88,828 research outputs found
An Analytical Study of Coupled Two-State Stochastic Resonators
The two-state model of stochastic resonance is extended to a chain of coupled
two-state elements governed by the dynamics of Glauber's stochastic Ising
model. Appropriate assumptions on the model parameters turn the chain into a
prototype system of coupled stochastic resonators. In a weak-signal limit
analytical expressions are derived for the spectral power amplification and the
signal-to-noise ratio of a two-state element embedded into the chain. The
effect of the coupling between the elements on both quantities is analysed and
array-enhanced stochastic resonance is established for pure as well as noisy
periodic signals. The coupling-induced improvement of the SNR compared to an
uncoupled element is shown to be limited by a factor four which is only reached
for vanishing input noise.Comment: 29 pages, 5 figure
Magnetotransport in disordered delta-doped heterostructures
We discuss theoretically how electrons confined to two dimensions in a
delta-doped heterostructure can arrange themselves in a droplet-like spatial
distribution due to disorder and screening effects when their density is low.
We apply this droplet picture to magnetotransport and derive the expected
dependence on electron density of several quantities relevant to this
transport, in the regimes of weak and moderate magnetic fields. We find good
qualitative and quantitative agreement between our calculations and recent
experiments on delta-doped heterostructures.Comment: 10 pages RevTeX, 2 figures, uses psfrag; published versio
Condensation temperature of interacting Bose gases with and without disorder
The momentum-shell renormalization group (RG) is used to study the
condensation of interacting Bose gases without and with disorder. First of all,
for the homogeneous disorder-free Bose gas the interaction-induced shifts in
the critical temperature and chemical potential are determined up to second
order in the scattering length. The approach does not make use of dimensional
reduction and is thus independent of previous derivations. Secondly, the RG is
used together with the replica method to study the interacting Bose gas with
delta-correlated disorder. The flow equations are derived and found to reduce,
in the high-temperature limit, to the RG equations of the classical
Landau-Ginzburg model with random-exchange defects. The random fixed point is
used to calculate the condensation temperature under the combined influence of
particle interactions and disorder.Comment: 7 pages, 2 figure
Quantum and thermal fluctuations in the SU(N) Heisenberg spin-glass model near the quantum critical point
We solve for the SU(N) Heisenberg spin-glass in the limit of large N focusing
on small S and T. We study the effect of quantum and thermal fluctuations in
the frequency dependent response function and observed interesting transfers of
spectral weight. We compute the T-dependence of the order parameter and the
specific heat and find an unusual T^2 behavior for the latter at low
temperatures in the spin-glass phase. We find a remarkable qualitative
agreement with various experiments on the quantum frustrated magnet
SrCr_{9p}Ga_{12-9p}O_{19}.Comment: 5 pages, 4 figures, submitted to Phys. Rev. Let
Deducing spectroscopic factors from wave-function asymptotics
In a coupled-channel model, we explore the effects of coupling between
configurations on the radial behavior of the wave function and, in particular,
on the spectroscopic factor (SF) and the asymptotic normalization coefficient
(ANC). We evaluate the extraction of a SF from the ratio of the ANC of the
coupled-channel model to that of a single-particle approximation of the wave
function. We perform this study within a core + n collective model, which
includes two states of the core that connect by a rotational coupling. To get
additional insights, we also use a simplified model that takes a delta function
for the coupling potential. Calculations are performed for 11Be. Fair agreement
is obtained between the SF inferred from the single-particle approximation and
the one obtained within the coupled-channel models. Significant discrepancies
are observed only for large coupling strength and/or large admixture, that is,
a small SF. This suggests that reliable SFs can be deduced from the
wave-function asymptotics when the structure is dominated by one configuration,
that is, for a large SF.Comment: Title correcte
Metallic behavior in Si/SiGe 2D electron systems
We calculate the temperature, density, and parallel magnetic field dependence
of low temperature electronic resistivity in 2D high-mobility Si/SiGe quantum
structures, assuming the conductivity limiting mechanism to be carrier
scattering by screened random charged Coulombic impurity centers. We obtain
comprehensive agreement with existing experimental transport data, compellingly
establishing that the observed 2D metallic behavior in low-density Si/SiGe
systems arises from the peculiar nature of 2D screening of long-range impurity
disorder. In particular, our theory correctly predicts the experimentally
observed metallic temperature dependence of 2D resistivity in the fully
spin-polarized system
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