is a consequence of a discrete change in the Cooper pair coherence length of the holes when the superconducting δ-barriers have a fractal structure.

Description: The low-temperature charge-density-wave (CDW) state in the layered organic metals α-(BEDT-TTF) 2 MHg(SCN) 4 has been studied by means of the Shubnikov–de Haas and de Haas–van Alphen effects. In addition to the dominant α-frequency, which is also observed in the normal state, both the magnetoresistance and magnetic torque possess a slowly oscillating component. These slow oscillations provide a firm evidence for the CDW-induced reconstruction of the original cylindrical Fermi surface. The α-oscillations of the interlayer magnetoresistance exhibit an anomalous phase inversion in the CDW state, whereas the de Haas–van Alphen signal maintains the normal phase. We argue that the anomaly may be attributed to the magnetic-breakdown origin of the α-oscillations in the CDW state. A theoretical model illustrating the possibility of a phase inversion in the oscillating interlayer conductivity in the presence of a spatially fluctuating magnetic breakdown gap is proposed.

Description: The influence of Coulomb interactions on the magnetic susceptibility, chemical potential and heat capacity in graphene is considered.

Description: The determining role of the Fermi surface geometry in the formation of long-period magnetic structures in heavy rare-earth metals was considered. It was shown that all these metals are on the verge of electronic topological transition of the Lifshitz type. It was experimentally confirmed that the electronic topological transition induced by elastic deformation results in a change in the type of magnetic ordering. A universal phase diagram “magnetic ordering type—hcp lattice parameters a and c ” was established for the rare-earth metals, in good agreement with the results of first-principles calculations.

Description: De Haas-van Alphen oscillations are studied for Fermi surfaces (FS) illustrating the model proposed by Pippard in the early sixties, namely the linear chain of orbits coupled by magnetic breakdown. This FS topology is relevant to many multiband quasi-two-dimensional (q-2D) organic metals such as κ-(BEDT–TTF) 2 Cu(NCS) 2 and θ-(BEDT–TTF) 4 CoBr 4 (C 6 H 4 Cl 2 ) which are considered in detail. Whereas the Lifshits-Kosevich model only involves a first order development of field- and temperature-dependent damping factors, second order terms may have significant contribution to the Fourier components amplitude for such q-2D systems at high magnetic field and low temperature. The strength of these second order terms depends on the relative value of the involved damping factors, which are in turns strongly dependent on parameters such as the magnetic breakdown field, effective masses and, most of all, effective Lande factors. In addition, the influence of field-dependent Onsager phase factors on the oscillation spectra is considered.

Description: Resistance measurements in pulsed magnetic fields up to 55 T as well as a first-principles DFT calculation of the Fermi surface for the organic metal (TSeT) 2 Cl have been performed to investigate its metal-semimetal phase transition. The results obtained are in line with the imperfect nesting that can be inferred from both the observed metallic behavior of the resistivity at low temperature and the previously reported Shubnikov-de Haas oscillations due to small carrier pockets. The DFT study points out the possibility that the LUMO bands of the TSeT donor may interact with the HOMO ones and modify the shape of the Fermi surface under pressure.

Description: The electronic structure, Fermi surface, angle dependence of the cyclotron masses and extremal cross sections of the Fermi surface of RIn 3 (R = Y, Lu, and Yb) compounds were investigated from first principles using the fully relativistic Dirac linear muffin-tin orbital method. The effect of the spin-orbit (SO) interaction and Coulomb repulsion U in a frame of the LDA + SO + U method on the Fermi surface, orbital dependence of the cyclotron masses, and extremal cross sections of the Fermi surface are examined in details. A good agreement with experimental data of cyclotron masses and extremal cross sections of the Fermi surface was achieved.

Description: We review the recent experimental advances on quantum phenomena in transport measurements of topological insulators with emphasis on quantum oscillation, weak antilocalization and Aharonov–Bohm effect and Altshuler–Aronov–Spivak effect. Following a brief introduction on the topic, we discuss the identification of the topological surface state based on quantum phenomena. Research prospect of topological insulators is described at the end of this article.

Description: Motivated by an interest to see if the field-induced (FI) phase in the charge-density wave (CDW) system is similar to the field-induced-SDW (FISDW) in (TMTSF) 2 X, (TMTSF: tetramethyltetraselenafulvalene), we examined the magnetic-field-induced phases in a quasi-one-dimensional (Q1D) organic conductor HMTSF–TCNQ (hexamethylene- tetraselenafulvalene- tetracyanoquinodimethane) under a pressure of 1.1 GPa, where the CDW occurring at 30 K is suppressed. The work was carried out by measurements of angular-dependent magnetoresistance oscillations and exploratory work on the Hall effect. It turned out that the FI-phase, most likely a FICDW for B 〉 0.1 T, accompany a quantum Hall effect, and the FI-phase transitions are controlled by the field component along the least conducting axis. Above 10 T, the lowest Landau level of the small 2D Fermi pocket (due to incomplete nesting of Fermi surface) exceeds the Fermi level, reaching the quantum limit. Although there are many differences between the CDW (HMTSF–TCNQ) and SDW ((TMTSF) 2 X) systems, a similar scenario for field-induced phases seems to hold.

Description: Angle-resolved photoemission spectroscopy (ARPES) enables direct observation of the Fermi surface and underlying electronic structure of crystals, which are the basic concepts necessary to describe all the electronic properties of solids and to reveal the nature of key electronic interactions involved. ARPES proved to be the most efficient for studies of quasi-2D metals, to which the most challenging and hence exciting compounds belong. This stimulated tremendously the development of ARPES in the recent years. The aim of this paper is to introduce the reader to the state-of-the-art ARPES experiment and to review the results of its application to such highly topical problems in solid state physics as high temperature superconductivity in cuprates and iron-based superconductors and electronic ordering in the transition metal dichalcogenides and manganites.

Description: The de Haas-van Alphen (dHvA) effect in the antiferromagnetic compound FeGe 2 is studied experimentally. A sharp suppression of the amplitude of the first harmonic of the dHvA oscillations is observed for a number of orbits on the Fermi surface of FeGe 2 . The change in the conditions for suppression of the amplitude ("spin zeroes") when FeGe 2 is doped with cobalt is determined. A method for analyzing the experimental data is proposed in which the g -factors of the conduction electrons are computed on the basis of calculations of the spin-polarized electron structure of the band antiferromagnet taking spin-orbital interactions into account. The exchange enhancement factor for spin paramagnetism in FeGe 2 is also calculated.

Description: We present an overview of our recent results on quantum magnetic oscillations in new functional materials. We begin with the Lifshitz and Kosevich approach for quasi-2D layered materials and obtain general formulas for the oscillatory parts of the grand thermodynamic potential and magnetization. Then we consider the oscillations of the Nernst–Ettingshausen coefficient which consists of thermal and magnetization parts. The difference between normal and Dirac carriers is also discussed. To conclude we consider a model for multilayer grapheme which allows to calculate exactly the Berry phase which remains undetermined in the Lifshitz–Kosevich approach. The magnetic oscillations of the density of states and capacitance for different number of the carbon layers are described.

Description: We investigate the monotonic growth of longitudinal interlayer magnetoresistance ( B z ), analytically and numerically in the self-consistent Born approximation. We show that in a weak magnetic field the monotonic part of ( B z ) is almost constant and starts to grow only above the crossover field B c , when the Landau levels (LL) become isolated, i.e., when the LL separation becomes greater than the LL broadening. In higher field B z ≫ B c , ( B z ) ∝ B z 1/2 in agreement with previous works. R ¯ z z R ¯ z z R ¯ z z

Description: Occupation numbers of natural orbitals capture the physics of strong electron correlations in momentum space. A natural orbital density functional theory based on the antisymmetrized geminal product provides these occupation numbers and the corresponding electron momentum density. A practical implementation of this theory approximates the natural orbitals by the Kohn–Sham orbitals and uses a mean-field approach to estimate pairing amplitudes leading to corrections for the independent particle model. The method is applied to weakly doped La 2 CuO 4 .

Description: Positron annihilation and Compton scattering are important probes of the Fermi surface. Relying on conservation of energy and momentum, being bulk sensitive and not limited by short electronic mean-free-paths, they can provide unique information in circumstances when other methods fail. Using a variety of examples, their contribution to knowledge about the electronic structure of a wide range of materials is demonstrated.

Description: Spin-lattice effects play an important role in many magnetic materials. In this short review, we give some examples of such effects studied in low-dimensional, frustrated as well as uranium-based antiferromagnets. Utilizing ultrasound measurements at low temperatures and high magnetic fields provides valuable information on the spin-strain interactions. Specifically phase transformations and critical phenomena in magnetic systems with strong spin-lattice interactions are fruitful grounds for sound-velocity and sound-attenuation measurements.

Description: In iron-based superconductors, unravelling the origin of the antiferromagnetism is a crucial step towards understanding the high -T c superconductivity as it is widely believed that the magnetic fluctuations play important roles in the formation of the Cooper pairs. Therefore, in this paper, we will briefly review experimental results related to the antiferromagnetic state in iron-based superconductors and focus on a review of the theoretical investigations which show applicability of the itinerant scenario to the observed antiferromagnetism and corresponding phase transitions in various families of the iron-based superconductors. A proposal of coupling between frustrated and unfrustrated bands for understanding the reduced magnetic moment typically observed in iron pnictides is also reviewed. While all the above theoretical investigations do not rule out a possible existence of localized electrons in iron-based superconductors, these results strongly indicate a close relation between itinerant electrons and the magnetically ordered state and point out the importance of taking into account the orbital degrees of freedom.

Description: The effect of spin-orbit interaction in the strongly correlated exactly solvable electron model with magnetic impurity is studied. The considered magnetic impurity reveals the property of a “mobile” one. It is shown that the asymptotics of correlation functions, calculated in the framework of the conformal field theory and finite size corrections of the Bethe ansatz exact solution, are strongly affected by both, the spin-orbit coupling, and by the magnetic impurity.

Description: The spin wave spectra of antiferromagnetic BiFeO 3 -type multiferronics are analyzed theoretically. The presence of a spatially modulated cycloidal antiferromagnetic structure leads to a countable number of frequency branches of two oscillatory modes (Goldstone and activation) for spin waves propagating along a cycloid. When there is no magnetic field and anisotropy, the magnon spectrum is characterized by the absence of frequency gaps. The spectral features of the spin oscillations with changing anisotropy and application of a magnetic field are identified and the limits on the existence of an antiferromagnetic cycloid are established up to its transformation into a conical structure. In the transverse direction the spin oscillations have a mixed character which indicates that the cycloid is stable with respect to bending throughout its domain of existence.

Description: The high-frequency properties of two-sublattice ferrimagnets are examined near the temperature T L at which the angular momentum of the sublattices is compensated. A diagram technique for spin operators is used, along with analytic expressions that are valid over the entire temperature range of the magnetically ordered state of the system. It is shown that near T L longitudinal fluctuations in the magnetization have a significant influence on the resonance properties of the magnet, with different temperature behaviors of the in-phase and out-of-phase precessions of the sublattice magnetization. As T L is approached the frequency of the in-phase precession increases simultaneously with a rise in the line width, while the frequency of the out-of-phase oscillations falls off significantly, so the two modes become equivalent. These results may be important for understanding the microscopic mechanisms of magnetic reversal by ultrashort laser pulses in rare earth ferrimagnets.

Description: Recently we proposed to describe the fascinating physics of copper carbodiimide, CuNCN, with help of the anisotropic triangular antiferromagnetic Heisenberg model with the parameters J a and J ab extending along the a , and a ± b lattice directions and a new frustrated Heisenberg antiferromagnetic model with exchange parameters J c , J a , and J ac , extending along the c , a , and a ± c ( c-a-ca model) directions assuming the resonating valence bond (RVB) type of the corresponding phases. Here we discuss possible RVB ground states of these models in the mean-field approximation and show that in either case it is a two-dimensional RVB state. The difference between the models is that in the ground state of the triangular model the quasiparticle spectrum features a finite (although exponentially small) energy gap for arbitrary weak J ab whereas that of the c-a-ca model shows two pseudogaps and a linear dependence of the quasiparticle density of states in the low-energy range.

Description: Spintronics of antiferromagnets is a new and rapidly developing field of the physics of magnetism. Even without macroscopic magnetization, antiferromagnets, similar to ferromagnetic materials are affected by spin-polarized current, and as in ferromagnets this phenomenon is based on a spin-dependent interaction between localized and free electrons. However, due to the nature of antiferromagnetic materials (complex magnetic structure, essential role of exchange interactions, absence of macroscopic magnetization) the study of possible spintronic effects requires new theoretical and experimental approaches. The purpose of this review is to systemize and describe recent developments in this area. After presenting the main features of structure and behavior of antiferromagnets various microscopic and phenomenological models for description of the current-induced phenomena in heterostructures containing ferro- and antiferromagnetic layers are considered. The questions related to an effect of antiferromagnetic ordering on an electric current, as well as the questions of possible creation of fully antiferromagnetic spin valves are discussed. In addition, we briefly discuss available experimental results and try to interpret them.

Description: Fluctuation induced conductivity (FIC) analysis on dc resistivity versus temperature data of as-prepared and oxygen post-annealed TlBa 2 (Ca 2- y Mg y )Cu 3 O 10-δ ( y = 0, 0.25, 0.5, 0.75, 1.0) superconductor samples are carried out by using Aslamazov–Larkin model for excess conductivity. The microscopic parameters such as zero temperature coherence length along c axis { ξ c ( 0 ) } , interlayer coupling ( J ), intergrain coupling (α), critical exponent (λ) and dimensionality of superconducting fluctuations are calculated with the help of aforementioned model. The crossover temperature ( T o ) is shifted towards higher temperature values with the increase of Mg contents up to y = 0.5 and then start to decrease but still remains greater than those values of undoped samples. The increase in ξ c (0) and J after Mg-doping at Ca sites shows the improvement of interplane coupling in TlBa 2 (Ca 2- y Mg y )Cu 3 O 10-δ samples. The appreciable change in all the microscopic parameters extracted from the FIC analysis indicates the optimization of oxygen in all the oxygen post-annealed samples. The increase in relative intensity of almost all the oxygen modes indicates the oxygen diffusion in the unit cell after oxygen post-annealing. The diffusion of oxygen can take place at intergranular sites along with intragranular sites, which increases the grains size, intergrain connectivity and carrier density in CuO 2 planes.

Description: The energy spectra of a Thue-Morse superlattice based on monolayer graphene containing a band gap are studied. The lattice consists of rectangular barriers located along the x axis. It is proposed that aperiodic Thue-Morse modulation be produced by a difference in the gap width in different elements of the superlattice. It is shown that effective splitting of the allowed bands (and, thereby, the formation of a series of gaps) under the influence of the aperiodic factor can be observed with both oblique and normal incidence of an electron wave on the superlattice. The spectra have periodicity that depends on the potential barrier height. In some segments of the spectrum, band splitting follows the Fibonacci inflation rule in each new generation. As in periodic graphene-based superlattices, a gap associated with a superlattice Dirac point is formed in all Thue-Morse generations. Its width depends substantially on the parameters of the problem; at the same time the position of this band on the energy axis depends weakly on the mass term in the hamiltonian and is independent of the superlattice period. The spectra have little dependence on the angle of incidence of the electron wave.

Description: This is a study of the low temperature kinetics of the sorption and subsequent desorption of 3 He, 4 He, H 2 , D 2 , Ne, and N 2 by bundles of single-wall carbon nanotubes with closed ends (c-SWNT). The characteristic times of the sorption and desorption processes are the same to within the experimental error. Annealing a sample of bundles at 500 °C greatly reduces the characteristic times and changes their temperature dependences. The effect of annealing decreased with increasing molecular weight of the dissolved gas. At temperatures below 16 K the sorption times for 3 He, 4 He, H 2 , and D 2 depends weakly on temperature, which suggests a tunnel character of the sorption for these impurities by nanotube bundles. The effect of γ-irradiation of nanotube bundles on the sorption and desorption of hydrogen is qualitatively similar to the effect of annealing.

Description: The structural state of as-received course-grained (CG) and extruded ultrafine-grained (UFG) billets of Al–3.8 at. % Li substitutional alloy as well as that of the samples of the same alloy subjected to tensile deformation until fracture at 4.2, 77, 295, and 350 K were studied by the microindentation method. The temperature dependence of microhardness was investigated in the temperature range of 77 to 295 K. The measurements revealed anisotropy of the micromechanical properties of the UFG samples due to the texture formed during the extrusion. It was shown that the samples which were pulled until failure exhibit correlation between the microhardness, which is a measure of local strain-hardening, and the magnitude of the local plastic shear deformation. This correlation breaks down for the UFG samples deformed at 295 and 350 K. In this case the microhardness was found to be independent on the magnitude of plastic shear deformation. The temperature dependences of microhardness for the CG and UFG samples were analyzed within the framework of thermally activated motion of dislocations through the network of local obstacles. Impurity atoms, small complexes of them, as well as forest dislocations represent the most probable energy barriers to the motion of dislocations in the alloy under investigation. Intensive plastic deformation in the process of hydrostatic extrusion produced no qualitative effect on the energy parameters of the obstacle spectrum.

Description: In-phase and quadrature components of the first harmonic response of polycrystalline YBa 2 Cu 3 O 7− x , were measured in a dc magnetic field. For small field modulation amplitude of 0.005 Oe, in-phase component of the first harmonic response of a sample was found to be proportional to the differential magnetic susceptibility. It was found that there is no pinning of Josephson vortices in a weakly coupled medium in magnetic fields below H c 2 J , where H c 2 J is the upper critical field of Josephson weak links. The obtained initial magnetization curve allowed us to describe the nonlinear part of the magnetization arising due to the weak bonds in ceramics. The size of a Josephson vortex and the critical parameters H p , H c 2 J and H c 1 g were evaluated.

Description: The temperature variations of the thermoelectric power coefficients of the Hg-based high-temperature superconductors HgBa 2 Ca n −1 Cu n O 2 n +2+δ ( n = 1, 2, 3) with oxygen doping and cation substitutions are analyzed in terms of a narrow conduction band model. The parameters of the band spectrum in the vicinity of the Fermi level are determined and the manner in which they transform is examined. A correlation is found between the effective band width and the superconducting transition temperature T c .

Description: The structure of hydrate cover layers of SiO 2 -DNA-Dox (where Dox: doxorubicin) and SiO 2 -DNA-Dox-C 60 fullerene hybrids was studied by means of low-temperature 1 H NMR spectroscopy in tetrachloromethane. The hydration properties of SiO 2 -DNA-Dox nanomaterials combined with fullerenes and their derivatives are extremely important for their further use as therapeutics in cancer treatment and for safety reasons. The findings reveal that the hydration properties of the hybrids differ from those of the solid DNA particulates or SiO 2 -DNA systems due to the existence of different types of water clusters, namely the weakly (WAW) and strongly associated water (SAW) clusters. For SAW clusters the radial distributions as well as the distributions of change in Gibbs free energy due to adsorptive interactions at the surfaces of the investigated systems were obtained.

Description: The widths of Cherenkov radiation spectrum lines of Josephson vortex trains in extended sandwiches are determined. Conditions are determined such that the line widths owing to a finite train length are comparable to or less than the line widths owing to thermal fluctuations.

Description: A 704 type 4 He cryostat is designed and the “National (China) Temperature Working Standard Group of the ITS-90 at 1.2–24 K” have been composed. A 703 type 3 He cryostat have been established and the working temperature of the “National Temperature Working Standard Group of the ITS-90 at 1.2–24 K” is extended to 0.65 K, which is the lowest limit temperature of ITS-90. The 5 rhodium-iron resistance thermometers (RIRTs) which compose the Working Standard Group have been transformed to the International Temperature Scale of 1990 (ITS-90). A set of indexing data is fitted and calculated as the indexing table of the standard facility of RIRT. 42 basis temperature points have been measured at 0.67–26.37 K. The results show that the temperature control level could reach 0.5 mK/30 min. The difference between the temperature value of the 5 RIRTs and the average is less than 1.1 mK, and the comparison measurements uncertainty is 0.95 mK.

Description: The transport properties of a composite consisting of the microparticles of superconductor MgB 2 and nanopowder of manganite La 0.7 Sr 0.3 MnO 3 (LSMO) were studied. It was found that when the amount of the ferromagnet component in the composite was above 10% by volume, a sharp increase in the resistivity of the composite occurred. This is caused by the disruption of percolation paths through the magnesium diboride microparticles, thus leading to the formation of a network of superconductor-ferromagnet-superconductor junctions. As a result, a significant broadening of the superconducting transition of the composite was observed while its ferromagnetic properties were maintained. The obtained data indicate that Cooper pairs can pass through the layer of a half-metal ferromagnet with high (about 100%) spin polarization of the charge carriers and thickness d  〉 10 nm.

Description: The possibility to reconstruct the electron-phonon interaction (EPI) function was demonstrated for S–c–N and S–c–S point contacts using the superconducting inelastic contribution to the excess current caused by Andreev reflection processes. Superconductors with both weak (Sn, Al) and strong (Pb, In) EPI were considered. It was shown that in the latter case it is necessary to account for the elastic component of current which is related to the frequency dependence of the superconducting energy gap arising due to electron-phonon renormalization of the energy spectrum of the superconductor.

Description: The dependence of the chemical potential jump coefficient on the evaporation coefficient of a binary mixture is studied in the case where the evaporating component is a Bose gas with a concentration assumed to be much lower than that of the carrier gas. An analytical solution of the problem yields an expression for the chemical potential jump for the case of a constant collision frequency of the Bose gas molecules.

Description: It is shown that the long-wave dynamics and magnetic properties of one-dimensional systems constructed of the inductively and capacitively coupled split-ring resonators are described by the regularized nonlinear dispersive Klein–Gordon equations. It is found that in such systems a high-frequency magnetic field excites dynamic solitons on a “pedestal”—stable breathers, oscillating in anti-phase with respect to the background of uniform oscillations, which means the existence of regions with a negative magnetic permeability in the system. If supplemented by a medium with negative permittivity, such a system forms a “left-handed” metamaterial in which the regions with the breather excitations are transparent to electromagnetic radiation. This makes it possible to observe them experimentally.

Description: We report on properties of Nb(/Ti)–carbon–(Ti/)Nb junctions fabricated on graphite flakes using e-beam lithography. The devices were characterized at temperatures above 1.8 K where a Josephson current was not observed, but the differential conductivity revealed features below the critical temperature of Nb, and overall metallic conductivity, in spite of a high-junctions resistance. Since the conductivity of graphite along the planes is essentially two-dimensional (2D), we use a theoretical model developed for metal/graphene junctions for interpretation of the results. The model involves two very different graphene “access” lengths. The shorter length characterizes ordinary tunneling between the three-dimensional Nb(/Ti) electrode and 2D graphene, while the second, much longer length, is associated with the Andreev reflections (AR) inside the junction and involves also “reflectionless” AR processes. The relevant transmission factors are small in the first case and much larger in the second, which explains the apparent contradiction of the observed behaviors.

Description: Equations for normal and superfluid liquids with spontaneous polarization are obtained by a phenomenological approach. It is shown that the propagation of sound waves in media with spontaneous polarization is accompanied by electric field oscillations. Corrections are calculated for the speeds of first and second sound in normal and superfluid polar liquids.

Description: The conditions for the transition from laminar to turbulent flow in superfluid 4 He are investigated experimentally, and the effect of acoustic emission with variable power on the critical velocity for the transition is studied. The quartz tuning fork method is used at temperatures of 2–0.3 K. The experiments are done over a wide range of pressures, from the saturated vapor pressure to 24.8 atm. It is found that at high temperatures ( T 〉 0.9 K) the critical velocity is determined by viscous friction and at low temperatures ( T 〈 0.5 K) by the effect of acoustic emission, which leads to a significant increase in the critical velocity for the transition to the turbulent state. The critical velocity depends on the power of the acoustic emission and the transition to the turbulent state of the superfluid is similar to that in ordinary liquids or gases. In the absence of any effects of acoustic emission, the critical transition velocity is essentially independent of temperature and the driving power is mainly determined by ballistic scattering of thermal excitations.

Description: Patterns of plastic deformation of nanocrystalline (NC) technical grade VT1-0 titanium, are studied in quasi-static tensile experiments, with the average size of the grain d , ranging from 35 nm to 2 μ m, at the temperature interval 4.2 K 〈 T 〈 395 K. The wide range of grain size, and grain size distribution, was made possible by cryomechanical grain fragmentation, which involves rolling at liquid nitrogen temperature, and subsequent annealing. At temperatures of T  ≲ 30 K, smooth deformation curves become wavy, and as the temperature is continually lowered to T jump ≈ 22 K, they become jagged. A correlation is found between the relative amplitude of the stress jump Δσ/σ and the rate of strain hardening θ = (∂σ/∂ e ) ė . A significant increase in plasticity is observed, especially noticeable at temperatures T ≲140 K, if a small fraction (≈15%) of submicron-sized grains is present. This is explained by a combination of dynamic grain growth under the influence of tensile stress, and nanotwinning activated in submicron grains. At cryogenic temperatures, abnormal grain growth favors nanotwinning during deformation. In nanometer-sized grains ( d ≲ 50 nm), twinning is not observed.

Description: Stationary Josephson current I c between d -wave superconductors was calculated for superconductor-insulator-superconductor structures with various rotations of crystals relatively to each other and the junction plane. The directionality of tunneling was taken into account. It was demonstrated that the temperature, T, dependences of I c can become nonmonotonic for certain electrode orientations due to the T -dependent ratio between the contributions of positive and negative I c components. This model gives an explanation to the effect observed in junctions involving cuprates and is an alternative to the well-known scenario that makes allowance for Andreev-Saint-James zero-energy levels.

Description: Using a one-dimensional model of the crystal lattice, the CsDy(MoO 4 ) 2 single crystal low-frequency phonon vibrational spectrum, is calculated using previously measured far-infrared (10–10 cm −1 ) light transmission spectra, in polarized light, at low temperatures (6–40 K). The calculation is done for CsDy(MoO 4 ) 2 structures both above and below the temperature of the structural phase transition ( T C  = 38 K). It is determined that the result of this transition is a change in the phonon vibration and light transmission spectra.

Description: The dynamics of magnetic flux trapped in low dc magnetic fields (of the order of the Earth's field) was for the first time studied experimentally in single-crystal YBCO samples with unidirectional twin boundaries in the temperature range near T c (0.8 〈 T / T c 〈 0.99). Strong pinning in the system of unidirectional planar defects was demonstrated, and a significant deviation from monotonous behavior was established for the averaged effective pinning potential U p ( T ) for the trapped flux of low density. In order to compare different methods of J c determination, the field dependences of the magnetization loop width M ( H ), which are related to the effective pinning and J c , were obtained, and resistive measurements on microbridges made from the same single crystals were carried out.

Description: Photoelectric relaxation spectroscopy (photoinduced current relaxation spectroscopy, PICTS) is used to study pure and La-doped single crystals of TlInS 2 . The characteristics of electrically active defects are determined including the capture cross section for charge carriers, the thermal activation energy, and the temperature interval within which the charge state of a defect varies. The pyroelectric properties of TlInS 2 :La are studied. The defect responsible for the anomalies in the pyrocurrent is identified.

Description: It is shown that the first-order isostructural magnetic phase transition between antiferromagnetic phases with different magnitudes of the antiferromagnetism vector induced by an external magnetic field in Ising antiferromagnets can be related to entropy. It is found that, depending on the temperature, the entropy jump and the corresponding heat release can change their signs at the phase transition point. In the low-temperature region of the metamagnetic first-order phase transition the entropy jump is positive, while the entropy jump is negative near the triple point for isostructural magnetic phase transitions.

Description: An exact diagonalization method is applied to solve the quantum-mechanical problem of spinless helium atom in an external electric field of arbitrary magnitude. The basis set for two-electron problem is built from different pair combinations ψ n a l a m a ( α r a ) ψ n b l b m b ( α r b ) of orthonormalized single-particle hydrogen-like wave functions ψ n m l ( r ) belonging to any possibly bound states of the individual a- and b-electrons in the Coulomb central field renormalized by the scale parameter α 〉 0. Within the selected basis the matrix elements of the total Hamiltonian allows an exact analytical representation in the form of finite numerical sums. The diagonalization procedure is performed by Jacobi algorithm for N  ×  N square Hermitian matrix built on the basis of dimension N  = 25. The systematics and the numerical values of the low-lying energy levels at zero field are in good agreement with known experimental data. The field dependences of low-lying levels (Stark effect) and polarizability in the ground state of helium atom are presented. It is shown that even extremely high external fields lead only to shifting or splitting of existing low levels, without disturbance of their systematics. Typically, no new low-energy excitation can be created under external electric field of moderate intensity. Radical reconstruction in spectrum of individual helium atoms can be expected in condensed helium phases where each atom is deeply affected by interaction fields from neighbors. This result should be taken into account at interpretation of electrodynamic experiments on superfluid helium.

Description: Stationary Josephson current I c between d -wave superconductors was calculated for superconductor-insulator-superconductor structures with various rotations of crystals relatively to each other and the junction plane. The directionality of tunneling was taken into account. It was demonstrated that the temperature, T, dependences of I c can become nonmonotonic for certain electrode orientations due to the T -dependent ratio between the contributions of positive and negative I c components. This model gives an explanation to the effect observed in junctions involving cuprates and is an alternative to the well-known scenario that makes allowance for Andreev-Saint-James zero-energy levels.

Description: Using a one-dimensional model of the crystal lattice, the CsDy(MoO 4 ) 2 single crystal low-frequency phonon vibrational spectrum, is calculated using previously measured far-infrared (10–10 cm −1 ) light transmission spectra, in polarized light, at low temperatures (6–40 K). The calculation is done for CsDy(MoO 4 ) 2 structures both above and below the temperature of the structural phase transition ( T C  = 38 K). It is determined that the result of this transition is a change in the phonon vibration and light transmission spectra.

Description: An analysis of the experimental data of the temperature dependence of metallic glass and superconductor magnetization. A common feature is the irreversibility of magnetization: below a certain temperature T *, under fixed field strength, magnetization varies during cooling within ( M FC ), and outside ( M ZFC ) of a magnetic field. It is demonstrated that for all samples, the line T *( H ) is universal, and corresponds to the theoretical dependence of de Almeida-Thouless ( T * ∼ H 2/3 ) in all studied ranges of magnetic fields.

Description: The EPR spectra of impurity Dy 3+ ions in YAl 3 (BO 3 ) 4 and EuAl 3 (BO 3 ) 4 aluminoborates were investigated. The signals from the ground and excited doublets were observed and the g -factors and the hyperfine interaction constants were determined. From the temperature dependence of the signal intensity of the excited doublet, the energy distances between the ground and excited doublets were obtained as (3.26 ± 0.13) cm –1 and (2.54 ± 0.16) cm –1 for YAl 3 (BO 3 ) 4 and EuAl 3 (BO 3 ) 4 , respectively. A broadening of the absorption line with increasing temperature is related with the strong spin-phonon interaction and is described by the Orbach–Aminov process via the excited doublet. It was found that the decrease in g -factor is caused by the spin-phonon interaction.

Description: Patterns of plastic deformation of nanocrystalline (NC) technical grade VT1-0 titanium, are studied in quasi-static tensile experiments, with the average size of the grain d , ranging from 35 nm to 2 μ m, at the temperature interval 4.2 K 〈 T 〈 395 K. The wide range of grain size, and grain size distribution, was made possible by cryomechanical grain fragmentation, which involves rolling at liquid nitrogen temperature, and subsequent annealing. At temperatures of T  ≲ 30 K, smooth deformation curves become wavy, and as the temperature is continually lowered to T jump ≈ 22 K, they become jagged. A correlation is found between the relative amplitude of the stress jump Δσ/σ and the rate of strain hardening θ = (∂σ/∂ e ) ė . A significant increase in plasticity is observed, especially noticeable at temperatures T ≲140 K, if a small fraction (≈15%) of submicron-sized grains is present. This is explained by a combination of dynamic grain growth under the influence of tensile stress, and nanotwinning activated in submicron grains. At cryogenic temperatures, abnormal grain growth favors nanotwinning during deformation. In nanometer-sized grains ( d ≲ 50 nm), twinning is not observed.

Description: The effect of combined deformation, including equal-channel multiangle pressing (ECMAP), low-temperature (77 K) drawing, and thermal processing, on the critical current density of superconducting wires based on Nb-Ti alloys is investigated. Reasonable parameters are established for deformation-thermal processing of NT-50 alloy using ECMAP that will ensure improved functionality of superconductors based on this alloy. Vortex pinning on structural inhomogeneities of these alloys is analyzed qualitatively.

Description: An exact diagonalization method is applied to solve the quantum-mechanical problem of spinless helium atom in an external electric field of arbitrary magnitude. The basis set for two-electron problem is built from different pair combinations ψ n a l a m a ( α r a ) ψ n b l b m b ( α r b ) of orthonormalized single-particle hydrogen-like wave functions ψ n m l ( r ) belonging to any possibly bound states of the individual a- and b-electrons in the Coulomb central field renormalized by the scale parameter α 〉 0. Within the selected basis the matrix elements of the total Hamiltonian allows an exact analytical representation in the form of finite numerical sums. The diagonalization procedure is performed by Jacobi algorithm for N  ×  N square Hermitian matrix built on the basis of dimension N  = 25. The systematics and the numerical values of the low-lying energy levels at zero field are in good agreement with known experimental data. The field dependences of low-lying levels (Stark effect) and polarizability in the ground state of helium atom are presented. It is shown that even extremely high external fields lead only to shifting or splitting of existing low levels, without disturbance of their systematics. Typically, no new low-energy excitation can be created under external electric field of moderate intensity. Radical reconstruction in spectrum of individual helium atoms can be expected in condensed helium phases where each atom is deeply affected by interaction fields from neighbors. This result should be taken into account at interpretation of electrodynamic experiments on superfluid helium.

Description: It is shown that the first-order isostructural magnetic phase transition between antiferromagnetic phases with different magnitudes of the antiferromagnetism vector induced by an external magnetic field in Ising antiferromagnets can be related to entropy. It is found that, depending on the temperature, the entropy jump and the corresponding heat release can change their signs at the phase transition point. In the low-temperature region of the metamagnetic first-order phase transition the entropy jump is positive, while the entropy jump is negative near the triple point for isostructural magnetic phase transitions.

Description: Photoelectric relaxation spectroscopy (photoinduced current relaxation spectroscopy, PICTS) is used to study pure and La-doped single crystals of TlInS 2 . The characteristics of electrically active defects are determined including the capture cross section for charge carriers, the thermal activation energy, and the temperature interval within which the charge state of a defect varies. The pyroelectric properties of TlInS 2 :La are studied. The defect responsible for the anomalies in the pyrocurrent is identified.

Description: An analysis of the experimental data of the temperature dependence of metallic glass and superconductor magnetization. A common feature is the irreversibility of magnetization: below a certain temperature T *, under fixed field strength, magnetization varies during cooling within ( M FC ), and outside ( M ZFC ) of a magnetic field. It is demonstrated that for all samples, the line T *( H ) is universal, and corresponds to the theoretical dependence of de Almeida-Thouless ( T * ∼ H 2/3 ) in all studied ranges of magnetic fields.

Description: Theoretical results and experimental data on flux creep in high-temperature superconductors (HTSC) are analyzed in this review paper. When reviewing experimental work, the main attention is paid to the most striking experimental results which have had a major impact on the study of flux creep in HTSC. On the other hand, the analysis of theoretical results is focused on the studies which explain the features of flux creep by introducing modifications to the Anderson-Kim (AK) theory, i.e., on the studies that have not received sufficient attention earlier. However, it turned out that the modified AK theory could explain a number of features of flux creep in HTSC: the scaling behavior of current-voltage curves in HTSC, the finite rate of flux creep at ultralow temperatures, the logarithmic dependence of the effective pinning potential on the transport current and its decrease with temperature. The harmonic potential field which is used in this approach makes it possible to solve accurately both the problem of viscous vortex motion and the problem of thermally activated flux creep in this magnetic field. Moreover, the energy distribution of pinning potential and the interaction of vortices with each other are also taken into account in the approach. Thus, the modification of the AK theory consists, essentially, in its refinement and achieving a more realistic approximation.

Description: The article presents study results of VUV and UV cathodoluminescence spectra, of solid ternary Kr-Xe-D 2 solutions, and Xe-D 2, Kr-D 2 , Xe-Kr solid binary cryoalloys. The object of study is the dependence of the spectral band intensity distribution, on the concentration of impurities, and the dosage dependence of luminescence. All emission bands were identified. It is demonstrated that the formation of emitting states is caused by a transfer of energy through the system of its own electron excitation matrices. An emission from (Xe 2 D)* complexes is observed, if Xe is present in ternary solid solutions. It is shown that the process of forming (Xe 2 D)*-complexes has multiple steps, and includes the formation of an intermediate [XeD 2 ]* non-stable state at the first stage, the nonradiative decay of which leads to the dissociation of the admixture molecular deuterium. It is suggested that the formation of [XeD 2 ]* complexes occurs due to the high proton affinity of Xe, and its negative affinity to the electron in the D 2 molecule, as a result of which, the intermediate complex acquires the features of a charge-transfer complex.

Description: Ceramic samples of Pr 0.6− x Nd x Sr 0.3 Mn 1.1 O 3−δ ( x = 0−0.6) were studied by x-ray diffraction, resistive, magnetic (χ ac and 55 Mn NMR), magnetoresistive and electron microscopy methods. It was shown that with increasing the concentration x , the type of unit cell distortion changes from orthorhombic ( x = 0–0.2) to pseudo-cubic ( x = 0.4–0.6), and the imperfection of the structure, which contains anion and cation vacancies, is increased. A decrease in the temperatures of metal–semiconductor ( T ms ) and ferromagnetic–paramagnetic ( T C ) phase transitions and an increase in the resistivity and activation energy with increasing x was explained by an increase in the concentration of vacancies, which weakens high-frequency electronic double-exchange Mn 3+ ↔ Mn 4+ . It was found that the compositions with a higher content of neodymium exhibit a transition to the antiferromagnetic state at temperatures below 130 K. Two types of magnetoresistive effects were observed. The magnitude of the first effect, which occurs near the phase transition temperatures T ms and T C , increases with concentration x . The magnitude of the second effect, which was observed at low temperatures, exceeds that of the first one. The magnetic phase diagrams which describe strong correlations between the composition, structure defects, phase transitions, and functional characteristics, including magnetoresistive effect, were constructed.

Description: Based on the assumption of a negative volume dependence of random exchange integrals, it is possible to switch to a compressible Sherrington–Kirkpatrick spin-glass model. Within the proposed model, temperature–pressure phase diagrams were calculated and pressure- and magnetic-field-induced first-order phase transitions from the initial paramagnetic and spin-glass states to the ferromagnetic state were predicted. It was shown that the application of pressure in the spin-glass state not only increases and shifts magnetic susceptibility, but also reduces the critical magnetic fields of irreversible induced phase transitions from the spin-glass to the ferromagnetic state. The obtained results are used to describe the spin-glass state in (Sm 1– x Gd x ) 0.55 Sr 0.45 MnO 3 .

Description: High-pressure magnetic susceptibility experiments can provide insights into the changes in magnetic behavior and electric properties which can accompany extreme compressions of material. Instrumentation plays an important role in the experimental work in this field since 1990s. Here we present a comprehensive review of the high-pressure instrumentation development for magnetic measurement from the engineering perspective in the last 20 years. Suitable nonmagnetic materials for high pressure cell are introduced initially. Then we focus on the existing cells developed for magnetic property measurement system (MPMS ® ) SQUID magnetometer from Quantum Design (USA). Two categories of high pressure cells for this system are discussed in detail respectively. Some high pressure cells with built-in magnetic measurement system are also reviewed.

Description: Exact analytical expressions for propagator of small-amplitude linear magnetostatic waves in ferromagnetic thin film between two antennae and their corresponding mutual impedance are obtained by solving the linearized torque equation of spin dynamics (Landau–Lifshitz equation) in magnetostatic approximation. This is done for the case of arbitrary orientation of uniform static magnetization of the film and full account for arbitrary magnetic anisotropy. The result also contains full description of the magnetostatic spin-wave spectrum.

Description: The effect of high hydrostatic pressures up to 17 kbar on the conductivity in the basal ab -plane of single crystal samples of Y 1– x Pr x Ba 2 Cu 3 O 7–δ with a system of unidirectional twin boundaries and lightly doped with praseodymium ( x = 0.05) is studied. It is found that, in contrast with undoped YBa 2 Cu 3 O 7-δ samples with an optimum oxygen content, high pressure leads to a double increase in the pressure derivative dT c / dP . Possible mechanisms for the effect of high pressure on T c are discussed taking the existence of singularities in the carrier electronic spectrum into account.

Description: The article presents study results of VUV and UV cathodoluminescence spectra, of solid ternary Kr-Xe-D 2 solutions, and Xe-D 2, Kr-D 2 , Xe-Kr solid binary cryoalloys. The object of study is the dependence of the spectral band intensity distribution, on the concentration of impurities, and the dosage dependence of luminescence. All emission bands were identified. It is demonstrated that the formation of emitting states is caused by a transfer of energy through the system of its own electron excitation matrices. An emission from (Xe 2 D)* complexes is observed, if Xe is present in ternary solid solutions. It is shown that the process of forming (Xe 2 D)*-complexes has multiple steps, and includes the formation of an intermediate [XeD 2 ]* non-stable state at the first stage, the nonradiative decay of which leads to the dissociation of the admixture molecular deuterium. It is suggested that the formation of [XeD 2 ]* complexes occurs due to the high proton affinity of Xe, and its negative affinity to the electron in the D 2 molecule, as a result of which, the intermediate complex acquires the features of a charge-transfer complex.

Description: The conditions for the transition from laminar to turbulent flow in superfluid 4 He are investigated experimentally, and the effect of acoustic emission with variable power on the critical velocity for the transition is studied. The quartz tuning fork method is used at temperatures of 2–0.3 K. The experiments are done over a wide range of pressures, from the saturated vapor pressure to 24.8 atm. It is found that at high temperatures ( T 〉 0.9 K) the critical velocity is determined by viscous friction and at low temperatures ( T 〈 0.5 K) by the effect of acoustic emission, which leads to a significant increase in the critical velocity for the transition to the turbulent state. The critical velocity depends on the power of the acoustic emission and the transition to the turbulent state of the superfluid is similar to that in ordinary liquids or gases. In the absence of any effects of acoustic emission, the critical transition velocity is essentially independent of temperature and the driving power is mainly determined by ballistic scattering of thermal excitations.

Description: Equations for normal and superfluid liquids with spontaneous polarization are obtained by a phenomenological approach. It is shown that the propagation of sound waves in media with spontaneous polarization is accompanied by electric field oscillations. Corrections are calculated for the speeds of first and second sound in normal and superfluid polar liquids.

Description: The dynamics of magnetic flux trapped in low dc magnetic fields (of the order of the Earth's field) was for the first time studied experimentally in single-crystal YBCO samples with unidirectional twin boundaries in the temperature range near T c (0.8 〈 T / T c 〈 0.99). Strong pinning in the system of unidirectional planar defects was demonstrated, and a significant deviation from monotonous behavior was established for the averaged effective pinning potential U p ( T ) for the trapped flux of low density. In order to compare different methods of J c determination, the field dependences of the magnetization loop width M ( H ), which are related to the effective pinning and J c , were obtained, and resistive measurements on microbridges made from the same single crystals were carried out.

Description: Theoretical results and experimental data on flux creep in high-temperature superconductors (HTSC) are analyzed in this review paper. When reviewing experimental work, the main attention is paid to the most striking experimental results which have had a major impact on the study of flux creep in HTSC. On the other hand, the analysis of theoretical results is focused on the studies which explain the features of flux creep by introducing modifications to the Anderson-Kim (AK) theory, i.e., on the studies that have not received sufficient attention earlier. However, it turned out that the modified AK theory could explain a number of features of flux creep in HTSC: the scaling behavior of current-voltage curves in HTSC, the finite rate of flux creep at ultralow temperatures, the logarithmic dependence of the effective pinning potential on the transport current and its decrease with temperature. The harmonic potential field which is used in this approach makes it possible to solve accurately both the problem of viscous vortex motion and the problem of thermally activated flux creep in this magnetic field. Moreover, the energy distribution of pinning potential and the interaction of vortices with each other are also taken into account in the approach. Thus, the modification of the AK theory consists, essentially, in its refinement and achieving a more realistic approximation.

Description: A reduction in the transmission of terahertz radiation through a periodically modulated slab of layered superconductor owing to diffraction of the incident wave and resonance excitation of eigenmodes is predicted and studied theoretically. The slab thickness is assumed to be much less than the skin depth and when there is no modulation the slab transmission is close to unity.

Description: Theoretical investigation of the spatial confinement, self-polarization and exciton–phonon interaction influence on the exciton state in plane double nanoheterostructure (nanofilm)–lead iodide in polymeric matrix is performed within the effective mass approximation for the electron and dielectric continuum for the phonons in the framework of infinitely deep single quantum well. It is shown that spatial confinement is the dominating feature determining the energy of the bottom of exciton ground band and its binding energy. The relationship of two others depends on nanofilm thickness: in ultrathin films the influence of self-polarization effect is essentially bigger than the role of exciton–phonon interaction.

Description: Plastic deformation is studied in single-crystal β-tin oriented for plastic slip in the (100) 〈010〉 system. Cases are examined in which the logarithmic low-temperature creep curves are preceded by dynamic movement of dislocations. For this purpose an attachment was developed for the deformation machine that made it possible to change the external load in a standard way over identical times on the order of 0.1 s. A computer was used to record the creep process with a time resolution of 0.04 s. The logarithmic creep coefficient α is found to decrease dramatically when the preceding dynamic component of the deformation increment is increased. Thus, correct study of low-temperature logarithmic creep and its mechanisms requires a correct choice of the experimental conditions that excludes the dynamic stage when possible.

Description: The absorption spectrum of thin film CsPbCl 3 in the 2–6 eV range is studied at temperatures of 90–500 K. Sudden changes show up in the temperature dependences of the parameters of the long-wavelength exciton band (spectral position E m ( T ), half width Γ( T ), and oscillator strength f ( T )) at the first order phase transitions at 310 and 320. No phase transitions in E m ( T ), Γ( T ), and f ( T ) are detected at low temperatures. The exciton excitations in CsPbCl 3 are found to have a three-dimensional character.

Description: Temperature dependent positron annihilation lifetime spectroscopy (PALS) measurements in the range of 50–300 K are carried out to study positronium formation in 40 KeV B + -ion implanted polymethylmethacrylate (B:PMMA) with two ion doses of 3.13 × 10 15 and 3.75 × 10 16 ions/cm 2 . The investigated samples show the various temperature trends of ortho -positronium ( o -Ps) lifetime τ 3 and intensity I 3 in PMMA before and after ion implantation. Two transitions in the vicinity of ∼150 and ∼250 K, ascribed to γ and β transitions, respectively, are observed in the PMMA and B:PMMA samples in consistent with reference data for pristine sample. The obtained results are compared with room temperature PALS study of PMMA with different molecular weight ( M w ) which known from literature. It is found that B + -ion implantation leads to decreasing M w in PMMA at lower ion dose. At higher ion dose the local destruction of polymeric structure follows to broadening of lifetime distribution (hole size distribution).

Description: (Submitted December 18, 2013) The condensation of indirect excitons in double quantum wells is studied in an electric field created by electrodes of different shapes. The finite value of the exciton lifetime, the pumping and nonuniformity of the electric field under the electrode are taken into account. It is shown that islands of exciton condensed phase emerge under electrodes when the pumping exceeds a certain threshold value. They appear first under the rim where the potential energy of excitons has a dip. Calculations predict a complicated evolution of the exciton density distribution: from the gaseous phase at low laser intensities to the condensed phase in the whole area under the electrode at larger intensities. Therefore, the configurations of the exciton condensed phase may be manipulated by choosing the setups with conductive electrodes of different shapes via forming specific potentials of the electrical field and controlled by the level of the laser irradiation.

Description: The response of copper-permalloy (Ni 80 Fe 20 ) point microcontacts to microwave irradiation (8–12 GHz) is investigated systematically. The effects of the external magnetic field strength, transport current flowing through the contact, and microwave intensity and frequency on the measured signal are studied. The contributions to the contact electrical resistance owing to giant and anisotropic magnetoresistance are analyzed. The experimentally determined positions of the resonance features are in good agreement with Kittel's formula for FMR in films in parallel magnetic fields. The resonance signal is observed against the background of an additional contribution owing to rectification of the rf field on the nonlinear current-voltage characteristic of the contact. Two mechanisms for the resonance response are discovered: synchronous self-detection from mixing of the rf current with temporal variations in the resistance and a bolometric response associated with the steady-state variation in the contact resistance when magnetization precession is excited. The amplitude of the resonance signal is linearly proportional to the transport current through the contact and to the intensity of the microwave irradiation.

Description: Nonlinear current-voltage characteristics and magnetoresistance of point contacts between a normal metal (N) and films of amorphous ferromagnet (F) Co 40 Fe 40 B 20 of different thickness, exchange-biased by antiferromagnetic Mn 80 Ir 20 are studied. A surface spin valve effect in the conductance of such F–N contacts is observed. The effect of exchange bias is found to be inversely proportional to the Co 40 Fe 40 B 20 film thickness. This behavior as well as other magneto-transport effects we observe on single exchange-pinned ferromagnetic films are similar in nature to those found in conventional three-layer spin-valves.

Description: This review is devoted to the studies of the resistive state in wide superconducting films induced by dc and ac currents in the absence of an external magnetic field. It is found experimentally that the current-voltage characteristics and their parameters are well described by the Aslamazov–Lempitskii theory of the resistive vortex state, which has long been lacking an adequate evidence to support it. There is experimental evidence that the vortex mechanism of resistivity is absent for currents exceeding the maximum current at which the resistive vortex state exists in wide films. The non-vortex nature of phase slip lines (PSL) in wide films is proved experimentally. It is shown that the phase-slip lines arise in a wide superconducting film subjected to an external microwave irradiation with the power exceeding some critical value. The resistance of these phase-slip lines depends on irradiation frequency. Results obtained in studies of PSL in wide films are similar to those obtained for the phase-slip centers in narrow channels. This suggests the identical nature of the phase-slip processes in these two phenomena. It is established that the current-carrying resistive state of a wide film arises due to two different mechanisms that occur one after the other: the penetration into the film and transverse motion of the Pearl vortices induced by the magnetic field of the transport current and the emergence of the phase-slip lines of the superconducting order parameter.

Description: Within the formalizm of Usadel equations the Josephson effect in dirty point contacts between single-band and three-band superconductors is investigated. The general expression for the Josephson current, which is valid for arbitrary temperatures, is obtained. We calculate current-phase relations for very low temperature and in the vicinity of the critical temperature. For three-band superconductors with broken time-reversal symmetry (BTRS) point contacts undergo frustration phenomena with different current-phase relations, corresponding to φ-contacts. For three-band superconductors without BTRS we have close to sinusoidal current-phase relations and absence of the frustration, excepting the case of very low temperature, where under certain conditions two ground states of the point contact are realized. Our results can be used as the potential probe for the detection of the possible BTRS state in three-band superconducting systems.

Description: We show that self sustained mechanical vibrations in a model magnetic shuttle device can be driven by both the charge and the spin accumulated on the movable central island of the device. Different scenarios for how spin- and charge-induced shuttle instabilities may develop are discussed and shown to depend on whether there is a Coulomb blockade of tunneling or not. The crucial role of electronic spin flips in a magnetically driven shuttle is established and shown to cause giant magnetoresistance and dynamic magnetostriction effects.

Description: Unconventional high-temperature superconductivity in MgB 2 :La 0:65 Sr 0:35 MnO 3 (MgB:LSMO) nanocomposite has been found recently [V. N. Krivoruchko and V. Yu. Tarenkov, Phys. Rev. B 86 , 10502 (2012)]. In this report, the symmetry of the nanocomposite superconducting order parameter and plausible pairing mechanisms have been studied by the point-contact Andreev-reflection (PCAR) spectroscopy. To clarify the experimental results obtained, we consider a model of a ferromagnetic superconductor, which assumes a coexistence of itinerant ferromagnetism and mixed-parity superconductivity. The Balian–Werthamer state, with quasiparticle gap topology of the same form as that of the ordinary s- wave state, fits the experimental data reasonably well. Utilizing the extended Eliashberg formalizm, we calculated the contribution of MgB 2 in the total composite's conductivity and estimated the magnitude of the electron–phonon effects originated from MgB 2 in I–V characteristics of the composite at above-gap energies. It was found that distinctive features observed in the PC spectra of the MgB:LSMO samples and conventionally attributed to the electron–phonon interaction cannot be related to the MgB 2 phonons. It is argued that the detected singularities may be a manifestation of the electron-spectrum renormalizations due to strong magnetoelastic (magnon–phonon) interaction in LSMO.

Description: The method of recording the current-voltage characteristic was used to study the processes occurring at the surface of a point contact conductance channel. The transport characteristics of the point contact in a liquid medium are studied for the first time. The current states of the conductance channel corresponding to the reversible, transient, and irreversible regimes of charge transport in a point contact are studied and identified. In the irreversible range of bias voltage, the famous cyclical effect of electromechanical switching is observed on the contact, which governs the growth and dissolution of dendritic point contacts of the test sample. The electric resistance of the point-contact structure changes over time, going through stages of increase, decrease, and stabilization. Subsequently, the stages of this process are repeated multiple times, reflecting the cyclical nature of the changes in the physiochemical properties of the test object. The current-voltage characteristic of the point contact has a step structure, due to shell effects. Copper point contact conductance histograms, formed spontaneously in the electric field under the influence of the shell effect, are built using the obtained curves. There is a demonstrated presence of preferred current states for the conductance channel, serving as evidence of the quantum nature of conductance changes in the process of dendritic point contact formation.

Description: The electronic gap structure of Nb 3 Sn was measured by the break-junction (BJ) tunneling technique. The superconducting gap values are estimated to be in the range 2Δ = 4–5.5 meV at T = 4.2 K as follows from the observed distinct conductance peaks. In addition to the superconducting gap structure, we observed reproducible hump-like structures at the biases of about ±20 and ±50 mV. Such a coexistence of gap and hump structures resembles the situation found in the high- T c copper-oxide superconductors. Above the superconducting critical temperature T c ∼ 18 K, the humps appear as the only gap-like structures. Their possible origin is discussed in connection to the structural phase transition occurring in Nb 3 Sn.

Description: Andreev-reflection spectroscopy of elemental superconductors in contact with nonmagnetic normal metals reveals that the strength of normal-reflection varies only slightly. This observation imposes strong constrictions on the three possible normal-reflection mechanisms: tunneling through a dielectric barrier, reflection due to the different electronic properties of the two electrodes, and diffusive transport caused by elastic scattering in the contact region. We discuss in detail the role played by Fermi-surface mismatch, represented by the different Fermi velocities on both sides of the contact interface. We find that it is at least not the dominant mechanism and possibly completely absent in the Andreev-reflection process.

Description: I ( V ) characteristics and their first derivatives of ScS and ScN-type (S is superconductor, c is constriction, N is normal metal) point contacts (PCs) based on Ba 1– x Na x Fe 2 As 2 ( x  = 0.25 and 0.35) were studied. ScS-type PCs with S = Nb,Ta, and Pb show Josephson-like resistively shunted I ( V ) curves with microwave induced Shapiro steps which satisfy relation 2 eV =  ℏ ω . The I c R N product ( I c is critical current, R N is normal state PC resistance) in these PCs is found to be up to 1.2 mV. All this data with the observed dependence of the I c on the microwave power of ScS PCs with Pb counter electrode indicates the presence of the singlet s -wave type pairing in Ba 1– x Na x Fe 2 As 2 . From the dV / dI ( V ) curves of ScN-type PCs demonstrating Andreev-reflection like features, the superconducting gap Δ ratio 2Δ/ k B T c  = 3.6 ± 1 for the compound with x  = 0.35 was evaluated. Analysis of these dV / dI ( V ) at high biases V , that is well above Δ, testifies transition to the thermal regime in PCs with a voltage increase.

Description: This is a theoretical investigation of the exchange bias phenomenon, and the properties of a thin magnetic film's magnetization hysteresis loop, on the rough surface of a hard-magnetic antiferromagnet. An interface model with a periodic structure of atomic steps is presented. These atomic steps are associated with a spatially inhomogeneous distribution of the ferromagnetic film magnetization, akin to a system of domain walls. This structure leads to a complicated external field dependence of magnetization: the hysteresis curve can assume an asymmetrical shape and “fall apart” into two hysteresis loops, divided by a “horizontal plateau,” or an area with constant field-independent magnetization. Such field dependence behavior has been recently observed experimentally in different ferro/antiferromagnet systems. The field dependence of magnetization has been obtained analytically using the long-wave approximation for various characteristics of ferromagnetic film (its thickness, values of exchange interaction, and magnetic anisotropy), and the interface (the period of the inhomogeneous structure, and exchange interaction through the interface). The analytical results are confirmed by numerical calculations for the corresponding discrete model with a more complex interface structure.

Description: Magnetization dependence on the external magnetic field was studied in a system with the Shastry–Sutherland lattice using exact diagonalization method at zero temperature within the xxz -model with spin S = 1/2. It was found that contrary to the classical Heisenberg model, the magnetization plateaus corresponding to m * = m / m sat = 1/4, 1/3, and 1/2 (here m sat is the saturation magnetization) exist even in the case of isotropic exchange interactions. The effect of exchange interaction on the width of the plateaus was studied. It was shown that depending on the ratio of exchange constants, there exist three regions corresponding to different types of the behavior of the system: the regions of a Neel-like antiferromagnet, a “dimer” antiferromagnet, and an intermediate region where the unique properties of Shastry–Sutherland lattices are most pronounced. The boundaries of these regions were also determined.

Description: Earlier studies of the properties of cryocondensed films of diluted solutions of ethanol in nitrogen have shown that a slight increase in the temperature of solid nitrogen, long before its sublimation, leads to changes in the vibrational spectrum of ethanol. The present work is a continuation of these studies and is focused on exploring the properties of thin films of cryovacuum condensates of dilute solutions of water and heavy water in nitrogen. The measurements were carried out in the temperature range from 12 to 40 K; the pressure in the vacuum chamber was below 5 × 10 −8 Torr. The concentration of water and heavy water in nitrogen was varied in the range from 0.5% to 3%. Based on the analysis of the vibrational spectra, it is suggested that the structure of the two-component film is a system of polyaggregates. An increase in the temperature of the matrix leads to the transformation of these polyaggregates into more stable states, as indicated by the changes in the fine structure of the bands. The presence of the absorption bands with the frequencies corresponding to the water monomers and dimers in a nitrogen matrix can be due to the fact that a fraction of the water molecules constituting polyaggregates might not be connected through hydrogen bonds with the neighboring molecules, forming broken chains. Thus, a population of quasi-free molecules with the corresponding absorption bands is formed. It is assumed that these unbound quasi-free molecules are mainly located in the subsurface layer of the clusters.

Description: Experimental observation of a weak low-frequency harmonic signal in a point (superconductor-constriction-superconductor, ScS) contact superconducting quantum interference device (rf-SQUID loop), being amplified due to the stochastic transitions between two or more metastable states of the loop, under the influence of applied noise flux of varying intensity (the effect of stochastic resonance, SR). In addition to the usual SR effect found in a bi-stable system with Gaussian noise, there were observed transitions between several metastable states of the multiwell SQUID loop potential, due to the influence of binary noise, which can be interpreted as a kind of noise “spectroscopy” of the loop's metastable states, with varying values of trapped magnetic flux.

Description: The isochoric thermal conductivity of solid tetrahydrofuran C 4 H 8 O (a cyclic hydrocarbon with pseudorotational motion of molecules) was studied using samples of different density in the temperature range 120−180 K. For all the samples, the isochoric thermal conductivity was found to decrease with temperature following a much weaker dependence than 1/ T . The experimental data were interpreted in terms of the Debye model of thermal conductivity considering heat transfer by both low-frequency phonons and “diffuse” modes. No significant contribution to the thermal conductivity by pseudorotation motion of the molecules was found.

Description: The dynamics of an easy-plane ferromagnet with spin S = 1 is studied at finite temperatures taking into account a significant quantum reduction in the spin. It is shown that in this case, besides the well-known standard transverse and longitudinal modes, an additional excitation branch appears because of the nonzero probability of transitions between excited levels of the magnetic ion at finite temperatures. This mode is strongly suppressed at low temperatures since the populations of the excited energy levels fall off exponentially with decreasing temperature. However, at finite temperatures lower than the Curie temperature T C but comparable to the energy of the exchange integral J , this mode can make a substantial contribution to energy absorption and exhibits high dispersion.

Description: A linear magneto-optical effect, birefringence of linearly polarized light that is directly proportional to magnetic field strength, is observed in magnetoelectric crystals of the orthophosphate LiNiPO 4 . This effect is especially pronounced in the incommensurate antiferromagnetic phase of the crystal. The linear magneto-optical effect indicates that the magnetic symmetry groups of the incommensurate, as well as the commensurate, phases of antiferromagnetic LiNiPO 4 lack an anti-inversion operation. This fact is consistent with the existence of ultraweak ferromagnetism in the crystal and indicates that this antiferromagnet has more complicated magnetic structures than those found in neutron diffraction experiments.

Description: The types of distributions of the structural and magnetic order parameters under torsion and pressure applied perpendicular to the “easy axis” of a ferromagnet are studied. It is found that these interactions cause two-sided single-phase and antiphase spatial amplitude modulations of the ordering parameters. Increased pressure leads to an increase in the amplitude of the modulations of the order parameter. Possible cases of resonance phenomena in the structural and magnetic subsystems are pointed out.

Description: Dynamic transitions in two-dimensional Wigner crystals over a liquid helium surface are studied under conditions of incomplete screening of the holding potential. The critical electric field that produces a transition is found to depend on the surface density of the electron layer and not on the holding field. It is assumed that the dynamic transition is associated primarily with the destruction of the spatial order of the system, which leads to delocalization of electrons and, thereby, to a change in the electro-ripplon interaction.

Description: Transport phenomena are examined in electron systems on liquid helium surfaces in strong nonquantizing nonuniform magnetic fields. For applied electric fields with frequencies low enough that an equilibrium distribution of the spins along the conducting surface can develop during the wave period, the electrical resistance is determined by different current carrier scattering processes than in the uniform case. Spin nonuniformity makes electron-electron collisions efficient with respect to momentum loss, so that galvanomagnetic effects differ substantially from the Drude-Lorentz theory. A nonstationary spin-electron effect is found in a direction perpendicular to the applied electric field. The evolution of the transport properties following application of a nonuniform magnetic field is discussed.

Description: The longitudinal relaxation rate of the total magnetization to its equilibrium value after ultrafast demagnetization by a femtosecond laser pulse is governed by two phenomena: the homogeneous evolution of magnetization and the motion of the wavefront into the demagnetized region. If the demagnetized region is sufficiently large, the homogeneous evolution dominates. However, as the diameter of the demagnetized region decreases, the relative contribution of the wavefront increases, and in the case when a femtosecond laser pulse is focused to a diffraction-limited minimum size, the wavefront can significantly increase the longitudinal relaxation rate.

Description: A quantum statistical description of the anomalous Hall effect is developed within the framework of the previously proposed thermodynamic mechanism of the anomalous Hall effect in weakly magnetic electron systems with spontaneous spin polarization. A qualitative explanation of the physical nature of the thermodynamic mechanism is followed by a general formulation of the quantum theory of the effect, based on accounting for the local-equilibrium currents. The behavior of the magnetic field dependences and quantum magnetic oscillations of the physical parameters characterizing the anomalous Hall effect is discussed.

Description: This is an analysis of the properties of quasi-local vibrations, and the conditions of the formation thereof, in a realistic model of the crystal lattice on a microscopic scale. The evolution of quasi-local vibrations with an increase in the concentration of impurity atoms, is examined. It is shown that the formation of boson peaks occurs mainly due to the additional dispersion of high-velocity acoustic phonons (connected to the atomic vibrations of the main lattice), caused by the scattering of these phonons by the quasi-local vibrations localized at the impurities. We demonstrate a connection between the boson peaks in disordered systems, and the first van Hove singularity, in regular crystal structures. We analyze the manifestation of quasi-local vibrations and boson peaks, as it relates to the behavior of low-temperature heat capacity, and how it changes with an increasing impurity concentration.