1,089 research outputs found
Universality of the single-particle spectra of cuprate superconductors
All the available data for the dispersion and linewidth of the
single-particle spectra above the superconducting gap and the pseudogap in
metallic cuprates for any doping has universal features. The linewidth is
linear in energy below a scale and constant above. The cusp in the
linewidth at mandates, due to causality, a "waterfall", i.e., a
vertical feature in the dispersion. These features are predicted by a recent
microscopic theory. We find that all data can be quantitatively fitted by the
theory with a coupling constant and an upper cutoff at
which vary by less than 50% among the different cuprates and for varying
dopings. The microscopic theory also gives these values to within factors of
O(2).Comment: 4 pages, 4 figures; accepted by Phys. Rev. Let
Multiresolution analysis in statistical mechanics. I. Using wavelets to calculate thermodynamic properties
The wavelet transform, a family of orthonormal bases, is introduced as a
technique for performing multiresolution analysis in statistical mechanics. The
wavelet transform is a hierarchical technique designed to separate data sets
into sets representing local averages and local differences. Although
one-to-one transformations of data sets are possible, the advantage of the
wavelet transform is as an approximation scheme for the efficient calculation
of thermodynamic and ensemble properties. Even under the most drastic of
approximations, the resulting errors in the values obtained for average
absolute magnetization, free energy, and heat capacity are on the order of 10%,
with a corresponding computational efficiency gain of two orders of magnitude
for a system such as a Ising lattice. In addition, the errors in
the results tend toward zero in the neighborhood of fixed points, as determined
by renormalization group theory.Comment: 13 pages plus 7 figures (PNG
Entanglement renormalization and gauge symmetry
A lattice gauge theory is described by a redundantly large vector space that
is subject to local constraints, and can be regarded as the low energy limit of
an extended lattice model with a local symmetry. We propose a numerical
coarse-graining scheme to produce low energy, effective descriptions of lattice
models with a local symmetry, such that the local symmetry is exactly preserved
during coarse-graining. Our approach results in a variational ansatz for the
ground state(s) and low energy excitations of such models and, by extension, of
lattice gauge theories. This ansatz incorporates the local symmetry in its
structure, and exploits it to obtain a significant reduction of computational
costs. We test the approach in the context of the toric code with a magnetic
field, equivalent to Z2 lattice gauge theory, for lattices with up to 16 x 16
sites (16^2 x 2 = 512 spins) on a torus. We reproduce the well-known ground
state phase diagram of the model, consisting of a deconfined and spin polarized
phases separated by a continuous quantum phase transition, and obtain accurate
estimates of energy gaps, ground state fidelities, Wilson loops, and several
other quantities.Comment: reviewed version as published in PRB; this version includes a new
section about the accuracy of the results several corrections and added
citation
Homogeneous versus Spiral Phases of Hole-doped Antiferromagnets: A Systematic Effective Field Theory Investigation
Using the low-energy effective field theory for magnons and holes -- the
condensed matter analog of baryon chiral perturbation theory for pions and
nucleons in QCD -- we study different phases of doped antiferromagnets. We
systematically investigate configurations of the staggered magnetization that
provide a constant background field for doped holes. The most general
configuration of this type is either constant itself or it represents a spiral
in the staggered magnetization. Depending on the values of the low-energy
parameters, a homogeneous phase, a spiral phase, or an inhomogeneous phase is
energetically favored. The reduction of the staggered magnetization upon doping
is also investigated.Comment: 35 pages, 5 figure
Relevance of multiband Jahn-Teller effects on the electron-phonon interaction in C
Assessing the effective relevance of multiband effects in the fullerides is
of fundamental importance to understand the complex superconducting and
transport properties of these compounds. In this paper we investigate in
particular the role of the multiband effects on the electron-phonon (el-ph)
properties of the bands coupled with the Jahn-Teller intra-molecular
vibrational modes in the C compounds. We show that, assuming
perfect degeneracy of the electronic bands, vertex diagrams arising from the
breakdown of the adiabatic hypothesis, are one order of magnitude smaller than
the non-crossing terms usually retained in the Migdal-Eliashberg (ME) theory.
These results permit to understand the robustness on ME theory found by
numerical calculations. The effects of the non degeneracy of the in
realistic systems are also analyzed. Using a tight-binding model we show that
the el-ph interaction is mainly dominated by interband scattering within a
single electronic band. Our results question the reliability of a degenerate
band modeling and show the importance of these combined effects in the
C family.Comment: 5 pages, 3 eps figure
Dualities in Spin Ladders
We introduce a set of discrete modular transformations and
in order to study the relationships between the different phases of
the Heisenberg ladders obtained with all possible exchange coupling constants.
For the 2 legged ladder we show that the phase is invariant under the
transformation, while the Haldane phase is invariant under .
These two phases are related by . Moreover there is a "mixed" phase,
that is invariant under , and which under becomes the RVB
phase, while under becomes the Haldane phase. For odd ladders there
exists only the transformation which, for strong coupling, maps the
effective antiferromagnetic spin 1/2 chain into the spin 3/2 chain.Comment: REVTEX file, 5 pages, 2 EPS figure
Extended Defects in the Potts-Percolation Model of a Solid: Renormalization Group and Monte Carlo Analysis
We extend the model of a 2 solid to include a line of defects. Neighboring
atoms on the defect line are connected by ?springs? of different strength and
different cohesive energy with respect to the rest of the system. Using the
Migdal-Kadanoff renormalization group we show that the elastic energy is an
irrelevant field at the bulk critical point. For zero elastic energy this model
reduces to the Potts model. By using Monte Carlo simulations of the 3- and
4-state Potts model on a square lattice with a line of defects, we confirm the
renormalization-group prediction that for a defect interaction larger than the
bulk interaction the order parameter of the defect line changes discontinuously
while the defect energy varies continuously as a function of temperature at the
bulk critical temperature.Comment: 13 figures, 17 page
Slave-boson approach to the infinite-U Anderson-Holstein impurity model
The infinite- Anderson-Holstein impurity model is studied with a focus on
the interplay between the strong electron correlation and the weak
electron-phonon interaction. The slave boson method has been employed in
combination with the large degeneracy expansion (1/N) technique. The charge and
spin susceptibilities and the phonon propagator are obtained in the
approximation scheme where the saddle point configuration and the Gaussian 1/N
fluctuations are taken into account. The spin susceptibility is found not to be
renormalized by electron-phonon interaction, while the charge susceptibility is
renormalized.
From the renormalized charge susceptibility the Kondo temperature is found to
increase by the electron-phonon interaction. It turns out that the bosonic 1/N
Gaussian fluctuations play a very crucial role, in particular, for the phonon
propagator.Comment: 12pages, 3 figures. Published in Physical Review
Anomalous impurity effects in nonadiabatic superconductors
We show that, in contrast with the usual electron-phonon Migdal-Eliashberg
theory, the critical temperature Tc of an isotropic s-wave nonadiabatic
superconductor is strongly reduced by the presence of diluted non-magnetic
impurities. Our results suggest that the recently observed Tc-suppression
driven by disorder in K3C60 [Phys. Rev. B vol.55, 3866 (1997)] and in
Nd(2-x)CexCuO(4-delta) [Phys. Rev. B vol.58, 8800 (1998)] could be explained in
terms of a nonadiabatic electron-phonon coupling. Moreover, we predict that the
isotope effect on Tc has an impurity dependence qualitatively different from
the one expected for anisotropic superconductors.Comment: 10 pages, euromacr.tex, europhys.sty, 6 figures. Replaced with
accepted version (Europhysics Letters
Unitary Chern-Simons matrix model and the Villain lattice action
We use the Villain approximation to show that the Gross-Witten model, in the
weak- and strong-coupling limits, is related to the unitary matrix model that
describes U(N) Chern-Simons theory on S^3. The weak-coupling limit corresponds
to the q->1 limit of the Chern-Simons theory while the strong-coupling regime
is related to the q->0 limit. In the latter case, there is a logarithmic
relationship between the respective coupling constants. We also show how the
Chern-Simons matrix model arises by considering two-dimensional Yang-Mills
theory with the Villain action. This leads to a U(1)^N theory which is the
Abelianization of 2d Yang-Mills theory with the heat-kernel lattice action. In
addition, we show that the character expansion of the Villain lattice action
gives the q deformation of the heat kernel as it appears in q-deformed 2d
Yang-Mills theory. We also study the relationship between the unitary and
Hermitian Chern-Simons matrix models and the rotation of the integration
contour in the corresponding integrals.Comment: 17 pages, Minor corrections to match the published versio
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