267 research outputs found
New algorithm and results for the three-dimensional random field Ising Model
The random field Ising model with Gaussian disorder is studied using a new
Monte Carlo algorithm. The algorithm combines the advantanges of the replica
exchange method and the two-replica cluster method and is much more efficient
than the Metropolis algorithm for some disorder realizations. Three-dimensional
sytems of size are studied. Each realization of disorder is simulated at
a value of temperature and uniform field that is adjusted to the phase
transition region for that disorder realization. Energy and magnetization
distributions show large variations from one realization of disorder to
another. For some realizations of disorder there are three well separated peaks
in the magnetization distribution and two well separated peaks in the energy
distribution suggesting a first-order transition.Comment: 24 pages, 23 figure
Magnetic phase diagram of the Hubbard model
The competition between commensurate and incommensurate spin-density-wave
phases in the infinite-dimensional single-band Hubbard model is examined with
quantum Monte Carlo simulation and strong and weak coupling approximations.
Quantum fluctuations modify the weak-coupling phase diagram by factors of order
unity and produce remarkable agreement with the quantum Monte Carlo data, but
strong-coupling theories (that map onto effective Falicov-Kimball models)
display pathological behavior. The single-band model can be used to describe
much of the experimental data in Cr and its dilute alloys with V and Mn.Comment: 12 pages plus 3 uuencoded postscript figures, ReVTe
Excitonic Correlations in the Intermetallic Fe2VAl
The intermetallic compound Fe2VAl looks non-metallic in transport and
strongly metallic in thermodynamic and photoemission data. It has in its band
structure a highly differentiated set of valence and conduction bands leading
to a semimetallic system with a very low density of carriers. The pseudogap
itself is due to interaction of Al states with the d orbitals of Fe and V, but
the resulting carriers have little Al character. The effects of generalized
gradient corrections to the local density band structure as well spin-orbit
coupling are shown to be significant, reducing the carrier density by a factor
of three. Doping of this nonmagnetic compound by 0.5 electrons per cell in a
virtual crystal fashion results in a moment of 0.5 bohr magnetons and destroys
the pseudogap. We assess the tendencies toward formation of an excitonic
condensate and toward an excitonic Wigner crystal, and find both to be
unlikely. We propose a model is which the observed properties result from
excitonic correlations arising from two interpenetrating lattices of
distinctive electrons (e_g on V) and holes (t_2g on Fe) of low density (one
carrier of each sign per 350 formula units).Comment: 8 2-column pages, 8 postscript figure
Spectral functions of the Falicov-Kimball model with electronic ferroelectricity
We calculate the angular resolved photoemission spectrum of the
Falicov-Kimball model with electronic ferroelectricity where - and
-electrons have different hoppings. In mix-valence regimes, the presence of
strong scattering processes between - excitons and a hole, created by
emission of an electron, leads to the formation of pseudospin polarons and
novel electronic structures with bandwidth scaling with that of -
excitons. Especially, in the two-dimensional case, we find that flat regions
exist near the bottom of the quasiparticle band in a wide range of the - and
-level energy difference.Comment: 5 pages, 5 figure
Effects of Pore Walls and Randomness on Phase Transitions in Porous Media
We study spin models within the mean field approximation to elucidate the
topology of the phase diagrams of systems modeling the liquid-vapor transition
and the separation of He--He mixtures in periodic porous media. These
topologies are found to be identical to those of the corresponding random field
and random anisotropy spin systems with a bimodal distribution of the
randomness. Our results suggest that the presence of walls (periodic or
otherwise) are a key factor determining the nature of the phase diagram in
porous media.Comment: REVTeX, 11 eps figures, to appear in Phys. Rev.
Tricritical behaviour in deterministic aperiodic Ising systems
We use a mixed-spin model, with aperiodic ferromagnetic exchange interactions
and crystalline fields, to investigate the effects of deterministic geometric
fluctuations on first-order transitions and tricritical phenomena. The
interactions and the crystal field parameters are distributed according to some
two-letter substitution rules. From a Migdal-Kadanoff real-space
renormalization-group calculation, which turns out to be exact on a suitable
hierarchical lattice, we show that the effects of aperiodicity are
qualitatively similar for tricritical and simple critical behaviour. In
particular, the fixed point associated with tricritical behaviour becomes fully
unstable beyond a certain threshold dimension (which depends on the
aperiodicity), and is replaced by a two-cycle that controls a weakened and
temperature-depressed tricritical singularity.Comment: Formatting improved. 7 pages, 2 figures (included). Journal reference
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Magnetic phases near the Van Hove singularity in s- and d-band Hubbard model
We investigate the magnetic instabilities of the nondegenerate (s-band) and a
degenerate (d-band) Hubbard model in two dimensions using many-body effects due
to the particle-particle diagrams and Hund's rule local correlations. The
density of states and the position of Van Hove singularity change depending on
the value of next-nearest neighbor hopping t'. The Stoner parameter is strongly
reduced in the s-band case, and ferromagnetism survives only if electron
density is small, and the band is almost flat at small momenta due to
next-nearest neighbor hopping. In contrast, for the d-band case the reduction
of the Stoner parameter which follows from particle-particle correlations is
much smaller and ferromagnetism survives to a large extent. Inclusion of local
spin-spin correlations has a limited destabilizing effect on the magnetic
states.Comment: 8 pages, 7 figure
Influence of Hybridization on the Properties of the Spinless Falicov-Kimball Model
Without a hybridization between the localized f- and the conduction (c-)
electron states the spinless Falicov-Kimball model (FKM) is exactly solvable in
the limit of high spatial dimension, as first shown by Brandt and Mielsch. Here
I show that at least for sufficiently small c-f-interaction this exact
inhomogeneous ground state is also obtained in Hartree-Fock approximation. With
hybridization the model is no longer exactly solvable, but the approximation
yields that the inhomogeneous charge-density wave (CDW) ground state remains
stable also for finite hybridization V smaller than a critical hybridization
V_c, above which no inhomogeneous CDW solution but only a homogeneous solution
is obtained. The spinless FKM does not allow for a ''ferroelectric'' ground
state with a spontaneous polarization, i.e. there is no nonvanishing
-expectation value in the limit of vanishing hybridization.Comment: 7 pages, 6 figure
Theory of Electronic Ferroelectricity
We present a theory of the linear and nonlinear optical characteristics of
the insulating phase of the Falicov-Kimball model within the self-consistent
mean-field approximation. The Coulomb attraction between the itinerant
d-electrons and the localized f-holes gives rise to a built-in coherence
between the d and f-states, which breaks the inversion symmetry of the
underlying crystal, leading to: (1) electronic ferroelectricity, (2)
ferroelectric resonance, and (3) a nonvanishing susceptibility for
second-harmonic generation. As experimental tests of such a built-in coherence
in mixed-valent compounds we propose measurements of the static dielectric
constant, the microwave absorption spectrum, and the dynamic second-order
susceptibility.Comment: 15 pages, 5 PostScript figures, submitted to Physical Review
Theory for Spin-Polarized Oscillations in Nonlinear Magneto-Optics due to Quantum Well States
Using an electronic tight-binding theory we calculate the nonlinear
magneto-optical response from an x-Cu/1Fe/Cu(001) film as a function of
frequency and Cu overlayer thickness (x=3 ... 25). We find very strong
spin-polarized quantum well oscillations in the nonlinear magneto-optical Kerr
effect (NOLIMOKE). These are enhanced by the large density of Fe states
close to the Fermi level acting as intermediate states for frequency doubling.
In good agreement with experiment we find two oscillation periods of 6-7 and 11
monolayers the latter being more pronounced.Comment: 12 pages, Revtex, 3 postscript figure
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