7,597 research outputs found
Disorder Effects in Fluctuating One-Dimensional Interacting Systems
The zero temperature localization of interacting electrons coupled to a
two-dimensional quenched random potential, and constrained to move on a
fluctuating one-dimensional string embedded in the disordered plane, is studied
using a perturbative renormalization group approach. In the reference frame of
the electrons the impurities are dynamical and their localizing effect is
expected to decrease. We consider several models for the string dynamics and
find that while the extent of the delocalized regime indeed grows with the
degree of string fluctuations, the critical interaction strength, which
determines the localization-delocalization transition for infinitesimal
disorder,does not change unless the fluctuations are softer than those of a
simple elastic string.Comment: 15 page
Evidence of Electron Fractionalization from Photoemission Spectra in the High Temperature Superconductors
In the normal state of the high temperature superconductors
Bi_2Sr_2CaCu_2O_{8+delta} and La_{2-x}Sr_{x}CuO_4, and in the related ``stripe
ordered'' material La_1.25Nd_0.6Sr_0.15CuO_4, there is sharp structure in the
measured single hole spectral function A(k,w) considered as a function of k at
fixed small binding energy w. At the same time, as a function of w at fixed k
on much of the putative Fermi surface, any structure in A(k,w), other than the
Fermi cutoff, is very broad. This is characteristic of the situation in which
there are no stable excitations with the quantum numbers of the electron, as is
the case in the one dimensional electron gas.Comment: Published versio
Anisotropy in the helicity modulus of a 3D XY-model: application to YBCO
We present a Monte Carlo study of the helicity moduli of an anisotropic
classical three-dimensional (3D) XY-model of YBCO in superconducting state. It
is found that both the ab-plane and the c-axis helicity moduli, which are
proportional to the inverse square of the corresponding magnetic field
penetration depth, vary linearly with temperature at low temperatures. The
result for the c-axis helicity modulus is in disagreement with the experiments
on high quality samples of YBCO. Thus we conclude that purely classical phase
fluctuations of the superconducting order parameter cannot account for the
observed c-axis electrodynamics of YBCO.Comment: 7 pages, 1 figur
Effect of inhomogeneity on s-wave superconductivity in the attractive Hubbard model
Inhomogeneous s-wave superconductivity is studied in the two-dimensional,
square lattice attractive Hubbard Hamiltonian using the Bogoliubov-de Gennes
(BdG) mean field approximation. We find that at weak coupling, and for
densities mainly below half-filling, an inhomogeneous interaction in which the
on-site interaction takes on two values, results in a larger
zero temperature pairing amplitude, and that the superconducting can also
be significantly increased, relative to a uniform system with on all
sites. These effects are observed for stripe, checkerboard, and even random
patterns of the attractive centers, suggesting that the pattern of
inhomogeneity is unimportant. Monte Carlo calculations which reintroduce some
of the fluctuations neglected within the BdG approach see the same effect, both
for the attractive Hubbard model and a Hamiltonian with d-wave pairing
symmetry.Comment: 5 pages, 4 figure
s-wave Superconductivity Phase Diagram in the Inhomogeneous Two-Dimensional Attractive Hubbard Model
We study s-wave superconductivity in the two-dimensional square lattice
attractive Hubbard Hamiltonian for various inhomogeneous patterns of
interacting sites. Using the Bogoliubov-de Gennes (BdG) mean field
approximation, we obtain the phase diagram for inhomogeneous patterns in which
the on-site attractive interaction U_i between the electrons takes on two
values, U_i=0 and -U/(1-f) (with f the concentration of non-interacting sites)
as a function of average electron occupation per site n, and study the
evolution of the phase diagram as f varies. In certain regions of the phase
diagram, inhomogeneity results in a larger zero temperature average pairing
amplitude (order parameter) and also a higher superconducting (SC) critical
temperature T_c, relative to a uniform system with the same mean interaction
strength (U_i=-U on all sites). These effects are observed for stripes,
checkerboard, and even random patterns of the attractive centers, suggesting
that the pattern of inhomogeneity is unimportant. The phase diagrams also
include regions where superconductivity is obliterated due to the formation of
various charge ordered phases. The enhancement of T_{c} due to inhomogeneity is
robust as long as the electron doping per site n is less than twice the
fraction of interacting sites [2(1-f)] regardless of the pattern. We also show
that for certain inhomogeneous patterns, when n = 2(1-f), increasing
temperature can work against the stability of existing charge ordered phases
for large f and as a result, enhance T_{c}.Comment: 16 pages, 11 figure
Static versus dynamic fluctuations in the one-dimensional extended Hubbard model
The extended Hubbard Hamiltonian is a widely accepted model for uncovering
the effects of strong correlations on the phase diagram of low-dimensional
systems, and a variety of theoretical techniques have been applied to it. In
this paper the world-line quantum Monte Carlo method is used to study spin,
charge, and bond order correlations of the one-dimensional extended Hubbard
model in the presence of coupling to the lattice. A static alternating lattice
distortion (the ionic Hubbard model) leads to enhanced charge density wave
correlations at the expense of antiferromagnetic order. When the lattice
degrees of freedom are dynamic (the Hubbard-Holstein model), we show that a
similar effect occurs even though the charge asymmetry must arise
spontaneously. Although the evolution of the total energy with lattice coupling
is smooth, the individual components exhibit sharp crossovers at the phase
boundaries. Finally, we observe a tendency for bond order in the region between
the charge and spin density wave phases.Comment: Corrected typos. (10 pages, 9 figures
Weak-coupling phase diagrams of bond-aligned and diagonal doped Hubbard ladders
We study, using a perturbative renormalization group technique, the phase
diagrams of bond-aligned and diagonal Hubbard ladders defined as sections of a
square lattice with nearest-neighbor and next-nearest-neighbor hopping. We find
that for not too large hole doping and small next-nearest-neighbor hopping the
bond-aligned systems exhibit a fully spin-gapped phase while the diagonal
systems remain gapless. Increasing the next-nearest-neighbor hopping typically
leads to a decrease of the gap in the bond-aligned ladders, and to a transition
into a gapped phase in the diagonal ladders. Embedding the ladders in an
antiferromagnetic environment can lead to a reduction in the extent of the
gapped phases. These findings suggest a relation between the orientation of
hole-rich stripes and superconductivity as observed in LSCO.Comment: Published version. The set of RG equations in the presence of
magnetization was corrected and two figures were replace
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