scholarly journals Isotope Effect in the Translation-Invariant Bipolaron Theory of High-Temperature Superconductivity

2020 ◽  
Vol 5 (4) ◽  
pp. 80
Author(s):  
Victor D. Lakhno
Keyword(s):  
High Temperature ◽  
Isotope Effect ◽  
High Temperature Superconductivity ◽  
High Temperature Superconductors ◽  
Superconducting Transition ◽  
Strong Electron ◽  
Translation Invariant ◽  
Electron Phonon Interaction ◽  
Theory Of Superconductivity

It is shown that the translation-invariant bipolaron theory of superconductivity can explain the dependence of the isotope coefficient in high-temperature superconductors on the critical temperature of a superconducting transition: in the case of strong electron–phonon interaction, the isotope coefficient is low when doping is optimal and high when it is weak. It is demonstrated that in the case of London penetration depth, the absolute value of the isotope coefficient behaves in the opposite way. A conclusion of the great role of non-adiabaticity in the case of weak doping is made. The criteria for d-wave phonon input into the isotope effect is established.

scholarly journals Mathematical foundations of the translation-invariant bipolaron theory of superconductivity

2021 ◽  
Author(s):  
Victor Dmitrievich Lakhno
Keyword(s):  
High Temperature Superconductors ◽  
Bogolyubov Transformation ◽  
Strong Electron ◽  
Translation Invariant ◽  
Angle Resolved Photoemission Spectroscopy ◽  
Electron Phonon Interaction ◽  
Mathematical Foundations ◽  
Senior Students ◽  
Theory Of Superconductivity

The monograph presents the theory of translation-invariant polarons and bipolarons based on the theory of squeezed vacuum wave functions. It is shown that the Tulub ansatz, which establishes a connection between the generalized Bogolyubov transformation with the unitary squeezed operator gives a solution to the spectral problem for a bipolaron. The solutions obtained are used to construct a theory of superconductivity based on the Froehlich Hamiltonian with a strong electron-phonon interaction. The role of Cooper pairs in it is played by TI bipolarons of spatially delocalized electrons with a small correlation length. The theory developed explains a large number of experiments on the thermodynamic, spectroscopic and transport characteristics of high-temperature superconductors, Josephson tunneling, angle-resolved photoemission spectroscopy, neutron scattering, etc. The book is intended for physicists and mathematicians who work in the field of the theory of condensed matter, as well as graduate students and senior students of universities.

scholarly journals Translational-Invariant Bipolarons and Superconductivity

2020 ◽  
Vol 5 (2) ◽  
pp. 30 ◽  
Author(s):  
Victor D. Lakhno
Keyword(s):  
Angular Dependence ◽  
High Temperature Superconductors ◽  
Einstein Condensation ◽  
Superconducting Transition ◽  
Natural Explanation ◽  
Strong Electron ◽  
Translation Invariant ◽  
Angle Resolved Photoemission Spectroscopy ◽  
Electron Phonon Interaction ◽  
Bose Einstein

A translation-invariant (TI) bipolaron theory of superconductivity based, like Bardeen–Cooper–Schrieffer theory, on Fröhlich Hamiltonian is presented. Here the role of Cooper pairs belongs to TI bipolarons which are pairs of spatially delocalized electrons whose correlation length of a coupled state is small. The presence of Fermi surface leads to the stabilization of such states in its vicinity and a possibility of their Bose–Einstein condensation (BEC). The theory provides a natural explanation of the existence of a pseudogap phase preceding the superconductivity and enables one to estimate the temperature of a transition T * from a normal state to a pseudogap one. It is shown that the temperature of BEC of TI bipolarons determines the temperature of a superconducting transition T c which depends not on the bipolaron effective mass but on the ordinary mass of a band electron. This removes restrictions on the upper limit of T c for a strong electron-phonon interaction. A natural explanation is provided for the angular dependence of the superconducting gap which is determined by the angular dependence of the phonon spectrum. It is demonstrated that a lot of experiments on thermodynamic and transport characteristics, Josephson tunneling and angle-resolved photoemission spectroscopy (ARPES) of high-temperature superconductors does not contradict the concept of a TI bipolaron mechanism of superconductivity in these materials. Possible ways of enhancing T c and producing new room-temperature superconductors are discussed on the basis of the theory suggested.

Role of structural instability in high-temperature superconductivity and its effect on isotope effect

1992 ◽  
Vol 128 (1) ◽  
pp. 105-110 ◽  
Author(s):  
A. Bussmann-Holder ◽  
A. R. Bishop ◽  
A. Migliori ◽  
Z. Fisk
Keyword(s):  
High Temperature ◽  
Isotope Effect ◽  
High Temperature Superconductivity ◽  
Structural Instability

Crystal Growth of High Temperature Superconductors

MRS Bulletin ◽  
1988 ◽  
Vol 13 (10) ◽  
pp. 56-61 ◽  
Author(s):  
H.J. Scheel ◽  
F. Licci
Keyword(s):  
High Temperature ◽  
Critical Current Density ◽  
Single Crystals ◽  
Critical Current ◽  
Current Density ◽  
Large Scale ◽  
High Temperature Superconductivity ◽  
High Temperature Superconductors ◽  
Superconducting Transition ◽  
Theoretical Understanding

The discovery of high temperature superconductivity (HTSC) in oxide compounds has confronted materials scientists with many challenging problems. These include the preparation of ceramic samples with critical current density of about 106 A/cm2 at 77 K and sufficient mechanical strength for large-scale electrotechnical and magnetic applications and the preparation of epitaxial thin films of high structural perfection for electronic devices.The main interest in the growth of single crystals is for the study of physical phenomena, which will help achieve a theoretical understanding of HTSC. Theorists still do not agree on the fundamental mechanisms of HTSC, and there is a need for good data on relatively defect-free materials in order to test the many models. In addition, the study of the role of defects like twins, grain boundaries, and dislocations in single crystals is important for understanding such parameters as the critical current density. The study of HTSC with single crystals is also expected to be helpful for finding optimum materials for the various applications and hopefully achieving higher values of the superconducting transition temperature Tc than the current maximum of about 125 K. It seems unlikely at present that single crystals will be used in commercial devices, but this possibility cannot be ruled out as crystal size and quality improve.

HIGH Tc STUDIES IN SVERDLOVSK

1988 ◽  
Vol 02 (06) ◽  
pp. 1331-1379 ◽  
Author(s):  
B. N. GOSHCHITSKII ◽  
V. L. KOZHEVNIKOV ◽  
M. V. SADOVSKII
Keyword(s):  
High Temperature ◽  
High Temperature Superconductivity ◽  
High Temperature Superconductors ◽  
Structural Phase ◽  
Nmr Relaxation ◽  
Structural Phase Transitions ◽  
High Tc ◽  
Review Reports ◽  
Academy Of Sciences

This review reports the main experimental results on superconducting lattice and electron properties of high-temperature superconductors of the type La 2−x Sr x CuO 4 and YBa 2 Cu 3 O 7 obtained at three institutes of the Ural Branch of the USSR Academy of Sciences in Sverdlovsk. Special attention is paid to investigations of structural phase transitions, heat capacity, optic and magnetic properties, NMR relaxation and the role of radiation disordering under the influence of fast neutron irradiation. In conclusion, a brief review of possible theories to explain high-temperature superconductivity in metal oxides is given.

scholarly journals Electronic Structure and Thermodynamic Properties of High Temperature Superconductors

2021 ◽  
Author(s):  
◽  
James Glenn Storey
Keyword(s):  
High Temperature ◽  
Transition Temperature ◽  
Isotope Effect ◽  
High Temperature Superconductivity ◽  
Hole Concentration ◽  
Ac Susceptibility ◽  
High Temperature Superconductors ◽  
Phonon Modes ◽  
Bulk Properties ◽  
Doping Dependence

<p>The generic doping dependence of the thermodynamic, electrodynamic and transport properties of high-temperature superconductors remains a puzzle despite many years of study. We are still awaiting a rigorous scientific theory that explains the resistance-free flow of electric current in these novel materials. In conventional superconductors, observations of the predicted dependence of the superconducting transition temperature on isotopic mass played a key role in identifying a phononic pairing mechanism. In order to elucidate the role of phonons in the high-Tc superconductors, the oxygen isotope effect in the separate components of the penetration depth tensor of the high-temperature superconductor YBa2Cu4O8 was determined from AC susceptibility measurements, performed on biaxially-aligned powders set in epoxy. The results, extracted after assuming values for the upper cut-off radii in the particle size distributions, show that the isotope effect in the bc-plane is negligible compared to those of the ab- and ac-planes. This suggests that the electrons prefer to couple to phonon modes in which the motion of the atoms is perpendicular to the plane of transport. The electronic entropy, superfluid density, Raman response, spin susceptibility and thermoelectric power were calculated from energy-momentum dispersions determined by angle-resolved photoemission spectroscopy (ARPES). An excellent match with experimental data was obtained. This is a highly significant result because it provides the first comprehensive link between these bulk properties and the ARPES measurements which are dominated by the outermost CuO2 layer. Thus, in most respects surface effects do not appear to seriously modify or obscure the band structure which governs bulk properties. The calculations reveal the presence of a van Hove singularity (vHs) at the Fermi level (EF ) in the heavily overdoped regime to be a universal feature of the cuprates. The evolution of these properties with temperature and doping can be fully explained by the retreat of EF from the vHs and the opening of a normal state pseudogap as doping is decreased. Consequently, the pairing potential amplitude is found to be a strongly decreasing function of hole concentration, similar to the doping dependence of the exchange interaction, J. The pairing interaction is possibly a universal function of the EF â EvHs with the maximum in the transition temperature (Tc) governed by the exact magnitude of the density of states on the flanks of the vHs. These are key new discoveries which may provide a route forward to solving the puzzle of high-temperature superconductivity.</p>

scholarly journals Fermi blockade of the strong electron-phonon interaction in optimally doped high temperature superconductors

2021 ◽  
Author(s):  
Andrey Mishchenko ◽  
Naoto Nagaosa ◽  
Nikolay Prokof'ev ◽  
Igor Tupitsyn
Keyword(s):  
High Temperature ◽  
High Temperature Superconductors ◽  
Two Dimensional ◽  
Phonon Interaction ◽  
Strong Electron ◽  
Electron Phonon Interaction ◽  
Holstein Model ◽  
Bold Line ◽  
Spin Polarized ◽  
Migdal's Theorem

Abstract We study how manifestations of strong electron-phonon interaction (EPI) depend on the carrier concentration by solving the two-dimensional Holstein model for the spin-polarized fermions using an approximation free bold-line diagrammatic Monte Carlo (BDMC) method. We show that the strong EPI, obviously present at very small Fermion concentration, is masked by the Fermi blockade effects and Migdal's theorem to the extent that it manifests itself as moderate one at large carriers densities. Suppression of strong EPI fingerprints is in agreement with experimental observations in doped high temperature superconductors.

scholarly journals Pseudogap Isotope Effect as a Probe of Bipolaron Mechanism in High Temperature Superconductors

Materials ◽  
2021 ◽  
Vol 14 (17) ◽  
pp. 4973
Author(s):  
Victor D. Lakhno
Keyword(s):  
Magnetic Field ◽  
High Temperature ◽  
Isotope Effect ◽  
High Temperature Superconductors ◽  
Translation Invariant ◽  
Pseudogap Phase ◽  
The Magnetic Field

A theory of a pseudogap phase of high-temperature superconductors where current carriers are translation invariant bipolarons is developed. A temperature of T* a transition from a pseudogap phase to a normal one is calculated. For the temperature of a transition to the pseudogap phase, the isotope coefficient is found. It is shown that the results obtained, in particular, the possibility of negative values of the isotope coefficient, are consistent with the experiment. New experiments on the influence of the magnetic field on the isotope coefficient are proposed.

scholarly journals High-temperature superconductivity using a model of hydrogen bonds

2018 ◽  
Vol 115 (22) ◽  
pp. 5709-5713 ◽  
Author(s):  
Daniel Kaplan ◽  
Yoseph Imry
Keyword(s):  
High Temperature ◽  
Matrix Element ◽  
Simple Model ◽  
Hydrogen Bonds ◽  
Isotope Effect ◽  
High Temperature Superconductivity ◽  
High Temperature Superconductors ◽  
Superconducting Gap ◽  
Gap Equation ◽  
The Matrix

Recently, there has been much interest in high-temperature superconductors and more recently in hydrogen-based superconductors. This work offers a simple model that explains the behavior of the superconducting gap based on naive BCS (Bardeen–Cooper–Schrieffer) theory and reproduces most effects seen in experiments, including the isotope effect and Tc enhancement as a function of pressure. We show that this is due to a combination of the factors appearing in the gap equation: the matrix element between the proton states and the level splitting of the proton.

scholarly journals Mechanism of High-Temperature Superconductivity in Correlated-Electron Systems

2019 ◽  
Vol 4 (2) ◽  
pp. 57 ◽  
Author(s):  
Takashi Yanagisawa
Keyword(s):  
High Temperature ◽  
High Temperature Superconductivity ◽  
High Temperature Superconductors ◽  
Energy Scale ◽  
Strongly Correlated ◽  
Electron Systems ◽  
Strong Electron ◽  
Correlated Electron Systems ◽  
Correlated Electron ◽  
Variational Monte Carlo Method

It is very important to elucidate the mechanism of superconductivity for achieving room temperature superconductivity. In the first half of this paper, we give a brief review on mechanisms of superconductivity in many-electron systems. We believe that high-temperature superconductivity may occur in a system with interaction of large-energy scale. Empirically, this is true for superconductors that have been found so far. In the second half of this paper, we discuss cuprate high-temperature superconductors. We argue that superconductivity of high temperature cuprates is induced by the strong on-site Coulomb interaction, that is, the origin of high-temperature superconductivity is the strong electron correlation. We show the results on the ground state of electronic models for high temperature cuprates on the basis of the optimization variational Monte Carlo method. A high-temperature superconducting phase will exist in the strongly correlated region.

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