1,056 research outputs found
Quantum Monte Carlo diagonalization for many-fermion systems
In this study we present an optimization method based on the quantum Monte
Carlo diagonalization for many-fermion systems. Using the Hubbard-Stratonovich
transformation, employed to decompose the interactions in terms of auxiliary
fields, we expand the true ground-state wave function. The ground-state wave
function is written as a linear combination of the basis wave functions. The
Hamiltonian is diagonalized to obtain the lowest energy state, using the
variational principle within the selected subspace of the basis functions. This
method is free from the difficulty known as the negative sign problem. We can
optimize a wave function using two procedures. The first procedure is to
increase the number of basis functions. The second improves each basis function
through the operators, , using the Hubbard-Stratonovich
decomposition. We present an algorithm for the Quantum Monte Carlo
diagonalization method using a genetic algorithm and the renormalization
method. We compute the ground-state energy and correlation functions of small
clusters to compare with available data
Ground state of the three-band Hubbard model
The ground state of the two-dimensional three-band Hubbard model in oxide
superconductors is investigated by using the variational Monte Carlo method.
The Gutzwiller-projected BCS and spin- density wave (SDW) functions are
employed in the search for a possible ground state with respect to dependences
on electron density. Antiferromagnetic correlations are considerably enhanced
near half-filling. It is shown that the d-wave state may exist away from
half-filling for both the hole and electron doping cases. The overall structure
of the phase diagram obtained by the calculations qualitatively agrees with
experimental indications. The superconducting condensation energy is in
reasonable agreement with the experimental value obtained from specific heat
and critical magnetic field measurements for optimally doped samples. The
inhomogeneous SDW state is also examined near 1/8-hole doping.Comment: 10 pages, 17 figure
Possible high superconductivity mediated by antiferromagnetic spin fluctuations in systems with Fermi surface pockets
We propose that if there are two small pocket-like Fermi surfaces, and the
spin susceptibility is pronounced around a wave vector {\bf Q} that bridges the
two pockets, the spin-singlet superconductivity mediated by spin fluctuations
may have a high transition temperature. Using the fluctuation exchange
approximation, this idea is confirmed for the Hubbard on a lattice with
alternating hopping integrals, for which is estimated to be almost an
order of magnitude larger than those for systems with a large connected Fermi
surface.Comment: 5 pages, uses RevTe
Nuclear fission: The "onset of dissipation" from a microscopic point of view
Semi-analytical expressions are suggested for the temperature dependence of
those combinations of transport coefficients which govern the fission process.
This is based on experience with numerical calculations within the linear
response approach and the locally harmonic approximation. A reduced version of
the latter is seen to comply with Kramers' simplified picture of fission. It is
argued that for variable inertia his formula has to be generalized, as already
required by the need that for overdamped motion the inertia must not appear at
all. This situation may already occur above T=2 MeV, where the rate is
determined by the Smoluchowski equation. Consequently, comparison with
experimental results do not give information on the effective damping rate, as
often claimed, but on a special combination of local stiffnesses and the
friction coefficient calculated at the barrier.Comment: 31 pages, LaTex, 9 postscript figures; final, more concise version,
accepted for publication in PRC, with new arguments about the T-dependence of
the inertia; e-mail: [email protected]
Superconductivity in the three-leg Hubbard ladder: a Quantum Monte Carlo study
Quantum Monte Carlo method is used to look into the superconductivity in the
three-leg Hubbard ladder. The enhanced correlation for the pairing across the
central and edge chains, which has been predicted in the weak-coupling
renormalization as an effect of coexistence of gapful and gapless spin modes,
is here shown to persist for intermediate interaction strengths.Comment: 10 pages, RevTeX, 3 figures in PostScript file
Fermi arc in doped high-Tc cuprates
We propose a -density wave induced by the spin-orbit coupling in the CuO
plane. The spectral function of high-temperature superconductors in the under
doped and lightly doped regions is calculated in order to explain the Fermi arc
spectra observed recently by angle-resolved photoemission spectroscopy. We take
into account the tilting of CuO octahedra as well as the on-site
Coulombrepulsive interaction; the tilted octahedra induce the staggered
transfer integral between orbitals and Cu orbitals, and
bring about nontrivial effects of spin-orbit coupling for the electrons in
the CuO plane. The spectral weight shows a peak at around (,) for
light doping and extends around this point forming an arc as the carrier
density increases, where the spectra for light doping grow continuously to be
the spectra in the optimally doped region. This behavior significantly agrees
with that of the angle-resolved photoemissionspectroscopy spectra. Furthermore,
the spin-orbit term and staggered transfer effectively induce a flux state, a
pseudo-gap with time-reversal symmetry breaking. We have a nodal metallic state
in the light-doping case since the pseudogap has a symmetry.Comment: 5 pages, 7 figure
Quantum Monte Carlo study of the pairing correlation in the Hubbard ladder
An extensive Quantum Monte Carlo calculation is performed for the two-leg
Hubbard ladder model to clarify whether the singlet pairing correlation decays
slowly, which is predicted from the weak-coupling theory but controversial from
numerical studies. Our result suggests that the discreteness of energy levels
in finite systems affects the correlation enormously, where the enhanced
pairing correlation is indeed detected if we make the energy levels of the
bonding and anti-bonding bands lie close to each other at the Fermi level to
mimic the thermodynamic limit.Comment: 10 pages, RevTeX, 5 figures in PostScript file
Persistent current of two-chain Hubbard model with impurities
The interplay between impurities and interactions is studied in the gapless
phase of two-chain Hubbard model in order to see how the screening of impurity
potentials due to repulsive interactions in single-chain model will be changed
by increasing the number of channels. Renormalization group calculations show
that charge stiffness, and hence persistent current, of the two-chain model are
less enhanced by interactions than single chain case.Comment: 4 Pages, RevTeX, No figures, Submitted to PR
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