169 research outputs found
Competition Between Antiferromagnetic Order and Spin-Liquid Behavior in the Two-Dimensional Periodic Anderson Model at Half-Filling
We study the two-dimensional periodic Anderson model at half-filling using
quantum Monte Carlo (QMC) techniques. The ground state undergoes a magnetic
order-disorder transition as a function of the effective exchange coupling
between the conduction and localized bands. Low-lying spin and charge
excitations are determined using the maximum entropy method to analytically
continue the QMC data. At finite temperature we find a competition between the
Kondo effect and antiferromagnetic order which develops in the localized band
through Ruderman-Kittel-Kasuya-Yosida interactions.Comment: Revtex 3.0, 10 pages + 5 figures, UCSBTH-94-2
Impurity correlations in dilute Kondo alloys
The single impurity Kondo model is often used to describe metals with dilute
concentrations (n_i) of magnetic impurities. Here we examine how dilute the
impurities must be for this to be valid by developing a virial expansion in
impurity density. The O(n_i^2) term is determined from results on the
2-impurity Kondo problem by averaging over the RKKY coupling. The non-trivial
fixed point of the 2-impurity problem could produce novel singularities in the
heat capacity of dilute alloys at O(n_i^2).Comment: 6 pages, no figure
RKKY interaction and Kondo screening cloud for correlated electrons
The RKKY law and the Kondo screening cloud around a magnetic impurity are
investigated for correlated electrons in 1D (Luttinger liquid). We find slow
algebraic distance dependences, with a crossover between both types of
behavior. Monte Carlo simulations have been developed to study this crossover.
In the strong coupling regime, the Knight shift is shown to increase with
distance due to correlations.Comment: 5 pages REVTeX, incl two figures, to appear in Phys.Rev.
Multi-Orbital Hubbard Model in Infinite Dimensions: Quantum Monte Carlo Calculation
Using Quantum Monte Carlo we compute thermodynamics and spectra for the
orbitally degenerate Hubbard model in infinite spatial dimensions. With
increasing orbital degeneracy we find in the one-particle spectra: broader
Hubbard bands (consistent with increased kinetic energy), a narrowing Mott gap,
and increasing quasi-particle spectral weight. In opposition, Hund's rule
exchange coupling decreases the critical on-site Coulomb energy for the Mott
transition. The metallic regime resistivity for two-fold degeneracy is
quadratic-in-temperature at low temperatures.Comment: 4 pages, 4 figures, to be published in PR
Exact Quantum Monte Carlo Process for the Statistics of Discrete Systems
We introduce an exact Monte Carlo approach to the statistics of discrete
quantum systems which does not rely on the standard fragmentation of the
imaginary time, or any small parameter. The method deals with discrete objects,
kinks, representing virtual transitions at different moments of time. The
global statistics of kinks is reproduced by explicit local procedures, the key
one being based on the exact solution for the biased two-level system.Comment: 4 pages, latex, no figures, English translation of the paper
Spin and charge dynamics of the ferromagnetic and antiferromagnetic two-dimensional half-filled Kondo lattice model
We present a detailed numerical study of spin and charge dynamics of the
two-dimensional Kondo lattice model with hopping t and exchange J. At T=0 and J
> 0, the competition between the RKKY interaction and Kondo effect triggers a
quantum phase transition between magnetically ordered and disordered
insulators: J_c/t = 1.45(5). The quasiparticle gap scales as |J|. S(q,\omega),
evolves smoothly from its strong coupling form with spin gap at q = (\pi,\pi)
to a spin wave form. At J>0, A(\vec{k},\omega) shows a dispersion relation
following that of hybridized bands. For J < J_c this feature is supplemented by
shadows thus pointing to a coexistence of Kondo screening and magnetism. For J
< 0 A(\vec{k},\omega) is similar to that of non-interacting electrons in a
staggered magnetic field. Spin, T_S, and charge, T_C, scales are defined. For
weak to intermediate couplings, T_S marks the onset of antiferromagnetic
fluctuations and follows a J^2 law. At strong couplings T_S scales as J. T_C
scales as J both at weak and strong couplings. At and slightly below T_C we
observe i) a rise in the resistivity as a function of decreasing temperature,
ii) a dip in the integrated density of states at the Fermi energy and iii) the
occurrence of hybridized bands in A(k,\omega). It is shown that in the weak
coupling limit, the charge gap of order J is of magnetic origin. The specific
heat shows a two peak structure, the low temperature peak being of magnetic
origin. Our results are compared to various mean-field theories.Comment: 30 pages, 24 figure
Phase Diagram of the Two-Channel Kondo Lattice
The phase diagram of the two-channel Kondo lattice model is examined with a
Quantum Monte Carlo simulation in the limit of infinite dimensions.
Commensurate (and incommensurate) antiferromagnetic and superconducting states
are found. The antiferromagnetic transition is very weak and continuous;
whereas the superconducting transition is discontinuous to an odd-frequency
channel-singlet and spin-singlet pairing state.Comment: 5 pages, LaTeX and 4 PS figures (see also cond-mat/9609146 and
cond-mat/9605109
Universal Conductivity in the Two dimensional Boson Hubbard Model
We use Quantum Monte Carlo to evaluate the conductivity of the
2--dimensional disordered boson Hubbard model at the superfluid-bose glass
phase boundary. At the critical point for particle density , we find
, where from
a finite size scaling analysis of the superfluid density. We obtain
from a direct calculation of the
current--current correlation function. Simulations at the critical points for
other particle densities, and , give similar values for
. We discuss possible origins of the difference in this value from that
recently obtained by other numerical approaches.Comment: 20 pages, figures available upon request. Tex with jnl3.tex and
reforder.tex macros. cond-mat/yymmnn
Molecular dynamics study of the fragmentation of silicon doped fullerenes
Tight binding molecular dynamics simulations, with a non orthogonal basis
set, are performed to study the fragmentation of carbon fullerenes doped with
up to six silicon atoms. Both substitutional and adsorbed cases are considered.
The fragmentation process is simulated starting from the equilibrium
configuration in each case and imposing a high initial temperature to the
atoms. Kinetic energy quickly converts into potential energy, so that the
system oscillates for some picoseconds and eventually breaks up. The most
probable first event for substituted fullerenes is the ejection of a C2
molecule, another very frequent event being that one Si atom goes to an
adsorbed position. Adsorbed Si clusters tend to desorb as a whole when they
have four or more atoms, while the smaller ones tend to dissociate and
sometimes interchange positions with the C atoms. These results are compared
with experimental information from mass abundance spectroscopy and the products
of photofragmentation.Comment: Seven two-column pages, six postscript figures. To be published in
Physical Review
Fine Structure and Fractional Aharonov-Bohm Effect
We find a fine structure in the Aharonov-Bohm effect, characterized by the
appearence of a new type of periodic oscillations having smaller fractional
period and an amplitude, which may compare with the amplitude of the
conventional Aharonov-Bohm effect. Specifically, at low density or strong
coupling on a Hubbard ring can coexist along with the conventional Aaronov-Bohm
oscillations with the period equal to an integer, measured in units of the
elementary flux quantum, two additional oscillations with periods and
. The integers and are the particles number and the number of
down-spin particles, respectively. {}From a solution of the Bethe ansatz
equations for electrons located on a ring in a magnetic field we show that
the fine structure is due to electron-electron and Zeeman interactions. Our
results are valid in the dilute density limit and for an arbitrary value of the
Hubbard repulsion Comment: 40 pages (Latex,Revtex) 12 figures by request, in Technical Reports
of ISSP , Ser. A, No.2836 (1994
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