3,247 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-$\sigma$-model approach.Comment: 7 pages, no figures; new references added; extended discussion on
vertex structure. To appear in Europhysics Letter

### 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

### 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

### 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.

### Two-dimensional quantum spin-1/2 Heisenberg model with competing interactions

We study the quantum spin-1/2 Heisenberg model in two dimensions, interacting
through a nearest-neighbor antiferromagnetic exchange ($J$) and a ferromagnetic
dipolar-like interaction ($J_d$), using double-time Green's function, decoupled
within the random phase approximation (RPA). We obtain the dependence of $k_B
T_c/J_d$ as a function of frustration parameter $\delta$, where $T_c$ is the
ferromagnetic (F) transition temperature and $\delta$ is the ratio between the
strengths of the exchange and dipolar interaction (i.e., $\delta = J/J_d$). The
transition temperature between the F and paramagnetic phases decreases with
$\delta$, as expected, but goes to zero at a finite value of this parameter,
namely $\delta = \delta_c = \pi /8$. At T=0 (quantum phase transition), we
analyze the critical parameter $\delta_c(p)$ for the general case of an
exchange interaction in the form $J_{ij}=J_d/r_{ij}^{p}$, where ferromagnetic
and antiferromagnetic phases are present.Comment: 4 pages, 1 figur

### Dipolar ground state of planar spins on triangular lattices

An infinite triangular lattice of classical dipolar spins is usually
considered to have a ferromagnetic ground state. We examine the validity of
this statement for finite lattices and in the limit of large lattices. We find
that the ground state of rectangular arrays is strongly dependent on size and
aspect ratio. Three results emerge that are significant for understanding the
ground state properties: i) formation of domain walls is energetically favored
for aspect ratios below a critical valu e; ii) the vortex state is always
energetically favored in the thermodynamic limit of an infinite number of
spins, but nevertheless such a configuration may not be observed even in very
large lattices if the aspect ratio is large; iii) finite range approximations
to actual dipole sums may not provide the correct ground sta te configuration
because the ferromagnetic state is linearly unstable and the domain wall energy
is negative for any finite range cutoff.Comment: Several short parts have been rewritten. Accepted for publication as
a Rapid Communication in Phys. Rev.

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