251 research outputs found
Study of the charge correlation function in one-dimensional Hubbard heterostructures
We study inhomogeneous one-dimensional Hubbard systems using the density
matrix renormalization group method. Different heterostructures are
investigated whose configuration is modeled varying parameters like the on-site
Coulomb potential and introducing local confining potentials. We investigate
their Luttinger liquid properties through the parameter K_rho, which
characterizes the decay of the density-density correlation function at large
distances. Our main goal is the investigation of possible realization of
engineered materials and the ability to manipulate physical properties by
choosing an appropriate spatial and/or chemical modulation.Comment: 6 pages, 7 figure
Charge transfer fluctuation, wave superconductivity, and the Raman phonon in the Cuprates: A detailed analysis
The Raman spectrum of the phonon in the superconducting cuprate
materials is investigated theoretically in detail in both the normal and
superconducting phases, and is contrasted with that of the phonon. A
mechanism involving the charge transfer fluctuation between the two oxygen ions
in the CuO plane coupled to the crystal field perpendicular to the plane is
discussed and the resulting electron-phonon coupling is evaluated. Depending on
the symmetry of the phonon the weight of different parts of the Fermi surface
in the coupling is different. This provides the opportunity to obtain
information on the superconducting gap function at certain parts of the Fermi
surface. The lineshape of the phonon is then analyzed in detail both in the
normal and superconducting states. The Fano lineshape is calculated in the
normal state and the change of the linewidth with temperature below T is
investigated for a pairing symmetry. Excellent agreement is
obtained for the phonon lineshape in YBaCuO. These
experiments, however, can not distinguish between and a
highly anisotropic -wave pairing.Comment: Revtex, 21 pages + 4 postscript figures appended, tp
Trapped interacting two-component bosons
In this paper we solve one dimensional trapped SU(2) bosons with repulsive
-function interaction by means of Bethe-ansatz method. The features of
ground state and low-lying excited states are studied by numerical and analytic
methods. We show that the ground state is an isospin "ferromagnetic" state
which differs from spin-1/2 fermions system. There exist three quasi-particles
in the excitation spectra, and both holon-antiholon and holon-isospinon
excitations are gapless for large systems. The thermodynamics equilibrium of
the system at finite temperature is studied by thermodynamic Bethe ansatz. The
thermodynamic quantities, such as specific heat etc. are obtained for the case
of strong coupling limit.Comment: 15 pages, 9 figure
NMR and Neutron Scattering Experiments on the Cuprate Superconductors: A Critical Re-Examination
We show that it is possible to reconcile NMR and neutron scattering
experiments on both LSCO and YBCO, by making use of the Millis-Monien-Pines
mean field phenomenological expression for the dynamic spin-spin response
function, and reexamining the standard Shastry-Mila-Rice hyperfine Hamiltonian
for NMR experiments. The recent neutron scattering results of Aeppli et al on
LSCO (x=14%) are shown to agree quantitatively with the NMR measurements of
and the magnetic scaling behavior proposed by Barzykin and Pines.
The reconciliation of the relaxation rates with the degree of
incommensuration in the spin fluctuation spectrum seen in neutron experiments
is achieved by introducing a new transferred hyperfine coupling between
oxygen nuclei and their next nearest neighbor spins; this leads to a
near-perfect cancellation of the influence of the incommensurate spin
fluctuation peaks on the oxygen relaxation rates of LSCO. The inclusion of the
new term also leads to a natural explanation, within the one-component
model, the different temperature dependence of the anisotropic oxygen
relaxation rates for different field orientations, recently observed by
Martindale . The measured significant decrease with doping of the
anisotropy ratio, in LSCO system, from
for to for LSCO (x=15%) is made compatible with the
doping dependence of the shift in the incommensurate spin fluctuation peaks
measured in neutron experiments, by suitable choices of the direct and
transferred hyperfine coupling constants and B.Comment: 24 pages in RevTex, 9 figures include
Line-distortion, Bandwidth and Path-length of a graph
We investigate the minimum line-distortion and the minimum bandwidth problems
on unweighted graphs and their relations with the minimum length of a
Robertson-Seymour's path-decomposition. The length of a path-decomposition of a
graph is the largest diameter of a bag in the decomposition. The path-length of
a graph is the minimum length over all its path-decompositions. In particular,
we show:
- if a graph can be embedded into the line with distortion , then
admits a Robertson-Seymour's path-decomposition with bags of diameter at most
in ;
- for every class of graphs with path-length bounded by a constant, there
exist an efficient constant-factor approximation algorithm for the minimum
line-distortion problem and an efficient constant-factor approximation
algorithm for the minimum bandwidth problem;
- there is an efficient 2-approximation algorithm for computing the
path-length of an arbitrary graph;
- AT-free graphs and some intersection families of graphs have path-length at
most 2;
- for AT-free graphs, there exist a linear time 8-approximation algorithm for
the minimum line-distortion problem and a linear time 4-approximation algorithm
for the minimum bandwidth problem
Electronic Correlations Near a Peierls-CDW Transition
Results of a phenomenological Monte carlo calculation for a 2D
electron-phonon Holstein model near a Peierls-CDW transition are presented.
Here the zero Matsubara frequency part of the phonon action is dominant and we
approximated it by a phenomenological form that as an Ising-like Peierls-CDW
transition. The resulting model is studied on a 32 by 32 lattice. The single
particle spectral weight A(k,\omega), the density of states N(\omega), and the
real part of the conductivity \sigma_1(\omega) all show evidence of a pseudogap
which develops in the low-energy electronic degrees of freedom as the
Peierls-CDW transition is approachedComment: 14 pages, 7 figure
Resistivity as a function of temperature for models with hot spots on the Fermi surface.
We calculate the resistivity as a function of temperature for two
models currently discussed in connection with high temperature
superconductivity: nearly antiferromagnetic Fermi liquids and models with van
Hove singularities on the Fermi surface. The resistivity is calculated
semiclassicaly by making use of a Boltzmann equation which is formulated as a
variational problem. For the model of nearly antiferromagnetic Fermi liquids we
construct a better variational solution compared to the standard one and we
find a new energy scale for the crossover to the behavior at
low temperatures. This energy scale is finite even when the spin-fluctuations
are assumed to be critical. The effect of additional impurity scattering is
discussed. For the model with van Hove singularities a standard ansatz for the
Boltzmann equation is sufficient to show that although the quasiparticle
lifetime is anomalously short, the resistivity .Comment: Revtex 3.0, 8 pages; figures available upon request. Submitted to
Phys. Rev. B
On the Bilayer Coupling in the Yttrium-Barium Family of High Temperature Superconductors
We present and solve a model for the susceptibility of two CuO2 planes
coupled by an interplane coupling J_perp and use the results to analyze a
recent "cross-relaxation" NMR experiment on Y2Ba4Cu7O15. We deduce that in this
material the product of J_perp and the maximum value of the in-plane
susceptibility chi_max varies from approximately 0.2 at T = 200 K to 0.4 at T =
120 K and that this implies the existence of a temperature dependent in-plane
spin correlation length. Using estimates of chi_max from the literature we find
5 meV < J_perp < 20 meV. We discuss the relation of the NMR results to neutron
scattering results which have been claimed to imply that in YBa2Cu3O_{6+x} the
two planes of a bilayer are perfectly anticorrelated. We also propose that the
recently observed 41 meV excitation in YBa2Cu3O7 is an exciton pulled down
below the superconducting gap by J_perp.Comment: 11 pages, 3 postscript figures (uuencoded and compressed
Theory of Thermal Conductivity in YBa_2Cu_3O_{7-\delta}
We calculate the electronic thermal conductivity in a d-wave superconductor,
including both the effect of impurity scattering and inelastic scattering by
antiferromagnetic spin fluctuations. We analyze existing experiments,
particularly with regard to the question of the relative importance of
electronic and phononic contributions to the heat current, and to the influence
of disorder on low-temperature properties. We find that phonons dominate heat
transport near T_c, but that electrons are responsible for most of the peak
observed in clean samples, in agreement with a recent analysis of Krishana et
al. In agreement with recent data on YBa_2(Cu_1-xZn_x)_3O_7-\delta the peak
position is found to vary nonmonotonically with disorder.Comment: 4 pages, 4 figures, to be published in Phys. Rev. Let
Spin Gaps and Bilayer Coupling in YBaCuO and YBaCuO
We investigate the relevance to the physics of underdoped
YBaCuO and YBaCuO of the quantum critical point
which occurs in a model of two antiferromagnetically coupled planes of
antiferromagnetically correlated spins. We use a Schwinger boson mean field
theory and a scaling analysis to obtain the phase diagram of the model and the
temperature and frequency dependence of various susceptibilities and relaxation
rates. We distinguish between a low coupled-planes regime in which
the optic spin excitations are frozen out and a high
decoupled-planes regime in which the two planes fluctuate independently. In the
coupled-planes regime the yttrium nuclear relaxation rate at low temperatures
is larger relative to the copper and oxygen rates than would be naively
expected in a model of uncorrelated planes. Available data suggest that in
YBaCuO the crossover from the coupled to the decoupled planes
regime occurs at or . The predicted correlation length is
of order 6 lattice constants at . Experimental data related to the
antiferromagnetic susceptibility of YBaCuO may be made consistent
with the theory, but available data for the uniform susceptibility are
inconsistent with the theory.Comment: RevTex 3.
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