3,836 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
Spin-wave interactions in quantum antiferromagnets
We study spin-wave interactions in quantum antiferromagnets by expressing the
usual magnon annihilation and creation operators in terms of Hermitian field
operators representing transverse staggered and ferromagnetic spin
fluctuations. In this parameterization, which was anticipated by Anderson in
1952, the two-body interaction vertex between staggered spin fluctuations
vanishes at long wavelengths. We derive a new effective action for the
staggered fluctuations only by tracing out the ferromagnetic fluctuations. To
one loop order, the renormalization group flow agrees with the
nonlinear--model approach.Comment: 7 pages, no figures; new references added; extended discussion on
vertex structure. To appear in Europhysics Letter
Magnetic properties of a metal-organic antiferromagnet on a distorted honeycomb lattice
For temperatures T well above the ordering temperature T*=3.0+-0.2K the
magnetic properties of the metal-organic material Mn[C10H6(OH)(COO)]2x2H20
built from Mn^2+ ions and 3-hydroxy-2-naphthoic anions can be described by a
S=5/2 quantum antiferromagnet on a distorted honeycomb lattice with two
different nearest neighbor exchange couplings J2 \approx 2J1 \approx 1.8K.
Measurements of the magnetization M(H,T) as a function of a uniform external
field H and of the uniform zero field susceptibility \chi(T) are explained
within the framework of a modified spin-wave approach which takes into account
the absence of a spontaneous staggered magnetization at finite temperatures.Comment: 11 pages, 11 figures; more thorough discussion of the dependence of
the correlation length on the uniform magnetic field adde
Symplectic N and time reversal in frustrated magnetism
Identifying the time reversal symmetry of spins as a symplectic symmetry, we
develop a large N approximation for quantum magnetism that embraces both
antiferromagnetism and ferromagnetism. In SU(N), N>2, not all spins invert
under time reversal, so we have introduced a new large N treatment which builds
interactions exclusively out of the symplectic subgroup [SP(N)] of time
reversing spins, a more stringent condition than the symplectic symmetry of
previous SP(N) large N treatments. As a result, we obtain a mean field theory
that incorporates the energy cost of frustrated bonds. When applied to the
frustrated square lattice, the ferromagnetic bonds restore the frustration
dependence of the critical spin in the Neel phase, and recover the correct
frustration dependence of the finite temperature Ising transition.Comment: added reference
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
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.
Anomalous Larmour Frequency Dependence of Proton Spin-Lattice Relaxation Time (T) in the Ferroelectric Glycine Phosphite
We report here the results of H NMR spin-lattice relaxation time (T)
studies in Glycine phosphite which is a ferroelectric below 224 K. The
experiments have been carried out in the temperature range from 200 K to 419 K
and at two Larmour frequencies of 11.40MHz and 23.56 MHz. We have noticed a
Larmour frequency dependence on the high temperature side of the T minimum
>. A model is proposed based on the BPP theory to explain the observation.Comment: 4 pages, 2 ps figs included, revtex forma
On the significance of quantum effects and interactions for the apparent universality of Bloch laws for M_s(T)
The apparent universality of Bloch's T^{3/2}-law for the temperature
dependence of the spontaneous magnetization, and of generalizations thereof, is
considered. It is argued that in the derivation one should not only consider
the exchange interaction between the spins, but also the other interactions
between them, leading to elliptical spin precession and deviations from the
parabolic dispersion of magnons. Also interaction effects are important to
explain the apparent universality of generalized Bloch law exponents e_B,
defined by M_s(T)= M_s(0)-const. x T^{e_B}, valid in a wide temperature range
T_1 < T < T_2, and for dimensionalities d = 1, 2, and 3. The above-mentioned
temperature range, the 'Bloch range', lies above the quantum range, where
magnetic long-range order (e.g. in d=2 dimensions) is nontrivially enforced by
the additional interactions, but below the thermal critical region, where
universal 'anomalous scaling dimensions' apply. In contrast, for the Bloch
temperature region, the universality is only apparent, i.e. a
crossover-phenomenon, and simple scaling considerations with 'normal
dimensions' apply.
However, due to interactions, the Bloch exponent e_B depends not only on the
dimensionality d of the system, but also on the spin quantum number s (mod
(1/2)) of the system, i.e. for given d the Bloch exponent e_B is different for
half-integer s and for integer s.Comment: LATEX, 27 pages (including 5 eps-figures); accepted by JMM
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
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