102 research outputs found
Strong Coupling Correction in Superfluid He in Aerogel
Effects of impurity scatterings on the strong coupling (SC) contribution,
stabilizing the ABM (axial) pairing state, to the quartic term of the
Ginzburg-Landau (GL) free energy of superfluid He are theoretically studied
to examine recent observations suggestive of an anomalously small SC effect in
superfluid He in aerogels. To study the SC corrections, two approaches are
used. One is based on a perturbation in the short-range repulsive interaction,
and the other is a phenomenological approach used previously for the bulk
liquid by Sauls and Serene [Phys.Rev.B 24, 183 (1981)]. It is found that the
impurity scattering favors the BW pairing state and shrinks the region of the
ABM pairing state in the T-P phase diagram. In the phenomenological approach,
the resulting shrinkage of the ABM region is especially substantial and, if
assuming an anisotropy over a large scale in aerogel, leads to justifying the
phase diagrams determined experimentally.Comment: 19 pages, 9 figures, Accepted for publication in Phys. Rev.
Nodes of the Gap Function and Anomalies in Thermodynamic Properties of Superfluid He
Departures of thermodynamic properties of three-dimensional superfluid He
from the predictions of BCS theory are analyzed. Attention is focused on
deviations of the ratios and
from their BCS values, where is the pairing gap at zero
temperature, is the critical temperature, and and are the
superfluid and normal specific heats. We attribute these deviations to the
momentum dependence of the gap function , which becomes well
pronounced when this function has a pair of nodes lying on either side of the
Fermi surface. We demonstrate that such a situation arises if the P-wave
pairing interaction , evaluated at the Fermi surface, has a sign
opposite to that anticipated in BCS theory. Taking account of the momentum
structure of the gap function, we derive a closed relation between the two
ratios that contains no adjustable parameters and agrees with the experimental
data. Some important features of the effective pairing interaction are inferred
from the analysis.Comment: 17 pages, 4 figure
On the Nagaoka polaron in the t-J model
It is widely believed that a single hole in the two (or three) dimensional
t-J model, for sufficiently small exchange coupling J, creates a ferromagnetic
bubble around itself, a finite J remnant of the ferromagnetic groundstate at
J=0 (the infinite U Hubbard model), first established by Nagaoka. We
investigate this phenomenon in two dimensions using the density matrix
renormalization group, for system sizes up to 9x9. We find that the polaron
forms for J/t<0.02-0.03 (a somewhat larger value than estimated previously).
Although finite-size effects appear large, our data seems consistent with the
expected 1.1(J/t)^{-1/4} variation of polarion radius. We also test the
Brinkman-Rice model of non-retracing paths in a Neel background, showing that
it is quite accurate, at larger J. Results are also presented in the case where
the
Heisenberg interaction is dropped (the t-J^z model). Finally we discuss a
"dressed polaron" picture in which the hole propagates freely inside a finite
region but makes only self-retracing excursions outside this region.Comment: 7 pages, 9 encapsulated figure
Effective-Medium Theory for the Normal State in Orientationally Disordered Fullerides
An effective-medium theory for studying the electronic structure of the
orientationally disordered A3C60 fullerides is developed and applied to study
various normal-state properties. The theory is based on a cluster-Bethe-lattice
method in which the disordered medium is modelled by a three-band Bethe
lattice, into which we embed a molecular cluster whose scattering properties
are treated exactly. Various single-particle properties and the
frequency-dependent conductivity are calculated in this model, and comparison
is made with numerical calculations for disordered lattices, and with
experiment.Comment: 12 pages + 2 figures, REVTeX 3.
Hole dynamics in a quantum antiferromagnet beyond the retraceable path approximation
The one-hole spectral weight for two chains and two dimensional lattices is
studied numerically using a new method of analysis of the spectral function
within the Lanczos iteration scheme: the Lanczos spectra decoding method. This
technique is applied to the model for , directly in the
infinite size lattice. By a careful investigation of the first 13 Lanczos steps
and the first 26 ones for the two dimensional and the two chain cases
respectively, we get several new features of the one-hole spectral weight. A
sharp incoherent peak with a clear momentum dispersion is identified, together
with a second broad peak at higher energy. The spectral weight is finite up to
the Nagaoka energy where it vanishes in a non-analytic way. Thus the lowest
energy of one hole in a quantum antiferromagnet is degenerate with the Nagaoka
energy in the thermodynamic limit.Comment: RevTeX 3.0, SISSA preprint 156/93/CM/MB, 10 pages + postscript file
appended, contains more accurate calculations in Fig.
Interaction Effect in the Kondo Energy of the Periodic Anderson-Hubbard Model
We extend the periodic Anderson model by switching on a Hubbard for the
conduction electrons. The nearly integral valent (Kondo) limit of the
Anderson--Hubbard model is studied with the Gutzwiller variational method. The
new formula for the Kondo energy contains the -dependent chemical
potential of the Hubbard subsystem in the exponent, and the correlation-induced
band narrowing in the prefactor. Both effects tend to suppress the Kondo scale,
which can be understood to result from the blocking of hybridization (this
behaviour is the opposite of that found for Kondo--Hubbard models). At
half-filling, we find a Brinkman--Rice-type transition which leads from a
small-gap Kondo insulator to a Mott insulator.Comment: 4 pages (ReVTeX), submitted for publicatio
Even and odd-frequency pairing correlations in 1-D t-J-h model: a comparative study
An equal time version of odd-frequency pairing for a generalized model
is introduced. It is shown that the composite operators describing binding of
Cooper pairs with magnetization fluctuations naturally appear in this approach.
The pairing correlations in both BCS and odd-frequency channels are
investigated exactly in 1D systems with up to 16 sites. Our results indicate
that at some range of parameters odd-frequency correlations become comparable,
however smaller than BCS pairing correlations. It is speculated that the spin
and density fluctuations in the frustrated model lead to the enhancement of the
odd gap susceptibilities. 4 postscript figure files are attached at the bottom
of the tex file.Comment: 6 pages + 4 figure
Quantum Monte Carlo treatment of elastic exciton-exciton scattering
We calculate cross sections for low energy elastic exciton-exciton scattering
within the effective mass approximation. Unlike previous theoretical
approaches, we give a complete, non-perturbative treatment of the four-particle
scattering problem. Diffusion Monte Carlo is used to calculate the essentially
exact energies of scattering states, from which phase shifts are determined.
For the case of equal-mass electrons and holes, which is equivalent to
positronium-positronium scattering, we find a_s = 2.1 a_x for scattering of
singlet-excitons and a_s= 1.5 a_x for triplet-excitons, where a_x is the
excitonic radius. The spin dependence of the cross sections arises from the
spatial exchange symmetry of the scattering wavefunctions. A significant
triplet-triplet to singlet-singlet scattering process is found, which is
similar to reported effects in recent experiments and theory for excitons in
quantum wells. We also show that the scattering length can change sign and
diverge for some values of the mass ratio m_h/m_e, an effect not seen in
previous perturbative treatments.Comment: 6 pages, 6 figures. Revision has updated figures, improved paper
structure, some minor correction
Temperature Dependence of Hall Response in Doped Antiferromagnets
Using finite-temperature Lanczos method the frequency-dependent Hall response
is calculated numerically for the t-J model on the square lattice and on
ladders. At low doping, both the high-frequency RH* and the d.c. Hall
coefficient RH0 follow qualitatively similar behavior at higher temperatures:
being hole-like for T > Ts~1.5J and weakly electron-like for T < Ts. Consistent
with experiments on cuprates, RH0 changes, in contrast to RH*, again to the
hole-like sign below the pseudogap temperature T*, revealing a strong
temperature variation for T->0.Comment: LaTeX, 4 pages, 4 figures, submitted to PR
Comparison of Variational Approaches for the Exactly Solvable 1/r-Hubbard Chain
We study Hartree-Fock, Gutzwiller, Baeriswyl, and combined
Gutzwiller-Baeriswyl wave functions for the exactly solvable one-dimensional
-Hubbard model. We find that none of these variational wave functions is
able to correctly reproduce the physics of the metal-to-insulator transition
which occurs in the model for half-filled bands when the interaction strength
equals the bandwidth. The many-particle problem to calculate the variational
ground state energy for the Baeriswyl and combined Gutzwiller-Baeriswyl wave
function is exactly solved for the~-Hubbard model. The latter wave
function becomes exact both for small and large interaction strength, but it
incorrectly predicts the metal-to-insulator transition to happen at infinitely
strong interactions. We conclude that neither Hartree-Fock nor Jastrow-type
wave functions yield reliable predictions on zero temperature phase transitions
in low-dimensional, i.e., charge-spin separated systems.Comment: 23 pages + 3 figures available on request; LaTeX under REVTeX 3.
- …