52 research outputs found
Efficient calculation of imaginary time displaced correlation functions in the projector auxiliary field quantum Monte-Carlo algorithm
The calculation of imaginary time displaced correlation functions with the
auxiliary field projector quantum Monte-Carlo algorithm provides valuable
insight (such as spin and charge gaps) in the model under consideration. One of
the authors and M. Imada [F.F. Assaad and M. Imada, J. Phys. Soc. Jpn. 65 189
(1996).] have proposed a numerically stable method to compute those quantities.
Although precise this method is expensive in CPU time. Here, we present an
alternative approach which is an order of magnitude quicker, just as precise,
and very simple to implement. The method is based on the observation that for a
given auxiliary field the equal time Green function matrix, , is a
projector: .Comment: 4 papes, 1 figure in eps forma
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
Huge metastability in high-T_c superconductors induced by parallel magnetic field
We present a study of the temperature-magnetic field phase diagram of
homogeneous and inhomogeneous superconductivity in the case of a
quasi-two-dimensional superconductor with an extended saddle point in the
energy dispersion under a parallel magnetic field. At low temperature, a huge
metastability region appears, limited above by a steep superheating critical
field (H_sh) and below by a strongly reentrant supercooling field (H_sc). We
show that the Pauli limit (H_p) for the upper critical magnetic field is
strongly enhanced due to the presence of the Van Hove singularity in the
density of states. The formation of a non-uniform superconducting state is
predicted to be very unlikely.Comment: 5 pages, 2 figures; to appear in Phys. Rev.
Kondo lattice model with a direct exchange interaction between localized moments
We study the Kondo lattice model with a direct antiferromagnetic exchange
interaction between localized moments. Ferromagnetically long-range ordered
state coexisting with the Kondo screening shows a continuous quantum phase
transition to the Kondo singlet state. We obtain the value of the critical
point where the magnetizations of the localized moments and the conduction
electrons vanish. The magnetization curves yield a universal critical exponent
independent of the filling factors and the strength of the interaction between
localized moments. It is shown that the direct exchange interaction between
localized moments introduces another phase transition from an antiferromagnetic
ordering to a ferromagnetic ordering for small Kondo exchange interaction. We
also explain the local minimum of the Kondo temperature in recent experiments.Comment: 6 pages, 5 figures, final versio
Kondo spin liquid and magnetically long-range ordered states in the Kondo necklace model
A simplified version of the symmetric Kondo lattice model, the Kondo necklace
model, is studied by using a representation of impurity and conduction electron
spins in terms of local Kondo singlet and triplet operators. Within a mean
field theory, a spin gap always appears in the spin triplet excitation spectrum
in 1D, leading to a Kondo spin liquid state for any finite values of coupling
strength (with as hopping and as exchange); in 2D and 3D cubic
lattices the spin gaps are found to vanish continuously around and , respectively, where quantum phase transitions
occur and the Kondo spin liquid state changes into an antiferromagnetically
long-range ordered state. These results are in agreement with variational Monte
Carlo, higher-order series expansion, and recent quantum Monte Carlo
calculations for the symmetric Kondo lattice modelComment: Revtex, four pages, three figures; to be published in Physical Review
B1, 1 July (2000
Numerical renormalization group study of the 1D t-J model
The one-dimensional (1D) model is investigated using the density matrix
renormalization group (DMRG) method. We report for the first time a
generalization of the DMRG method to the case of arbitrary band filling and
prove a theorem with respect to the reduced density matrix that accelerates the
numerical computation. Lastly, using the extended DMRG method, we present the
ground state electron momentum distribution, spin and charge correlation
functions. The anomaly of the momentum distribution function first
discussed by Ogata and Shiba is shown to disappear as increases. We also
argue that there exists a density-independent beyond which the system
becomes an electron solid.Comment: Wrong set of figures were put in the orginal submissio
Finite temperature properties of the 2D Kondo lattice model
Using recently developed Lanczos technique we study finite-temperature
properties of the 2D Kondo lattice model at various fillings of the conduction
band. At half filling the quasiparticle gap governs physical properties of the
chemical potential and the charge susceptibility at small temperatures. In the
intermediate coupling regime quasiparticle gap scales approximately linearly
with Kondo coupling. Temperature dependence of the spin susceptibility reveals
the existence of two different temperature scales. A spin gap in the
intermediate regime leads to exponential drop of the spin susceptibility at low
temperatures. Unusual scaling of spin susceptibility is found for temperatures
above 0.6 J. Charge susceptibility at finite doping reveals existence of heavy
quasiparticles. A new low energy scale is found at finite doping.Comment: REVTeX, 7 pages, 7 figure
Thermodynamics of the half-filled Kondo lattice model around the atomic limit
We present a perturbation theory for studying thermodynamic properties of the
Kondo spin liquid phase of the half-filled Kondo lattice model. The grand
partition function is derived to calculate chemical potential, spin and charge
susceptibilities and specific heat. The treatment is applicable to the model
with strong couplings in any dimensions (one, two and three dimensions). The
chemical potential equals zero at any temperatures, satisfying the requirement
of the particle-hole symmetry. Thermally activated behaviors of the
spin(charge) susceptibility due to the spin(quasiparticle) gap can be seen and
the two-peak structure of the specific heat is obtained. The same treatment to
the periodic Anderson model around atomic limit is also briefly discussed.Comment: 5 pages, 3 figures, to appear in Phys. Rev.
Ordering and Fluctuation of Orbital and Lattice Distortion in Perovskite Manganese Oxides
Roles of orbital and lattice degrees of freedom in strongly correlated
systems are investigated to understand electronic properties of perovskite Mn
oxides such as La_{1-x}Sr_{x}MnO_{3}. An extended double-exchange model
containing Coulomb interaction, doubly degenerate orbitals and Jahn-Teller
coupling is derived under full polarization of spins with two-dimensional
anisotropy. Quantum fluctuation effects of Coulomb interaction and orbital
degrees of freedom are investigated by using the quantum Monte Carlo method. In
undoped states, it is crucial to consider both the Coulomb interaction and the
Jahn-Teller coupling in reproducing characteristic hierarchy of energy scales
among charge, orbital-lattice and spin degrees of freedom in experiments. Our
numerical results quantitatively reproduce the charge gap amplitude as well as
the stabilization energy and the amplitude of the cooperative Jahn-Teller
distortion in undoped compounds. Upon doping of carriers, in the absence of the
Jahn-Teller distortion, critical enhancement of both charge compressibility and
orbital correlation length is found with decreasing doping concentration. These
are discussed as origins of strong incoherence in charge dynamics. With the
Jahn-Teller coupling in the doped region, collapse of the Jahn-Teller
distortion and instability to phase separation are obtained and favorably
compared with experiments. These provide a possible way to understand the
complicated properties of lightly doped manganites.Comment: 22 pages RevTeX including 25 PS figures, submitted to Phys.Rev.B,
replaced version; two figures are replaced by Fig.17 with minor changes in
the tex
Quantum Monte Carlo study of the 3D attractive Hubbard model
We study the three-dimensional (3D) attractive Hubbard model by means of the
Determinant Quantum Monte Carlo method. This model is a prototype for the
description of the smooth crossover between BCS superconductivity and
Bose-Einstein condensation. By detailed finite-size scaling we extract the
finite-temperature phase diagram of the model. In particular, we interpret the
observed behavior according to a scenario of two fundamental temperature
scales; T* associated with Cooper pair formation and Tc with condensation
(giving rise to long-range superconducting order). Our results also indicate
the presence of a recently conjectured phase transition hidden by the
superconducting state. A comparison with the 2D case is briefly discussed,
given its relevance for the physics of high-Tc cuprate superconductors.Comment: 4 pages, 4 Postscript figure
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