3,753 research outputs found
Enhanced stability of bound pairs at nonzero lattice momenta
A two-body problem on the square lattice is analyzed. The interaction
potential consists of strong on-site repulsion and nearest-neighbor attraction.
Exact pairing conditions are derived for s-, p-, and d-symmetric bound states.
The pairing conditions are strong functions of the total pair momentum K. It is
found that the stability of pairs increases with K. At weak attraction, the
pairs do not form at the -point but stabilize at lattice momenta close
to the Brillouin zone boundary. The phase boundaries in the momentum space,
which separate stable and unstable pairs are calculated. It is found that the
pairs are formed easier along the direction than along the
direction. This might lead to the appearance of ``hot pairing
spots" on the Kx and Ky axes.Comment: 7 RevTEX pages, 5 figure
Isotope effect on the electron band structure of doped insulators
Applying a continuous-time quantum Monte-Carlo algorithm we calculate the
exact coherent band dispersion and the density of states of a two dimensional
lattice polaron in the region of parameters where any approximation might fail.
We find an isotope effect on the band structure, which is different for
different wave-vectors of the Brillouin zone and depends on the radius and
strength of the electron-phonon interaction. An isotope effect on the electron
spectral function is also discussed.Comment: 4 pages, 3 figure
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
High temperature superconductivity and charge segregation in a model with strong long-range electron-phonon and Coulomb interactions
An analytical method of studying strong long-range electron-phonon and
Coulomb interactions in complex lattices is presented. The method is applied to
a perovskite layer with anisotropic coupling of holes to the vibrations of
apical atoms. Depending on the relative strength of the polaronic shift, Ep,
and the inter-site Coulomb repulsion, Vc, the system is either a polaronic
Fermi liquid, Vc > 1.23 Ep, a bipolaronic superconductor, 1.16 Ep < Vc < 1.23
Ep, or a charge segregated insulator, Vc < 1.16 Ep. In the superconducting
window, the carriers are mobile bipolarons with a remarkably low effective
mass. The model describes the key features of the underdoped superconducting
cuprates.Comment: 5 pages, 2 figures (1 color
Hamiltonian Analysis of non-chiral Plebanski Theory and its Generalizations
We consider non-chiral, full Lorentz group-based Plebanski formulation of
general relativity in its version that utilizes the Lagrange multiplier field
Phi with "internal" indices. The Hamiltonian analysis of this version of the
theory turns out to be simpler than in the previously considered in the
literature version with Phi carrying spacetime indices. We then extend the
Hamiltonian analysis to a more general class of theories whose action contains
scalars invariants constructed from Phi. Such theories have recently been
considered in the context of unification of gravity with other forces. We show
that these more general theories have six additional propagating degrees of
freedom as compared to general relativity, something that has not been
appreciated in the literature treating them as being not much different from
GR.Comment: 10 page
Polaron effective mass from Monte Carlo simulations
A new Monte Carlo algorithm for calculating polaron effective mass is
proposed. It is based on the path-integral representation of a partial
partition function with fixed total quasi-momentum. Phonon degrees of freedom
are integrated out analytically resulting in a single-electron system with
retarded self-interaction and open boundary conditions in imaginary time. The
effective mass is inversely proportional to the covariance of total energy
calculated on an electron trajectory and the square distance between ends of
the trajectory. The method has no limitations on values of model parameters and
on the size and dimensionality of the system although large statistics is
required for stable numerical results. The method is tested on the
one-dimensional Holstein model for which simulation results are presented.Comment: 4 pages + 1 figure, RevTeX. Accepted for publication as a Rapid
Communication in Phys.Rev.
Photoemission spectroscopy and sum rules in dilute electron-phonon systems
A family of exact sum rules for the one-polaron spectral function in the
low-density limit is derived. An algorithm to calculate energy moments of
arbitrary order of the spectral function is presented. Explicit expressions are
given for the first two moments of a model with general electron-phonon
interaction, and for the first four moments of the Holstein polaron. The sum
rules are linked to experiments on momentum-resolved photoemission
spectroscopy. The bare electronic dispersion and the electron-phonon coupling
constant can be extracted from the first and second moments of spectrum. The
sum rules could serve as constraints in analytical and numerical studies of
electron-phonon models.Comment: 4 page
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
- …