2,274 research outputs found
High Temperature Superconductivity: the explanation
Soon after the discovery of the first high temperature superconductor by
Georg Bednorz and Alex Mueller in 1986 the late Sir Nevill Mott answering his
own question "Is there an explanation?" [Nature v 327 (1987) 185] expressed a
view that the Bose-Einstein condensation (BEC) of small bipolarons, predicted
by us in 1981, could be the one. Several authors then contemplated BEC of real
space tightly bound pairs, but with a purely electronic mechanism of pairing
rather than with the electron-phonon interaction (EPI). However, a number of
other researchers criticized the bipolaron (or any real-space pairing) scenario
as incompatible with some angle-resolved photoemission spectra (ARPES), with
experimentally determined effective masses of carriers and unconventional
symmetry of the superconducting order parameter in cuprates. Since then the
controversial issue of whether the electron-phonon interaction (EPI) is crucial
for high-temperature superconductivity or weak and inessential has been one of
the most challenging problems of contemporary condensed matter physics. Here I
outline some developments in the bipolaron theory suggesting that the true
origin of high-temperature superconductivity is found in a proper combination
of strong electron-electron correlations with a significant finite-range
(Froehlich) EPI, and that the theory is fully compatible with the key
experiments.Comment: 8 pages, 2 figures, invited comment to Physica Script
Hall effect and resistivity in underdoped cuprates
The behaviour of the Hall ratio as a function of temperature is
one of the most intriguing normal state properties of cuprate superconductors.
One feature of all the data is a maximum of in the normal state that
broadens and shifts to temperatures well above with decreasing doping. We
show that a model of preformed pairs-bipolarons provides a selfconsistent
quantitative description of together with in-plane resistivity and
uniform magnetic susceptibility for a wide range of doping.Comment: 4 pages, 2 figures, the model and fits were refine
Coherent `ab' and `c' transport theory of high- cuprates
We propose a microscopic theory of the `'-axis and in-plane transport of
copper oxides based on the bipolaron theory and the Boltzmann kinetics. The
fundamental relationship between the anisotropy and the spin susceptibility is
derived, . The
temperature and doping dependence of the in-plane, and
out-of-plane, resistivity and the spin susceptibility,
are found in a remarkable agreement with the experimental data in underdoped,
optimally and overdoped for the entire temperature
regime from up to . The normal state gap is explained and its
doping and temperature dependence is clarified.Comment: 12 pages, Latex, 3 figures available upon reques
Diamagnetism of real-space pairs above Tc in hole doped cuprates
The nonlinear normal state diamagnetism reported by Lu Li et al. [Phys. Rev.
B 81, 054510 (2010)] is shown to be incompatible with an acclaimed Cooper
pairing and vortex liquid above the resistive critical temperature. Instead it
is perfectly compatible with the normal state Landau diamagnetism of real-space
composed bosons, which describes the nonlinear magnetization curves in less
anisotropic cuprates La-Sr-Cu-O (LSCO) and Y-Ba-Cu-O (YBCO) as well as in
strongly anisotropic bismuth-based cuprates in the whole range of available
magnetic fields.Comment: 4 pages, 4 figure
Optimal interlayer hopping and high temperature Bose–Einstein condensation of local pairs in quasi 2D superconductors
Both FeSe and cuprate superconductors are quasi 2D materials with high transition temperatures and local fermion pairs. Motivated by such systems, we investigate real space pairing of fermions in an anisotropic lattice model with intersite attraction, V, and strong local Coulomb repulsion, U, leading to a determination of the optimal conditions for superconductivity from Bose–Einstein condensation. Our aim is to gain insight as to why high temperature superconductors tend to be quasi 2D. We make both analytically and numerically exact solutions for two body local pairing applicable to intermediate and strong V. We find that the Bose–Einstein condensation temperature of such local pairs pairs is maximal when hopping between layers is intermediate relative to in-plane hopping, indicating that the quasi 2D nature of unconventional superconductors has an important contribution to their high transition temperatures
Vortex matter in the charged Bose liquid at absolute zero
The Gross-Pitaevskii-type equation is solved for the charge Bose liquid in
the external magnetic field at zero temperature. There is a vortex lattice with
locally broken charge neutrality. The boson density is modulated in real space
and each vortex is charged. Remarkably, there is no upper critical field at
zero temperature, so the density of single flux-quantum vortices monotonously
increases with the magnetic field up to B=infinity and no indication of a phase
transition. The size of each vortex core decreases as about 1/sqrt(B) keeping
the system globally charge neutral. If bosons are composed of two fermions, a
phase transition to a spin-polarized Fermi liquid at some magnetic field larger
than the pair-breaking field is predicted.Comment: 4 pages, 4 figures, references update
Bose-Einstein condensation of strongly correlated electrons and phonons in cuprate superconductors
The long-range Froehlich electron-phonon interaction has been identified as
the most essential for pairing in high-temperature superconductors owing to
poor screening, as is now confirmed by optical, isotope substitution, recent
photoemission and some other measurements. I argue that low energy physics in
cuprate superconductors is that of superlight small bipolarons, which are
real-space hole pairs dressed by phonons in doped charge-transfer Mott
insulators. They are itinerant quasiparticles existing in the Bloch states at
low temperatures as also confirmed by continuous-time quantum Monte-Carlo
algorithm (CTQMC) fully taking into account realistic Coulomb and long-range
Froehlich interactions. Here I suggest that a parameter-free evaluation of Tc,
unusual upper critical fields, the normal state Nernst effect, diamagnetism,
the Hall-Lorenz numbers and giant proximity effects strongly support the
three-dimensional (3D) Bose-Einstein condensation of mobile small bipolarons
with zero off-diagonal order parameter above the resistive critical temperature
Tc at variance with phase fluctuation scenarios of cuprates.Comment: 35 pages, 10 figures, to appear in the special volume of Journal of
Physics: Condensed Matte
Giant enhancement of anisotropy by electron-phonon interaction
Anisotropic electron-phonon interaction is shown to lead to the anisotropic
polaron effect. The resulting anisotropy of the polaron band is an exponential
function of the electron-phonon coupling and might be as big as . This
also makes anisotropy very sensitive to small changes of coupling and implies
wide variations of anisotropy among compounds of similar structure. The isotope
effect on mass anisotropy is predicted. Polaron masses are obtained by an exact
Quantum Monte Carlo method. Implications for high-temperature superconductors
are briefly discussed.Comment: 5 pages, 4 figure
TBA for non-perturbative moduli spaces
Recently, an exact description of instanton corrections to the moduli spaces
of 4d N=2 supersymmetric gauge theories compactified on a circle and Calabi-Yau
compactifications of Type II superstring theories was found. The equations
determining the instanton contributions turn out to have the form of
Thermodynamic Bethe Ansatz. We explore further this relation and, in
particular, we identify the contact potential of quaternionic string moduli
space with the free energy of the integrable system and the Kahler potential of
the gauge theory moduli space with the Yang-Yang functional. We also show that
the corresponding S-matrix satisfies all usual constraints of 2d integrable
models, including crossing and bootstrap, and derive the associated Y-system.
Surprisingly, in the simplest case the Y-system is described by the MacMahon
function relevant for crystal melting and topological strings.Comment: 25 pages, 1 figur
AKSZ-BV Formalism and Courant Algebroid-induced Topological Field Theories
We give a detailed exposition of the Alexandrov-Kontsevich-Schwarz-
Zaboronsky superfield formalism using the language of graded manifolds. As a
main illustarting example, to every Courant algebroid structure we associate
canonically a three-dimensional topological sigma-model. Using the AKSZ
formalism, we construct the Batalin-Vilkovisky master action for the model.Comment: 13 pages, based on lectures at Rencontres mathematiques de Glanon
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