1,226 research outputs found
Zero-temperature magnetism in the periodic Anderson model in the limit of large dimensions
We study the magnetism in the periodic Anderson model in the limit of large
dimensions by mapping the lattice problem into an equivalent local impurity
self-consistent model. Through a recently introduced algorithm based on the
exact diagonalization of an effective cluster hamiltonian, we obtain solutions
with and without magnetic order in the half-filled case. We find the exact
AFM-PM phase boundary which is shown to be of order and obeys
We calculate the local staggered moments and the
density of states to gain insights on the behavior of the AFM state as it
evolves from itinerant to a local-moment magnetic regimeComment: 9 pages + 9 figures, to appear in Phys. Rev. B, 1 Sept. 1995 issu
On insertion-deletion systems over relational words
We introduce a new notion of a relational word as a finite totally ordered
set of positions endowed with three binary relations that describe which
positions are labeled by equal data, by unequal data and those having an
undefined relation between their labels. We define the operations of insertion
and deletion on relational words generalizing corresponding operations on
strings. We prove that the transitive and reflexive closure of these operations
has a decidable membership problem for the case of short insertion-deletion
rules (of size two/three and three/two). At the same time, we show that in the
general case such systems can produce a coding of any recursively enumerable
language leading to undecidabilty of reachability questions.Comment: 24 pages, 8 figure
Transfer of Spectral Weight in Spectroscopies of Correlated Electron Systems
We study the transfer of spectral weight in the photoemission and optical
spectra of strongly correlated electron systems. Within the LISA, that becomes
exact in the limit of large lattice coordination, we consider and compare two
models of correlated electrons, the Hubbard model and the periodic Anderson
model. The results are discussed in regard of recent experiments. In the
Hubbard model, we predict an anomalous enhancement optical spectral weight as a
function of temperature in the correlated metallic state which is in
qualitative agreement with optical measurements in . We argue that
anomalies observed in the spectroscopy of the metal are connected to the
proximity to a crossover region in the phase diagram of the model. In the
insulating phase, we obtain an excellent agreement with the experimental data
and present a detailed discussion on the role of magnetic frustration by
studying the resolved single particle spectra. The results for the periodic
Anderson model are discussed in connection to recent experimental data of the
Kondo insulators and . The model can successfully explain
the different energy scales that are associated to the thermal filling of the
optical gap, which we also relate to corresponding changes in the density of
states. The temperature dependence of the optical sum rule is obtained and its
relevance for the interpretation of the experimental data discussed. Finally,
we argue that the large scattering rate measured in Kondo insulators cannot be
described by the periodic Anderson model.Comment: 19 pages + 29 figures. Submitted to PR
Mott transition at large orbital degeneracy: dynamical mean-field theory
We study analytically the Mott transition of the N-orbital Hubbard model
using dynamical mean-field theory and a low-energy projection onto an effective
Kondo model. It is demonstrated that the critical interaction at which the
insulator appears (Uc1) and the one at which the metal becomes unstable (Uc2)
have different dependence on the number of orbitals as the latter becomes
large: Uc1 ~ \sqrt{N} while Uc2 ~ N. An exact analytical determination of the
critical coupling Uc2/N is obtained in the large-N limit. The metallic solution
close to this critical coupling has many similarities at low-energy with the
results of slave boson approximations, to which a comparison is made. We also
discuss how the critical temperature associated with the Mott critical endpoint
depends on the number of orbitals.Comment: 13 pages. Minor changes in V
Optical Conductivity in Mott-Hubbard Systems
We study the transfer of spectral weight in the optical spectra of a strongly
correlated electron system as a function of temperature and interaction
strength. Within a dynamical mean field theory of the Hubbard model that
becomes exact in the limit of large lattice coordination, we predict an
anomalous enhancement of spectral weight as a function of temperature in the
correlated metallic state and report on experimental measurements which agree
with this prediction in . We argue that the optical conductivity
anomalies in the metal are connected to the proximity to a crossover region in
the phase diagram of the model.Comment: 12 pages and 4 figures, to appear in Phys. Rev. Lett., v 75, p 105
(1995
Quantum and thermal fluctuations in the SU(N) Heisenberg spin-glass model near the quantum critical point
We solve for the SU(N) Heisenberg spin-glass in the limit of large N focusing
on small S and T. We study the effect of quantum and thermal fluctuations in
the frequency dependent response function and observed interesting transfers of
spectral weight. We compute the T-dependence of the order parameter and the
specific heat and find an unusual T^2 behavior for the latter at low
temperatures in the spin-glass phase. We find a remarkable qualitative
agreement with various experiments on the quantum frustrated magnet
SrCr_{9p}Ga_{12-9p}O_{19}.Comment: 5 pages, 4 figures, submitted to Phys. Rev. Let
Melting transition of an Ising glass driven by magnetic field
The quantum critical behavior of the Ising glass in a magnetic field is
investigated. We focus on the spin glass to paramagnet transition of the
transverse degrees of freedom in the presence of finite longitudinal field. We
use two complementary techniques, the Landau theory close to the T=0 transition
and the exact diagonalization method for finite systems. This allows us to
estimate the size of the critical region and characterize various crossover
regimes. An unexpectedly small energy scale on the disordered side of the
critical line is found, and its possible relevance to experiments on metallic
glasses is briefly discussed.Comment: 4 pages, 3 figure
Dynamical Mean Field Theory of the Antiferromagnetic Metal to Antiferromagnetic Insulator Transition
We study the antiferromagnetic metal to antiferromagnetic insulator using
dynamical mean field theory and exact diagonalization methods. We find two
qualitatively different behaviors depending on the degree of magnetic
correlations. For strong correlations combined with magnetic frustration, the
transition can be described in terms of a renormalized slater theory, with a
continuous gap closure driven by the magnetism but strongly renormalized by
correlations. For weak magnetic correlations, the transition is weakly first
order.Comment: 4 pages, uses epsfig,4 figures,notational errors rectifie
The infinite-range quantum random Heisenberg magnet
We study with exact diagonalization techniques the Heisenberg model for a
system of SU(2) spins with S=1/2 and random infinite-range exchange
interactions. We calculate the critical temperature T_g for the spin-glass to
paramagnetic transition. We obtain T_g ~ 0.13, in good agreement with previous
quantum Monte Carlo and analytical estimates. We provide a detailed picture for
the different kind of excitations which intervene in the dynamical response
chi''(w,T) at T=0 and analyze their evolution as T increases. We also calculate
the specific heat Cv(T). We find that it displays a smooth maximum at TM ~
0.25, in good qualitative agreement with experiments. We argue that the fact
that TM>Tg is due to a quantum disorder effect.Comment: 17 pages, 14 figure
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