ALBERT

Beschreibung: This is a review of results from studies of the effect of artificially restricted geometry (the size effect) on the superconducting properties of nanoparticles of low-melting metals (Hg, Pb, Sn, In). Restricted geometrical conditions are created by embedding molten metals under high pressure into nanoporous matrices of two types: channel structures based on chrysotile asbestos and porous alkali-borosilicate glasses. Chrysotile asbestos is a system of parallel nanotubes with channel diameters ranging from 2 to 20 nm and an aspect ratio (channel length to diameter) of up to 10 7 . The glasses are a random dendritic three-dimensional system of interconnected channels with a technologically controllable mean diameter of 2–30 nm. Temperature dependences of the resistance and heat capacity in the region of the superconducting transition and the dependences of the critical temperature on the mean pore diameter are obtained. The critical magnetic fields are also determined.

Beschreibung: The electronic structure of FeSe, the simplest iron-based superconductor (Fe-SC), conceals a potential of dramatic increase of T c that realizes under pressure or in a single layer film. This is also the system where nematicity, the phenomenon of a keen current interest, is most easy to study since it is not accompanied by the antiferromagnetic transition like in all other Fe-SC's. Here we overview recent experimental data on electronic structure of FeSe-based superconductors: isovalently doped crystals, intercalates, and single layer films, trying to clarify its topology and possible relation of this topology to superconductivity. We argue that the marked differences between the experimental and calculated band structures for all FeSe compounds can be described by a hoping selective renormalization model for a spin/orbital correlated state that may naturally explain both the evolution of the band structure with temperature and nematicity.

Beschreibung: A comprehensive study of superconductor-constriction-superconductor contacts, obtained using the “break junction” technique in layered superconductors. Depending on the constriction transparency, tunneling and SnS-Andreev spectroscopies could be used to directly determine the values of the superconducting gaps, characteristic BCS ratios and temperature dependences of the gaps in cuprates, magnesium diboride and iron pnictides and chalcogenides. Based on these results we can estimate the gap anisotropy and the electron-boson coupling constants. The advantages and drawbacks of “break junction” technique are discussed, and we demonstrate that this method is powerful enough for the study of optical phonon modes in high-temperature superconducting cuprates and for creating contacts with selective transparency in Mg 1- x Al x B 2 compounds.

Beschreibung: Results are presented from a theoretical study of the possibility of hole carrier localization and metal-insulator transitions which show up in the temperature dependences of the magnetic susceptibility χ( T ) of doped copper-oxide (cuprate) compounds. The criteria for metal-insulator transitions owing to strong hole-lattice interactions and the formation of very narrow polaron bands in these materials with reduced doping level x are analyzed. It is shown that these kinds of metal-insulator transitions occur in underdoped La 2- x Sr x CuO 4 and YBa 2 Cu 3 O 6+ x cuprates (i.e., for x ranging from 0.04 to 0.12). The characteristic temperature dependences χ( T ) of the HTSC cuprates are found for different doping levels. These results are in good agreement with experimental data on metal-insulator transitions and the magnetic susceptibility of the HTSC cuprates.

Beschreibung: The possibility of an anomalous structure in the differential conductivity of tunnel junctions based on high-temperature superconductors as a result of degradation of their surface layer is analyzed. This feature is in the form of two peaks near an energy gap separated by a region of suppressed conductivity. One peak is usually high and sharp, while the other is much more spread out. Differential conductivity and shot noise spectra in contacts of a normal injector with s - and d -type superconductors are calculated and compared. It is shown that combined measurements of these two characteristics can provide new information on the kinetics of transport processes in these structures.

Beschreibung: During recent years the interest to dynamics of quantum systems has grown considerably. Quantum many body systems out of equilibrium often manifest behavior, different from the one predicted by standard statistical mechanics and thermodynamics in equilibrium. Since the dynamics of a many-body quantum system typically involve many excited eigenstates, with a non-thermal distribution, the time evolution of such a system provides an unique way for investigation of non-equilibrium quantum statistical mechanics. Last decade such new subjects like quantum quenches, thermalization, pre-thermalization, equilibration, generalized Gibbs ensemble, etc. are among the most attractive topics of investigation in modern quantum physics. One of the most interesting themes in the study of dynamics of quantum many-body systems out of equilibrium is connected with the recently proposed important concept of dynamical quantum phase transitions. During the last few years a great progress has been achieved in studying of those singularities in the time dependence of characteristics of quantum mechanical systems, in particular, in understanding how the quantum critical points of equilibrium thermodynamics affect their dynamical properties. Dynamical quantum phase transitions reveal universality, scaling, connection to the topology, and many other interesting features. Here we review the recent achievements of this quickly developing part of low-temperature quantum physics. The study of dynamical quantum phase transitions is especially important in context of their connection to the problem of the modern theory of quantum information, where namely non-equilibrium dynamics of many-body quantum system plays the major role.

Beschreibung: An influence of the electron-phonon interaction on excitation spectrum and damping in a narrow band electron subsystem of cuprates has been investigated. Within the framework of the t-J model an approach to solving a problem of account of both strong electron correlations and local electron–phonon binding with characteristic Einstein mode ω 0 in the normal state has been presented. In approximation Hubbard-I it was found an exact solution for the polaron bands. We established that in the low-dimensional system with a pure kinematic part of Hamiltonian a complicated excitation spectrum is realized. It is determined mainly by peculiarities of the lattice Green's function. In the definite area of the electron concentration and hopping integrals a correlation gap may be possible on the Fermi level. Also, in specific cases it is observed a doping evolution of the Fermi surface. We found that the strong electron–phonon binding enforces a degree of coherence of electron-polaron excitations near the Fermi level and spectrum along the nodal direction depends on wave vector module weakly. It corresponds to ARPES data. A possible origin of the experimentally observed kink in the nodal direction of cuprates is explained by fine structure of the polaron band to be formed near the mode −ω 0 .

Beschreibung: A review of experimental and theoretical studies of transport phenomena in strongly anisotropic organic conductors is presented. Considerable attention is paid to the phenomena that are specific to quasi-2D and quasi-1D conductive structures and have no analogues both in ordinary metals and in truly 2D or 1D conducting systems. Angular magnetoresistance oscillations, de Haas–van Alphen and Shubnikov–de Haas phenomena, high-temperature quantum oscillations of the magnetoresistance, and high-frequency resonances, including those arising due to the motion of electrons open trajectories, are discussed. The resonant angular oscillations of high-frequency conductivity and weakly damped electromagnetic waves in quasi-2D organic conductors under strong spatial dispersion are considered.

Beschreibung: We investigated the properties of a system where the itinerant electrons coexist and interact with the preformed local pairs. Using the nonperturbative continuous unitary transformation technique we show that Andreev-type scattering between these charge carriers gives rise to the enhanced diamagnetic response and is accompanied by appearance of the Drude peak inside the pseudogap regime ω ≤ 2 Δ p g . Both effects are caused by the short-range superconducting correlations above the transition temperature T c . In fact, the residual diamagnetism has been detected by the torque magnetometry in the lanthanum and bismuth cuprate superconductors at temperatures up to ∼1.5 T c . In this work we show how the superconducting correlations can be observed in the ac and dc conductivity.

Beschreibung: The reflection of transverse-electric and transverse-magnetic polarized waves from the surface of a semi-infinite layered superconductor in the presence of external dc magnetic field is studied theoretically. The superconducting layers are assumed to be perpendicular to the boundary of the sample. Due to a strong anisotropy of the irradiated superconductor surface, a transformation of the wave polarization occurs upon the reflection. It is shown that, despite a relatively small penetration depth of the dc magnetic field, it qualitatively affects the field distribution of the electromagnetic wave and, therefore, the reflection and transformation coefficients. Thus, the external dc magnetic field can serve as an efficient tool to control the transformation of the wave polarization. The analytical expressions for the reflection and transformation coefficients are obtained and the parameters at which the most effective transformation of the transverse-electric to transverse-magnetic waves and vice versa occurs are found.