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.

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 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.

INTRIGUING ROLE OF HOLE-COOPER-PAIRS IN SUPERCONDUCTORS AND SUPERFLUIDS

2008 ◽  
Vol 22 (25n26) ◽  
pp. 4367-4378 ◽  
Author(s):  
M. GRETHER ◽  
M. de LLANO ◽  
S. RAMÍREZ ◽  
O. ROJO
Keyword(s):  
Free Energy ◽  
Helmholtz Free Energy ◽  
Bcs Theory ◽  
Einstein Condensation ◽  
Cooper Pairs ◽  
Superconducting Transition ◽  
Electron Phonon Interaction ◽  
Bose Einstein ◽  
Hole Cooper Pairs

The role in superconductors of hole-Cooper-pairs (CPs) are examined and contrasted with the more familiar electron-CPs, with special emphasis on their “background” effect in enhancing superconducting transition temperatures Tc — even when electron-CPs drive the transition. Both kinds of CPs are, of course, present at all temperatures. An analogy is drawn between the hole CPs in any many-fermion system with the antibosons in a relativistic ideal Bose gas that appear in substantial numbers only at higher and higher temperatures. Their indispensable role in yielding a lower Helmholtz free energy equilibrium state is established. For superconductors, the problem is viewed in terms of a generalized Bose-Einstein condensation (GBEC) theory that is an extension of the Friedberg-T.D. Lee 1989 boson-fermion BEC theory of high-Tc superconductors in that the GBEC theory includes hole CPs as well as electron-CPs — thereby containing as well as further extending BCS theory to higher temperatures with the same weak-coupling electron-phonon interaction parameters. We show that the Helmholtz free energy of both 2e- and 2h-CP pure condensates has a positive second derivative, and are thus stable equilibrium states. Finally, it is conjectured that the role of hole pairs in ultra-cold fermionic atom gases will likely be negligible because the very low densities involved imply a “shallow” Fermi sea.

The combined effect of lattice’s uniaxial strains and electron–phonon interaction’s screening on TBEC of the intersite bipolarons

2016 ◽  
Vol 30 (26) ◽  
pp. 1650186
Author(s):  
B. Yavidov ◽  
SH. Djumanov ◽  
T. Saparbaev ◽  
O. Ganiyev ◽  
S. Zholdassova ◽  
...  
Keyword(s):  
Ideal Gas ◽  
Einstein Condensation ◽  
Chain Model ◽  
One Dimensional ◽  
Electron Phonon Interaction ◽  
Model Lattice ◽  
Experimental Values ◽  
Bose Einstein ◽  
Derivatives Of ◽  
The Ideal

Having accepted a more generalized form for density-displacement type electron–phonon interaction (EPI) force we studied the simultaneous effect of uniaxial strains and EPI’s screening on the temperature of Bose–Einstein condensation [Formula: see text] of the ideal gas of intersite bipolarons. [Formula: see text] of the ideal gas of intersite bipolarons is calculated as a function of both strain and screening radius for a one-dimensional chain model of cuprates within the framework of Extended Holstein–Hubbard model. It is shown that the chain model lattice comprises the essential features of cuprates regarding of strain and screening effects on transition temperature [Formula: see text] of superconductivity. The obtained values of strain derivatives of [Formula: see text] [Formula: see text] are in qualitative agreement with the experimental values of [Formula: see text] [Formula: see text] of La[Formula: see text]Sr[Formula: see text]CuO4 under moderate screening regimes.

scholarly journals Bose-Einstein condensation superconductivity induced by disappearance of the nematic state

2020 ◽  
Vol 6 (45) ◽  
pp. eabb9052
Author(s):  
Takahiro Hashimoto ◽  
Yuichi Ota ◽  
Akihiro Tsuzuki ◽  
Tsubaki Nagashima ◽  
Akiko Fukushima ◽  
...  
Keyword(s):  
Ultracold Atoms ◽  
Degrees Of Freedom ◽  
Photoemission Spectroscopy ◽  
Einstein Condensation ◽  
Energy Bands ◽  
Iron Based Superconductors ◽  
Bose Einstein Condensation ◽  
Angle Resolved Photoemission Spectroscopy ◽  
Nematic State ◽  
Bose Einstein

The crossover from the superconductivity of the Bardeen-Cooper-Schrieffer (BCS) regime to the Bose-Einstein condensation (BEC) regime holds a key to understanding the nature of pairing and condensation of fermions. It has been mainly studied in ultracold atoms, but in solid systems, fundamentally previously unknown insights may be obtained because multiple energy bands and coexisting electronic orders strongly affect spin and orbital degrees of freedom. Here, we provide evidence for the BCS-BEC crossover in iron-based superconductors FeSe1 − xSx from laser-excited angle-resolved photoemission spectroscopy. The system enters the BEC regime with x = 0.21, where the nematic state that breaks the orbital degeneracy is fully suppressed. The substitution dependence is opposite to the expectation for single-band superconductors, which calls for a new mechanism of BCS-BEC crossover in this system.

THE EFFECT OF STRONG ELECTRON-PHONON INTERACTIONS ON THE INFRARED REFLECTANCE IN HIGH-Tc SUPERCONDUCTORS

1991 ◽  
Vol 05 (20) ◽  
pp. 1361-1365 ◽  
Author(s):  
S. Y. TIAN ◽  
M. H. LI ◽  
Z. X. ZHAO
Keyword(s):  
High Temperature Superconductors ◽  
Peak Position ◽  
Superconducting Gap ◽  
High Tc Superconductors ◽  
Phonon Interaction ◽  
Phonon Softening ◽  
Strong Electron ◽  
High Tc ◽  
Electron Phonon Interaction ◽  
And Shifts

We show that the model of charge-transfer fluctuation with LO phonons can unify the interpretation of the infrared features and the phonon softening at about 55 meV measured by neutron scattering experiments in high temperature superconductors. Based upon this model we demonstrate that reflectivity ratio of superconducting state to normal state is strongly modified as a result of strong electron-phonon interaction which is different from Holstein mechanism. A broad peak in the ratio reflectivity develops around the peak position of different neutron phonon density of states. This peak becomes stronger and shifts to a higher frequency as the coupling increases at lower temperatures. This situation complicates the interpretation of the ratio reflectivity peak as the superconducting gap.

scholarly journals Translation-Invariant Bipolarons and Charge Density Waves in High-Temperature Superconductors

2021 ◽  
Vol 9 ◽  
Author(s):  
Victor D. Lakhno
Keyword(s):  
Charge Density ◽  
High Temperature Superconductors ◽  
Bose Condensate ◽  
Density Wave ◽  
Charge Density Waves ◽  
Superconducting Transition ◽  
Density Waves ◽  
Translation Invariant ◽  
Pseudogap Phase ◽  
Pair Density

A correlation is established between the theories of superconductivity based on the concept of charge density waves (CDWs) and the translation invariant (TI) bipolaron theory. It is shown that CDWs are originated from TI-bipolaron states in the pseudogap phase due to the Kohn anomaly and form a pair density wave (PDW) for wave vectors corresponding to nesting. Emerging in the pseudogap phase, CDWs coexist with superconductivity at temperatures below those of superconducting transition, while their wave amplitudes decrease as a Bose condensate is formed from TI bipolarons, vanishing at zero temperature.

scholarly journals Gate-controlled BCS-BEC crossover in a two-dimensional superconductor

Science ◽  
2021 ◽  
Vol 372 (6538) ◽  
pp. 190-195 ◽  
Author(s):  
Yuji Nakagawa ◽  
Yuichi Kasahara ◽  
Takuya Nomoto ◽  
Ryotaro Arita ◽  
Tsutomu Nojima ◽  
...  
Keyword(s):  
Carrier Density ◽  
Zirconium Nitride ◽  
Einstein Condensation ◽  
Two Dimensional ◽  
Superconducting Transition ◽  
Pseudogap Phase ◽  
Fermion Systems ◽  
Fermi Temperature ◽  
Crossover Behavior ◽  
Bose Einstein

Bardeen-Cooper-Schrieffer (BCS) superfluidity and Bose-Einstein condensation (BEC) are the two extreme limits of the ground state of the paired fermion systems. We report crossover behavior from the BCS limit to the BEC limit realized by varying carrier density in a two-dimensional superconductor, electron-doped zirconium nitride chloride. The phase diagram, established by simultaneous measurements of resistivity and tunneling spectra under ionic gating, demonstrates a pseudogap phase in the low-doping regime. The ratio of the superconducting transition temperature and Fermi temperature in the low–carrier density limit is consistent with the theoretical upper bound expected in the BCS-BEC crossover regime. These results indicate that the gate-doped semiconductor provides an ideal platform for the two-dimensional BCS-BEC crossover without added complexities present in other solid-state systems.

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