559 research outputs found
Disorder effect on the localization/delocalization in incommensurate potential
The interplay between incommensurate (IC) and random potentials is studied in
a two-dimensional symplectic model with the focus on
localization/delocalization problem. With the IC potential only, there appear
wavefunctions localized along the IC wavevector while extended perpendicular to
it. Once the disorder potential is introduced, these turn into two-dimensional
anisotropic metallic states beyond the scale of the elastic mean free path, and
eventually becomes localized in both directions at a critical strength of the
disorder. Implications of these results to the experimental observation of the
IC-induced localization is discussed.Comment: 4 pages, 3 figures (7 files), RevTe
Metal-insulator transition in half-filling two-orbital Hubbard model on triangular lattice
We have investigated the half-filling two-orbital Hubbard model on a
triangular lattice by means of the dynamical mean-field theory (DMFT). The
densities of states and optical conductivity clearly show the occurence of
metal-insulating transition (MIT) at U, U=18.2, 16.8, 6.12 and 5.85
for J=0, 0.01U, U/4 and U/3, respectively. The distinct continuities of double
occupation of electrons, local square moments and local susceptibility of the
charge, the spin and the orbital at J > 0 suggest that the MIT is the
first-order; however at J=0, the MIT is the second-order in the half-filling
two-orbital Hubbard model on triangular lattices. We attribute the first-order
nature of the MIT to the low symmetry of the systems with finite Hund's
coupling J.Comment: 5 figures,13 pages, published versio
Spin Hall effect of conserved current: Conditions for a nonzero spin Hall current
We study the spin Hall effect taking into account the impurity scattering
effect as general as possible with the focus on the definition of the spin
current. The conserved bulk spin current (Shi et al. [Phys. Rev. Lett. 96,
076604 (2006)]) satisfying the continuity equation of spin is considered in
addition to the conventional one defined by the symmetric product of the spin
and velocity operators. Conditions for non-zero spin Hall current are
clarified. In particular, it is found that (i) the spin Hall current is
non-zero in the Rashba model with a finite-range impurity potential, and (ii)
the spin Hall current vanishes in the cubic Rashba model with a
-function impurity potential.Comment: 5 pages, minor change from the previous versio
Effective mass staircase and the Fermi liquid parameters for the fractional quantum Hall composite fermions
Effective mass of the composite fermion in the fractional quantum Hall
system, which is of purely interaction originated, is shown, from a numerical
study, to exhibit a curious nonmonotonic behavior with a staircase correlated
with the number (=2,4,...) of attached flux quanta. This is surprising since
the usual composite-fermion picture predicts a smooth behavior. On top of that,
significant interactions are shown to exist between composite fermions, where
the excitation spectrum is accurately reproduced in terms of Landau's Fermi
liquid picture with negative (i.e., Hund's type) orbital and spin exchange
interactions.Comment: 4 pages, 3 figures, REVTe
Exchange interactions and magnetic properties of the layered vanadates CaV2O5, MgV2O5, CaV3O7 and CaV4O9
We have performed ab-initio calculations of exchange couplings in the layered
vanadates CaV2O5, MgV2O5, CaV3O7 and CaV4O9. The uniform susceptibility of the
Heisenberg model with these exchange couplings is then calculated by quantum
Monte Carlo method; it agrees well with the experimental measurements. Based on
our results we naturally explain the unusual magnetic properties of these
materials, especially the huge difference in spin gap between CaV2O5 and
MgV2O5, the unusual long range order in CaV3O7 and the "plaquette resonating
valence bond (RVB)" spin gap in CaV4O9
Gauge Theory of Composite Fermions: Particle-Flux Separation in Quantum Hall Systems
Fractionalization phenomenon of electrons in quantum Hall states is studied
in terms of U(1) gauge theory. We focus on the Chern-Simons(CS) fermion
description of the quantum Hall effect(QHE) at the filling factor
, and show that the successful composite-fermions(CF) theory
of Jain acquires a solid theoretical basis, which we call particle-flux
separation(PFS). PFS can be studied efficiently by a gauge theory and
characterized as a deconfinement phenomenon in the corresponding gauge
dynamics. The PFS takes place at low temperatures, , where
each electron or CS fermion splinters off into two quasiparticles, a fermionic
chargeon and a bosonic fluxon. The chargeon is nothing but Jain's CF, and the
fluxon carries units of CS fluxes. At sufficiently low temperatures , fluxons Bose-condense uniformly and (partly)
cancel the external magnetic field, producing the correlation holes. This
partial cancellation validates the mean-field theory in Jain's CF approach.
FQHE takes place at as a joint effect of (i) integer QHE of
chargeons under the residual field and (ii) Bose condensation of
fluxons. We calculate the phase-transition temperature and the CF
mass. PFS is a counterpart of the charge-spin separation in the t-J model of
high- cuprates in which each electron dissociates into holon and
spinon. Quasiexcitations and resistivity in the PFS state are also studied. The
resistivity is just the sum of contributions of chargeons and fluxons, and
changes its behavior at , reflecting the change of
quasiparticles from chargeons and fluxons at to electrons at
.Comment: 18 pages, 7 figure
Magneto-optics induced by the spin chirality in itinerant ferromagnet NdMoO
It is demonstrated both theoretically and experimentally that the spin
chirality associated with a noncoplanar spin configuration produces a
magneto-optical effect. Numerical study of the two-band Hubbard model on a
triangle cluster shows that the optical Hall conductivity
is proportional to the spin chirality. The detailed comparative experiments on
pyrochlore-type molybdates MoO with Nd (Ising-like moments)
and Gd (Heisenberg-like ones) clearly distinguishes the two mechanisms,
i.e., spin chirality and spin-orbit interactions. It is concluded that for
=Nd, is dominated by the spin chirality for the dc
() and the incoherent intraband optical transitions between
Mo atoms.Comment: 4 pages, 5 figures. submitted to Phys. Rev.
Effective gauge field theory of the t-J model in the charge-spin separated state and its transport properties
We study the slave-boson t-J model of cuprates with high superconducting
transition temperatures, and derive its low-energy effective field theory for
the charge-spin separated state in a self-consistent manner. The phase degrees
of freedom of the mean field for hoppings of holons and spinons can be regarded
as a U(1) gauge field, . The charge-spin separation occurs below certain
temperature, , as a deconfinement phenomenon of the dynamics of
. Below certain temperature , the spin-gap
phase develops as the Higgs phase of the gauge-field dynamics, and
acquires a mass . The effective field theory near takes the
form of Ginzburg-Landau theory of a complex scalar field coupled with
, where represents d-wave pairings of spinons. Three
dimensionality of the system is crucial to realize a phase transition at
.
By using this field theory, we calculate the dc resistivity . At , is proportional to . At , it deviates
downward from the -linear behavior as . When the system is near (but not) two dimensional, due to the compactness
of the phase of the field , the exponent deviates from its
mean-field value 1/2 and becomes a nonuniversal quantity which depends on
temperature and doping. This significantly improves the comparison with the
experimental data
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