735 research outputs found
Observations of attenuation at 20.6, 31.65 and 90.0 GHz: Preliminary results from Wallops Island, VA
Ground based radiometric observations of atmospheric attenuation at 20.6, 31.65, and 90.0 GHz were made at Wallops Island, Virginia during April and May 1989. Early results from the analysis of the data set are compared with previous observations from California and Colorado. The relative attenuation ratios observed at each frequency during clear, cloudy, and rainy conditions are shown. Plans for complete analysis of the data are described
Direct vs. indirect optical recombination in Ge films grown on Si substrates
The optical emission spectra from Ge films on Si are markedly different from
their bulk Ge counterparts. Whereas bulk Ge emission is dominated by the
material's indirect gap, the photoluminescence signal from Ge films is mainly
associated with its direct band gap. Using a new class of Ge-on-Si films grown
by a recently introduced CVD approach, we study the direct and indirect
photoluminescence from intrinsic and doped samples and we conclude that the
origin of the discrepancy is the lack of self-absorption in thin Ge films
combined with a deviation from quasi-equilibrium conditions in the conduction
band. The latter is confirmed by a simple model suggesting that the deviation
from quasi-equilibrium is caused by the much shorter recombination lifetime in
the films relative to bulk Ge
The Peierls substitution in an engineered lattice potential
Artificial gauge fields open new possibilities to realize quantum many-body
systems with ultracold atoms, by engineering Hamiltonians usually associated
with electronic systems. In the presence of a periodic potential, artificial
gauge fields may bring ultracold atoms closer to the quantum Hall regime. Here,
we describe a one-dimensional lattice derived purely from effective
Zeeman-shifts resulting from a combination of Raman coupling and radiofrequency
magnetic fields. In this lattice, the tunneling matrix element is generally
complex. We control both the amplitude and the phase of this tunneling
parameter, experimentally realizing the Peierls substitution for ultracold
neutral atoms.Comment: 6 pages, 5 figure
Stability of pulse-like earthquake ruptures
Pulse-like ruptures arise spontaneously in many elastodynamic rupture
simulations and seem to be the dominant rupture mode along crustal faults.
Pulse-like ruptures propagating under steady-state conditions can be
efficiently analysed theoretically, but it remains unclear how they can arise
and how they evolve if perturbed. Using thermal pressurisation as a
representative constitutive law, we conduct elastodynamic simulations of
pulse-like ruptures and determine the spatio-temporal evolution of slip, slip
rate and pulse width perturbations induced by infinitesimal perturbations in
background stress. These simulations indicate that steady-state pulses driven
by thermal pressurisation are unstable. If the initial stress perturbation is
negative, ruptures stop; conversely, if the perturbation is positive, ruptures
grow and transition to either self-similar pulses (at low background stress) or
expanding cracks (at elevated background stress). Based on a dynamic
dislocation model, we develop an elastodynamic equation of motion for slip
pulses, and demonstrate that steady-state slip pulses are unstable if their
accrued slip is a decreasing function of the uniform background stress
. This condition is satisfied by slip pulses driven by thermal
pressurisation. The equation of motion also predicts quantitatively the growth
rate of perturbations, and provides a generic tool to analyse the propagation
of slip pulses. The unstable character of steady-state slip pulses implies that
this rupture mode is a key one determining the minimum stress conditions for
sustainable ruptures along faults, i.e., their ``strength''. Furthermore, slip
pulse instabilities can produce a remarkable complexity of rupture dynamics,
even under uniform background stress conditions and material properties
Number of Common Sites Visited by N Random Walkers
We compute analytically the mean number of common sites, W_N(t), visited by N
independent random walkers each of length t and all starting at the origin at
t=0 in d dimensions. We show that in the (N-d) plane, there are three distinct
regimes for the asymptotic large t growth of W_N(t). These three regimes are
separated by two critical lines d=2 and d=d_c(N)=2N/(N-1) in the (N-d) plane.
For d<2, W_N(t)\sim t^{d/2} for large t (the N dependence is only in the
prefactor). For 2<d<d_c(N), W_N(t)\sim t^{\nu} where the exponent \nu=
N-d(N-1)/2 varies with N and d. For d>d_c(N), W_N(t) approaches a constant as
t\to \infty. Exactly at the critical dimensions there are logaritmic
corrections: for d=2, we get W_N(t)\sim t/[\ln t]^N, while for d=d_c(N),
W_N(t)\sim \ln t for large t. Our analytical predictions are verified in
numerical simulations.Comment: 5 pages, 3 .eps figures include
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