3,451 research outputs found
Mean-field theories for disordered electrons: Diffusion pole and Anderson localization
We discuss conditions to be put on mean-field-like theories to be able to
describe fundamental physical phenomena in disordered electron systems. In
particular, we investigate options for a consistent mean-field theory of
electron localization and for a reliable description of transport properties.
We argue that a mean-field theory for the Anderson localization transition must
be electron-hole symmetric and self-consistent at the two-particle (vertex)
level. We show that such a theory with local equations can be derived from the
asymptotic limit to high spatial dimensions. The weight of the diffusion pole,
i. e., the number of diffusive states at the Fermi energy, in this mean-field
theory decreases with the increasing disorder strength and vanishes in the
localized phase. Consequences of the disclosed behavior for our understanding
of vanishing of electron diffusion are discussed.Comment: REVTeX4, 11 pages, no figure
Low-frequency noise in tunneling through a single spin
We propose measurements of low-frequency noise in the tunneling current
through a single molecule with a spin as an experimental probe for identifying
a mechanism of the spin-dependent tunneling. A specific tail near the zero
frequency in the noise spectrum is predicted; the amplitude and the width of
being of the same order of magnitude as the recently reported peak in the noise
spectrum at the spin Larmor frequency. The ratio of the spectrum amplitudes at
zero- and Larmor frequencies is shown to be a convenient tool for testing
theoretical predictions.Comment: 4 pages, 3 figures. In the replaced version some mistakes are fixe
PROBLEMS OF THE HIGHER HARMONIOUS IN MODERN DISTRIBUTIVE NETWORKS, POSSIBILITY OF THE DEVELOPED DEVICE OF HIGHER HARMONIOUS REGISTRATION
Dyson-Maleev representation of nonlinear sigma-models
For nonlinear sigma-models in the unitary symmetry class, the non-linear
target space can be parameterized with cubic polynomials. This choice of
coordinates has been known previously as the Dyson-Maleev parameterization for
spin systems, and we show that it can be applied to a wide range of
sigma-models. The practical use of this parameterization includes
simplification of diagrammatic calculations (in perturbative methods) and of
algebraic manipulations (in non-perturbative approaches). We illustrate the use
and specific issues of the Dyson-Maleev parameterization with three examples:
the Keldysh sigma-model for time-dependent random Hamiltonians, the
supersymmetric sigma-model for random matrices, and the supersymmetric
transfer-matrix technique for quasi-one-dimensional disordered wires. We
demonstrate that nonlinear sigma-models of unitary-like symmetry classes C and
B/D also admit the Dyson-Maleev parameterization.Comment: 16 pages, 1 figur
Quantum criticality of dipolar spin chains
We show that a chain of Heisenberg spins interacting with long-range dipolar
forces in a magnetic field h perpendicular to the chain exhibits a quantum
critical point belonging to the two-dimensional Ising universality class.
Within linear spin-wave theory the magnon dispersion for small momenta k is
[Delta^2 + v_k^2 k^2]^{1/2}, where Delta^2 \propto |h - h_c| and v_k^2 \propto
|ln k|. For fields close to h_c linear spin-wave theory breaks down and we
investigate the system using density-matrix and functional renormalization
group methods. The Ginzburg regime where non-Gaussian fluctuations are
important is found to be rather narrow on the ordered side of the transition,
and very broad on the disordered side.Comment: 6 pages, 5 figure
Nonequilibrium orbital magnetization of strongly localized electrons
The magnetic response of strongly localized electrons to a time-dependent
vector potential is considered. The orbital magnetic moment of the system, away
from steady-state conditions, is obtained. The expression involves the
tunneling and phonon-assisted hopping currents between localized states. The
frequency and temperature dependence of the orbital magnetization is analyzed
as function of the admittances connecting localized levels. It is shown that
quantum interference of the localized wave functions contributes to the moment
a term which follows adiabatically the time-dependent perturbation.Comment: RevTeX 3.
Spin-wave interaction in two-dimensional ferromagnets with dipolar forces
We discuss the spin-wave interaction in two-dimensional (2D) Heisenberg
ferromagnet (FM) with dipolar forces at using 1/S expansion. A
comprehensive analysis is carried out of the first 1/S corrections to the
spin-wave spectrum. In particular, similar to 3D FM discussed in our previous
paper A.V. Syromyatnikov, PRB {\bf 74}, 014435 (2006), we obtain that the
spin-wave interaction leads to the {\it gap} in the spectrum
renormalizing greatly the bare gapless spectrum at small momenta .
Expressions for the spin-wave damping are derived
self-consistently and it is concluded that magnons are well-defined
quasi-particles in both quantum and classical 2D FMs at small . We observe
thermal enhancement of both and at small momenta. In particular, a peak appears in and
at small and at any given direction of
. If the height of the peak in is not larger than a value proportional to , where is the
spin-wave stiffness. In the case of large spins the peak in
cannot be greater than that of the classical
2D FM found at which height is small only {\it numerically}: for the simple square lattice. Frustrating
next-nearest-neighbor exchange coupling increases in classical 2D FM only slightly. We find expressions for spin Green's
functions and the magnetization. The latter differs from the well-known result
by S.V. Maleev, Sov. Phys. JETP {\bf 43}, 1240 (1976). The effect of the
exchange anisotropy is also discussed briefly
Quantum Field Theory for the Three-Body Constrained Lattice Bose Gas -- Part I: Formal Developments
We develop a quantum field theoretical framework to analytically study the
three-body constrained Bose-Hubbard model beyond mean field and non-interacting
spin wave approximations. It is based on an exact mapping of the constrained
model to a theory with two coupled bosonic degrees of freedom with polynomial
interactions, which have a natural interpretation as single particles and
two-particle states. The procedure can be seen as a proper quantization of the
Gutzwiller mean field theory. The theory is conveniently evaluated in the
framework of the quantum effective action, for which the usual symmetry
principles are now supplemented with a ``constraint principle'' operative on
short distances. We test the theory via investigation of scattering properties
of few particles in the limit of vanishing density, and we address the
complementary problem in the limit of maximum filling, where the low lying
excitations are holes and di-holes on top of the constraint induced insulator.
This is the first of a sequence of two papers. The application of the formalism
to the many-body problem, which can be realized with atoms in optical lattices
with strong three-body loss, is performed in a related work [14].Comment: 21 pages, 5 figure
Spin Diffusion in Double-Exchange Manganites
The theoretical study of spin diffusion in double-exchange magnets by means
of dynamical mean-field theory is presented. We demonstrate that the
spin-diffusion coefficient becomes independent of the Hund's coupling JH in the
range of parameters JH*S >> W >> T, W being the bandwidth, relevant to colossal
magnetoresistive manganites in the metallic part of their phase diagram. Our
study reveals a close correspondence as well as some counterintuitive
differences between the results on Bethe and hypercubic lattices. Our results
are in accord with neutron scattering data and with previous theoretical work
for high temperatures.Comment: 4.0 pages, 3 figures, RevTeX 4, replaced with the published versio
Neutron diffraction, magnetization and ESR studies of pseudocubic Nd(0.75)Ba(0.25)MnO3 and its unusual critical behavior above Tc
Results of structural neutron diffraction study, magnetization and ESR
measure-ments are presented for insulating Nd0.75Ba0.25MnO3, Tc = 129 K. The
crystal structure is refined in the range 4.2-300 K. The compound is found to
exhibit the Jahn-Teller (JT) transition at 250 K. The field cooled (FC)
magnetization data are in a reasonable agreement with the predictions for a 3D
isotropic ferromagnet above Tc. However, these measurements reveal a difference
between the FC and zero FC data in the paramagnetic region. ESR results are
also in a correspondence with behavior of a cubic ferromagnet above T* = 143 K.
It is shown that an anisotropic exchange coupling of the Mn and Nd magnetic
moments can give a substantial contribution in ESR linewidth masking its
critical enhan-cement. The different temperature treatments of the sample
reveal a temperature hysteresis of the ESR spectra below T* indicating an
anomalous response in the paramagnetic region. The study of phase transition in
this manganite suggests change in its character from the second to first order
at T*. The conventional free energy including the magnetization and magnetic
field is not found to describe the first order transition. This suggests that
the charge, orbital and JT phonon degrees of freedom, in addition to
magnetization, may be the critical variables, the unusual character of the
transition being determined by their coupling. Unconventional critical behavior
is attributed to orbital liquid metallic phase that coexists with the initial
orbital ordered phase below T*.Comment: 18 pages, 5 figures, submitted to Phys. Rev.
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