7,425 research outputs found
Filling dependence of a new type of charge ordered liquid on a triangular lattice system
We study the recently reported characteristic gapless charge ordered state in
a spinless fermion system on a triangular lattice under strong inter-site
Coulomb interactions. In this state the charges are spontaneously divided into
solid and liquid component, and the former solid part aligns in a Wigner
crystal manner while the latter moves among them like a pinball. We show that
such charge ordered liquid is stable over a wide range of filling, ,
and examine its filling dependent nature.Comment: 3 pages 3 figure
Comment on ``Spin Dependent Hopping and Colossal Negative Magnetoresistance in Epitaxial Films in Fields up to 50 T''
Recently Wagner et al. [Phys. Rev. Lett. Vol. 81, P. 3980 (1998)] proposed
that Mott's original model be modified to incorporate a hopping barrier which
depends on the misorientation between the spins of electrons at the initial and
the final states in an elementary process. They further claimed that using the
model they can explain the observed scaling behavior--
negative-magnetoresistivity scaling proportional to the Brillouin function
in the ferromagnetic state and to in the paramagnetic
state. In this comment we argue that the modification needed for Mott's
original model is different from that proposed by Wagner et al. and further
show that our picture will successfully explain the observed scaling in the two
regimes.Comment: 1 pag
A Monte Carlo Method for Fermion Systems Coupled with Classical Degrees of Freedom
A new Monte Carlo method is proposed for fermion systems interacting with
classical degrees of freedom. To obtain a weight for each Monte Carlo sample
with a fixed configuration of classical variables, the moment expansion of the
density of states by Chebyshev polynomials is applied instead of the direct
diagonalization of the fermion Hamiltonian. This reduces a cpu time to scale as
compared to for the
diagonalization in the conventional technique; is the dimension
of the Hamiltonian. Another advantage of this method is that parallel
computation with high efficiency is possible. These significantly save total
cpu times of Monte Carlo calculations because the calculation of a Monte Carlo
weight is the bottleneck part. The method is applied to the double-exchange
model as an example. The benchmark results show that it is possible to make a
systematic investigation using a system-size scaling even in three dimensions
within a realistic cpu timescale.Comment: 6 pages including 4 figure
Impurity Effect on Ferromagnetic Transition in Double-Exchange Systems
Effect of randomness in the double-exchange model is studied. Large
fluctuations and spatial random distribution of impurities are taken into
account in an essentially exact manner by using the Monte Carlo calculation.
The randomness suppresses the ferromagnetism by reducing the coherence of
itinerant electrons. The suppression is significant in the critical region
where the fluctuations are dominant. Temperature dependences of the
magnetization are estimated for finite-size clusters. A characteristic
temperature for phase transition is estimated from the inflection
point, which is expected to give a good approximation for the critical
temperature in the thermodynamic limit. Our results suggest that the
ferromagnetism becomes unstable more rapidly than predicted in the previous
theoretical results by the coherent-potential approximation.Comment: 7 pages including 4 figures, submitted to Proc. ISSP
Photo-induced insulator-metal transition of a spin-electron coupled system
The photo-induced metal-insulator transition is studied by the numerical
simulation of real-time quantum dynamics of a double-exchange model. The
spatial and temporal evolutions of the system during the transition have been
revealed including (i) the threshold behavior with respect to the intensity and
energy of light, (ii) multiplication of particle-hole (p-h) pairs by a p-h pair
of high energy, and (iii) the space-time pattern formation such as (a) the
stripe controlled by the polarization of light, (b) coexistence of metallic and
insulating domains, and (c) dynamical spontaneous symmetry-breaking associated
with the spin spiral formation imposed by the conservation of total spin for
small energy-dissipation rates
Universality Class of Ferromagnetic Transition in Three-Dimensional Double-Exchange System - O(N) Monte Carlo Study -
Curie temperature and exponents are studied for the three-dimensional
double-exchange model. Applying the O(N) Monte Carlo algorithm, we perform
systematic finite-size scaling analyses on the data up to sites. The
obtained values of the critical exponents are consistent with those of the
Heisenberg universality class, and clearly distinct from the mean-field values.Comment: 3 pages including 2 figure
Competing Orders and Disorder-induced Insulator to Metal Transition in Manganites
Effects of disorder on the two competing phases, i.e., the ferromagnetic
metal and the commensurate charge/lattice ordered insulator, are studied by
Monte Carlo simulation. The disorder suppresses the charge/lattice ordering
more strongly than the ferromagnetic order, driving the commensurate insulator
to the ferromagnetic metal near the phase boundary in the pure case. Above the
ferromagnetic transition temperature, on the contrary, the disorder makes the
system more insulating, which might cause an enhanced colossal
magnetoresistance as observed in the half-doped or Cr-substituted manganites.
No indication of the percolation or the cluster formation is found, and there
remain the charge/lattice fluctuations instead which are enhanced toward the
transition temperature.Comment: 5 pages including 4 figure
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