50,652 research outputs found

### Ionization of hydrogen atoms by electron impact at 1eV, 0.5eV and 0.3eV above threshold

We present here triple differential cross sections for ionization of hydrogen
atoms by electron impact at 1eV, 0.5eV and 0.3eV energy above threshold,
calculated in the hyperspherical partial wave theory. The results are in very
good agreement with the available semiclassical results of Deb and Crothers
\cite{DC02} for these energies. With this, we are able to demonstrate that the
hyperspherical partial wave theory yields good cross sections from 30 eV
\cite{DPC03} down to near threshold for equal energy sharing kinematics.Comment: 6 pages, 9 figure

### Band-Structure Effects in the Spin Relaxation of Conduction Electrons

Spin relaxation of conduction electrons in metals is significantly influenced
by the Fermi surface topology. Electrons near Brillouin zone boundaries,
special symmetry points, or accidental degeneracy lines have spin flip rates
much higher than an average electron. A realistic calculation and analytical
estimates show that these regions dominate the spin relaxation, explaining why
polyvalent metals have much higher spin relaxation rates (up to three orders of
magnitude) than similar monovalent metals. This suggests that spin relaxation
in metals can be tailored by band-structure modifications like doping,
alloying, reducing the dimensionality, etc.Comment: 10 pages, 2 figures; to appear in the 43rd MMM Conference Proceedings
published in the JA

### Driven Heisenberg Magnets: Nonequilibrium Criticality, Spatiotemporal Chaos and Control

We drive a $d$-dimensional Heisenberg magnet using an anisotropic current.
The continuum Langevin equation is analysed using a dynamical renormalization
group and numerical simulations. We discover a rich steady-state phase diagram,
including a critical point in a new nonequilibrium universality class, and a
spatiotemporally chaotic phase. The latter may be `controlled' in a robust
manner to target spatially periodic steady states with helical order.Comment: 7 pages, 2 figures. Published in Euro. Phys. Let

### Energy relaxation of an excited electron gas in quantum wires: many-body electron LO-phonon coupling

We theoretically study energy relaxation via LO-phonon emission in an excited
one-dimensional electron gas confined in a GaAs quantum wire structure. We find
that the inclusion of phonon renormalization effects in the theory extends the
LO-phonon dominated loss regime down to substantially lower temperatures. We
show that a simple plasmon-pole approximation works well for this problem, and
discuss implications of our results for low temperature electron heating
experiments in quantum wires.Comment: 10 pages, RevTex, 4 figures included. Also available at
http://www-cmg.physics.umd.edu/~lzheng

### Dynamic Scaling in a 2+1 Dimensional Limited Mobility Model of Epitaxial Growth

We study statistical scale invariance and dynamic scaling in a simple
solid-on-solid 2+1 - dimensional limited mobility discrete model of
nonequilibrium surface growth, which we believe should describe the low
temperature kinetic roughening properties of molecular beam epitaxy. The model
exhibits long-lived ``transient'' anomalous and multiaffine dynamic scaling
properties similar to that found in the corresponding 1+1 - dimensional
problem. Using large-scale simulations we obtain the relevant scaling
exponents, and compare with continuum theories.Comment: 5 pages, 4 ps figures included, RevTe

### Magnetic Percolation and the Phase Diagram of the Disordered RKKY model

We consider ferromagnetism in spatially randomly located magnetic moments, as
in a diluted magnetic semiconductor, coupled via the carrier-mediated indirect
exchange RKKY interaction. We obtain via Monte Carlo the magnetic phase diagram
as a function of the impurity moment density $n_{i}$ and the relative carrier
concentration $n_{c}/n_{i}$. As evidenced by the diverging correlation length
and magnetic susceptibility, the boundary between ferromagnetic (FM) and
non-ferromagnetic (NF) phases constitutes a line of zero temperature critical
points which can be viewed as a magnetic percolation transition. In the dilute
limit, we find that bulk ferromagnetism vanishes for $n_{c}/n_{i}>.1$. We also
incorporate the local antiferromagnetic direct superexchange interaction
between nearest neighbor impurities, and examine the impact of a damping factor
in the RKKY range function.Comment: 5 pages, 3 figures; figure formatting modified, typos fixe

### Critical behavior of diluted magnetic semiconductors: the apparent violation and the eventual restoration of the Harris criterion for all regimes of disorder

Using large-scale Monte Carlo calculations, we consider strongly disordered
Heisenberg models on a cubic lattice with missing sites (as in diluted magnetic
semiconductors such as Ga_{1-x}Mn_{x}As). For disorder ranging from weak to
strong levels of dilution, we identify Curie temperatures and calculate the
critical exponents nu, gamma, eta, and beta finding, per the Harris criterion,
good agreement with critical indices for the pure Heisenberg model where there
is no disorder component. Moreover, we find that thermodynamic quantities (e.g.
the second moment of the magnetization per spin) self average at the
ferromagnetic transition temperature with relative fluctuations tending to zero
with increasing system size. We directly calculate effective critical exponents
for T > T_{c}, yielding values which may differ significantly from the critical
indices for the pure system, especially in the presence of strong disorder.
Ultimately, the difference is only apparent, and eventually disappears when T
is very close to T_{c}.Comment: 11 pages, 9 figure

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