224 research outputs found
Studies on the Mating Behavior of the House Fly, Musca Domestica L.
Author Institution: Entomology Research Division, Agric, Res. Serv., U.S.D.A. Gainesville, Fla
A Maximum Entropy Method of Obtaining Thermodynamic Properties from Quantum Monte Carlo Simulations
We describe a novel method to obtain thermodynamic properties of quantum
systems using Baysian Inference -- Maximum Entropy techniques. The method is
applicable to energy values sampled at a discrete set of temperatures from
Quantum Monte Carlo Simulations. The internal energy and the specific heat of
the system are easily obtained as are errorbars on these quantities. The
entropy and the free energy are also obtainable. No assumptions as to the
specific functional form of the energy are made. The use of a priori
information, such as a sum rule on the entropy, is built into the method. As a
non-trivial example of the method, we obtain the specific heat of the
three-dimensional Periodic Anderson Model.Comment: 8 pages, 3 figure
Thermodynamic properties of the one-dimensional Kondo insulators studied by the density matrix renormalization group method
Thermodynamic properties of the one-dimensional Kondo lattice model at
half-filling are studied by the density matrix renormalization group method
applied to the quantum transfer matrix. Spin susceptibility, charge
susceptibility, and specific heat are calculated down to T=0.1t for various
exchange constants. The obtained results clearly show crossover behavior from
the high temperature regime of nearly independent localized spins and
conduction electrons to the low temperature regime where the two degrees of
freedom couple strongly. The low temperature energy scales of the charge and
spin susceptibilities are determined and shown to be equal to the quasiparticle
gap and the spin gap, respectively, for weak exchange couplings.Comment: 4 pages, 3 Postscript figures, REVTeX, submitted to J. Phys. Soc. Jp
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
Magnetic impurities coupled to quantum antiferromagnets in one dimension
Magnetic impurities coupled antiferromagnetically to a one-dimensional
Heisenberg model are studied by numerical diagonalization of chains of finite
clusters. By calculating the binding energy and the correlation function, it is
shown that a local singlet develops around each impurity. This holds true for
systems with a single impurity, with two impurities, and for impurities forming
a lattice. The local character of the singlet is found to be little affected by
the presence of other impurity spins. A small effective interaction is found
between a pair of impurity spins, which oscillates depending on impurity
distances. For impurity lattices, the energy spectrum shows a gap which is
found to be much smaller than the binding energy per impurity if the coupling
constants are small. For larger coupling constants, it increases to the same
order of magnitude as the binding energy, indicating that a local singlet is
broken to create excited states. Impurity lattices with ferromagnetic couplings
are also studied and their connection to the Haldane problem is discussed.Comment: 25 pages, plain TeX, 17 figures available on request, to be publised
in Phys. Rev.
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
Conformal Field Theory Approach to the 2-Impurity Kondo Problem: Comparison with Numerical Renormalization Group Results
Numerical renormalization group and conformal field theory work indicate that
the two impurity Kondo Hamiltonian has a non-Fermi liquid critical point
separating the Kondo-screening phase from the inter-impurity singlet phase when
particle-hole (P-H) symmetry is maintained. We clarify the circumstances under
which this critical point occurs, pointing out that there are two types of P-H
symmetry. Only one of them guarantees the occurance of the critical point. Much
of the previous numerical work was done on models with the other type of P-H
symmetry. We analyse this critical point using the boundary conformal field
theory technique. The finite-size spectrum is presented in detail and compared
with about 50 energy levels obtained using the numerical renormalization group.
Various Green's functions, general renormalization group behaviour, and a
hidden are analysed.Comment: 38 pages, RevTex. 2 new sections clarify the circumstances under
which a model will exhibit the non-trivial critical point (hence potentially
resolving disagreements with other Authors) and explain the hidden SO(7)
symmetry of the model, relating it to an alternative approach of Sire et al.
and Ga
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