2,624 research outputs found
Breakdown of the Wiedemann-Franz law in strongly-coupled electron-phonon system, application to the cuprates
With the superconducting cuprates in mind, a set of unitary transformations
was used to decouple electrons and phonons in the strong-coupling limit. While
phonons remain almost unrenormalised, electrons are transformed into itinerent
singlet and triplet bipolarons and thermally excited polarons. The
triplet/singlet exchange energy and the binding energy of the bipolarons are
thought to account for the spin and charge pseudogaps in the cuprates,
respectively. We calculated the Hall Lorenz number of the system to show that
the Wiedemann-Franz law breaks down due to the interference of the polaron and
bipolaron contributions to heat flow. The model provides a quantitative fit to
magnetotransport data in the cuprates. Furthermore we are able to extract the
phonon component of the thermal conductivity with the use of experimental data
and the model. Our results further validate the use of a charged Bose gas model
to describe normal and superconducting properties of unconventional
superconductors.Comment: 9 pages, 6 figures. Submitted to Physical Review
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
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
index for four-dimensional field theories
In theories with supersymmetry on , BPS bound states can decay
across walls of marginal stability in the space of Coulomb branch parameters,
leading to discontinuities in the BPS indices . We consider a
supersymmetric index which receives contributions from 1/2-BPS states,
generalizing the familiar Witten index . We expect
to be smooth away from loci where massless particles appear, thanks to
contributions from the continuum of multi-particle states. Taking inspiration
from a similar phenomenon in the hypermultiplet moduli space of string
vacua, we conjecture a formula expressing in terms of the BPS indices
, which is continuous across the walls and exhibits the
expected contributions from single particle states at large . This gives
a universal prediction for the contributions of multi-particle states to the
index . This index is naturally a function on the moduli space after
reduction on a circle, closely related to the canonical hyperk\"ahler metric
and hyperholomorphic connection on this space.Comment: 7 pages; v2: introduction expanded, minor corrections, differs from
published version in PRL in that supplemental material is included as an
Appendi
A mechanism for pair formation in strongly correlated systems
We start from a Hamiltonian describing non-interacting fermions and add
bosons to the model, with a Jaynes-Cummings-like interaction between the bosons
and fermions. Because of the specific form of the interaction the model can be
solved exactly. In the ground state, part of the electrons form bound pairs
with opposite momentum and spin. The model also shows a gap in the kinetic
energy of the fermions, but not in the spectrum of the full Hamiltonian. This
gap is not of a mean-field nature, but is due to the Pauli exclusion principle.Comment: 13 pages, corrected some notations and made some clarification
Plebanski Theory and Covariant Canonical Formulation
We establish an equivalence between the Hamiltonian formulation of the
Plebanski action for general relativity and the covariant canonical formulation
of the Hilbert-Palatini action. This is done by comparing the symplectic
structures of the two theories through the computation of Dirac brackets. We
also construct a shifted connection with simplified Dirac brackets, playing an
important role in the covariant loop quantization program, in the Plebanski
framework. Implications for spin foam models are also discussed.Comment: 18 page
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
Cut-and-Join operator representation for Kontsevich-Witten tau-function
In this short note we construct a simple cut-and-join operator representation
for Kontsevich-Witten tau-function that is the partition function of the
two-dimensional topological gravity. Our derivation is based on the Virasoro
constraints. Possible applications of the obtained expression are discussed.Comment: 5 pages, minor correction
Theory of Superconducting of doped fullerenes
We develop the nonadiabatic polaron theory of superconductivity of
taking into account the polaron band narrowing and realistic
electron-phonon and Coulomb interactions. We argue that the crossover from the
BCS weak-coupling superconductivity to the strong-coupling polaronic and
bipolaronic superconductivity occurs at the BCS coupling constant independent of the adiabatic ratio, and there is nothing ``beyond'' Migdal's
theorem except small polarons for any realistic electron-phonon interaction. By
the use of the polaronic-type function and the ``exact'' diagonalization in the
truncated Hilbert space of vibrons (``phonons'') we calculate the ground state
energy and the electron spectral density of the molecule. This
allows us to describe the photoemission spectrum of in a wide
energy region and determine the electron-phonon interaction. The strongest
coupling is found with the high-frequency pinch mode and with the
Frenkel exciton. We clarify the crucial role of high-frequency bosonic
excitations in doped fullerenes which reduce the bare bandwidth and the Coulomb
repulsion allowing the intermediate and low-frequency phonons to couple two
small polarons in a Cooper pair. The Eliashberg-type equations are solved for
low-frequency phonons. The value of the superconducting , its pressure
dependence and the isotope effect are found to be in a remarkable agreement
with the available experimental data.Comment: 20 pages, Latex, 4 figures available upon reques
Projected Spin Networks for Lorentz connection: Linking Spin Foams and Loop Gravity
In the search for a covariant formulation for Loop Quantum Gravity, spin
foams have arised as the corresponding discrete space-time structure and, among
the different models, the Barrett-Crane model seems the most promising. Here,
we study its boundary states and introduce cylindrical functions on both the
Lorentz connection and the time normal to the studied hypersurface. We call
them projected cylindrical functions and we explain how they would naturally
arise in a covariant formulation of Loop Quantum Gravity.Comment: Latex, 15 page
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