410 research outputs found
Curve crossing in linear potential grids: the quasidegeneracy approximation
The quasidegeneracy approximation [V. A. Yurovsky, A. Ben-Reuven, P. S.
Julienne, and Y. B. Band, J. Phys. B {\bf 32}, 1845 (1999)] is used here to
evaluate transition amplitudes for the problem of curve crossing in linear
potential grids involving two sets of parallel potentials. The approximation
describes phenomena, such as counterintuitive transitions and saturation
(incomplete population transfer), not predictable by the assumption of
independent crossings. Also, a new kind of oscillations due to quantum
interference (different from the well-known St\"uckelberg oscillations) is
disclosed, and its nature discussed. The approximation can find applications in
many fields of physics, where multistate curve crossing problems occur.Comment: LaTeX, 8 pages, 8 PostScript figures, uses REVTeX and psfig,
submitted to Physical Review
Destruction of Superconductivity by Impurities in the Attractive Hubbard Model
We study the effect of U=0 impurities on the superconducting and
thermodynamic properties of the attractive Hubbard model on a square lattice.
Removal of the interaction on a critical fraction of of the sites results in the destruction of off-diagonal long range order
in the ground state. This critical fraction is roughly independent of filling
in the range , although our data suggest that might be somewhat larger below half-filling than at . We also
find that the two peak structure in the specific heat is present at both
below and above the value which destroys long range pairing order. It is
expected that the high peak associated with local pair formation should be
robust, but apparently local pairing fluctuations are sufficient to generate a
low temperature peak
Counterintuitive transitions in the multistate Landau-Zener problem with linear level crossings
We generalize the Brundobler-Elser hypothesis in the multistate Landau-Zener
problem to the case when instead of a state with the highest slope of the
diabatic energy level there is a band of states with an arbitrary number of
parallel levels having the same slope. We argue that the probabilities of
counterintuitive transitions among such states are exactly zero.Comment: 9 pages, 5 figure
Electrical Conductivity of Fermi Liquids. I. Many-body Effect on the Drude Weight
On the basis of the Fermi liquid theory, we investigate the many-body effect
on the Drude weight. In a lattice system, the Drude weight is modified by
electron-electron interaction due to Umklapp processes, while it is not
renormalized in a Galilean invariant system. This is explained by showing that
the effective mass for is defined through the current, not
velocity, of quasiparticle. It is shown that the inequality is required
for the stability against the uniform shift of the Fermi surface. The result of
perturbation theory applied for the Hubbard model indicates that as a
function of the density is qualitatively modified around half filling
by Umklapp processes.Comment: 20 pages, 2 figures; J. Phys. Soc. Jpn. Vol.67, No.
Specific Heat of the 2D Hubbard Model
Quantum Monte Carlo results for the specific heat c of the two dimensional
Hubbard model are presented. At half-filling it was observed that
at very low temperatures. Two distinct features were also identified: a low
temperature peak related to the spin degrees of freedom and a higher
temperature broad peak related to the charge degrees of freedom. Away from
half-filling the spin induced feature slowly disappears as a function of hole
doping while the charge feature moves to lower temperature. A comparison with
experimental results for the high temperature cuprates is discussed.Comment: 6 pages, RevTex, 11 figures embedded in the text, Submitted to Phys.
Rev.
Nearly universal crossing point of the specific heat curves of Hubbard models
A nearly universal feature of the specific heat curves C(T,U) vs. T for
different U of a general class of Hubbard models is observed. That is, the
value C_+ of the specific heat curves at their high-temperature crossing point
T_+ is almost independent of lattice structure and spatial dimension d, with
C_+/k_B \approx 0.34. This surprising feature is explained within second order
perturbation theory in U by identifying two small parameters controlling the
value of C_+: the integral over the deviation of the density of states
N(\epsilon) from a constant value, characterized by \delta N=\int d\epsilon
|N(\epsilon)-1/2|, and the inverse dimension, 1/d.Comment: Revtex, 9 pages, 6 figure
Insulator-Metal Transition in the One and Two-Dimensional Hubbard Models
We use Quantum Monte Carlo methods to determine Green functions,
, on lattices up to for the 2D Hubbard model
at . For chemical potentials, , within the Hubbard gap, , and at {\it long} distances, , with critical behavior: , . This result stands in agreement with the
assumption of hyperscaling with correlation exponent and dynamical
exponent . In contrast, the generic band insulator as well as the
metal-insulator transition in the 1D Hubbard model are characterized by and .Comment: 9 pages (latex) and 5 postscript figures. Submitted for publication
in Phys. Rev. Let
Resonance Patterns of an Antidot Cluster: From Classical to Quantum Ballistics
We explain the experimentally observed Aharonov-Bohm (AB) resonance patterns
of an antidot cluster by means of quantum and classical simulations and Feynman
path integral theory. We demonstrate that the observed behavior of the AB
period signals the crossover from a low B regime which can be understood in
terms of electrons following classical orbits to an inherently quantum high B
regime where this classical picture and semiclassical theories based on it do
not apply.Comment: 5 pages revtex + 2 postscript figure
Fermionic R-Operator and Algebraic Structure of 1D Hubbard Model: Its application to quantum transfer matrix
The algebraic structure of the 1D Hubbard model is studied by means of the
fermionic R-operator approach. This approach treats the fermion models directly
in the framework of the quantum inverse scattering method. Compared with the
graded approach, this approach has several advantages. First, the global
properties of the Hamiltonian are naturally reflected in the algebraic
properties of the fermionic R-operator. We want to note that this operator is a
local operator acting on fermion Fock spaces. In particular, SO(4) symmetry and
the invariance under the partial particle hole transformation are discussed.
Second, we can construct a genuinely fermionic quantum transfer transfer matrix
(QTM) in terms of the fermionic R-operator. Using the algebraic Bethe Ansatz
for the Hubbard model, we diagonalize the fermionic QTM and discuss its
properties.Comment: 22 pages, no figure
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