2,190 research outputs found
Optical constants of silicon carbide for astrophysical applications. II. Extending optical functions from IR to UV using single-crystal absorption spectra
Laboratory measurements of unpolarized and polarized absorption spectra of
various samples and crystal stuctures of silicon carbide (SiC) are presented
from 1200--35,000 cm ( 8--0.28 m) and used to improve
the accuracy of optical functions ( and ) from the infrared (IR) to the
ultraviolet (UV). Comparison with previous 6--20 m
thin-film spectra constrains the thickness of the films and verifies that
recent IR reflectivity data provide correct values for in the IR region. We
extract and needed for radiative transfer models using a new
``difference method'', which utilizes transmission spectra measured from two
SiC single-crystals with different thicknesses. This method is ideal for
near-IR to visible regions where absorbance and reflectance are low and can be
applied to any material. Comparing our results with previous UV measurements of
SiC, we distinguish between chemical and structural effects at high frequency.
We find that for all spectral regions, 3C (-SiC) and the polarization of 6H (a type of -SiC) have almost identical
optical functions that can be substituted for each other in modeling
astronomical environments. Optical functions for of 6H SiC
have peaks shifted to lower frequency, permitting identification of this
structure below m. The onset of strong UV absorption for pure
SiC occurs near 0.2 m, but the presence of impurities redshifts the rise
to 0.33 m. Optical functions are similarly impacted. Such large
differences in spectral characteristics due to structural and chemical effects
should be observable and provide a means to distinguish chemical variation of
SiC dust in space.Comment: 46 pages inc. 8 figures and 2 full tables. Also 6 electronic-only
data files. Accepted by Ap
Crystallization in a dense suspension of self-propelled particles
Using Brownian dynamics computer simulations we show that a two-dimensional
suspension of self-propelled ("active") colloidal particles crystallizes at
sufficiently high densities. Compared to the equilibrium freezing of passive
particles the freezing density is both significantly shifted and depends on the
structural or dynamical criterion employed. In non-equilibrium the transition
is accompanied by pronounced structural heterogeneities. This leads to a
transition region between liquid and solid in which the suspension is globally
ordered but unordered liquid-like "bubbles" still persist
Half Cycle Pulse Train Induced State Redistribution of Rydberg Atoms
Population transfer between low lying Rydberg states independent of the
initial state is realized using a train of half-cycle pulses with pulse
durations much less than the classical orbit period. We demonstrate
experimentally the transfer of population from initial states around n=50 down
to n<40 as well as up to the continuum. The measured population transfer
matches well to a model of the process for 1D atoms.Comment: V2: discussion extended, version accepted for publication in Physical
Review
The quasi-free-standing nature of graphene on H-saturated SiC(0001)
We report on an investigation of quasi-free-standing graphene on 6H-SiC(0001)
which was prepared by intercalation of hydrogen under the buffer layer. Using
infrared absorption spectroscopy we prove that the SiC(0001) surface is
saturated with hydrogen. Raman spectra demonstrate the conversion of the buffer
layer into graphene which exhibits a slight tensile strain and short range
defects. The layers are hole doped (p = 5.0-6.5 x 10^12 cm^(-2)) with a carrier
mobility of 3,100 cm^2/Vs at room temperature. Compared to graphene on the
buffer layer a strongly reduced temperature dependence of the mobility is
observed for graphene on H-terminated SiC(0001)which justifies the term
"quasi-free-standing".Comment: 3 pages, 3 figures, accepted for publication in Applied Physics
Letter
Probability density functions of work and heat near the stochastic resonance of a colloidal particle
We study experimentally and theoretically the probability density functions
of the injected and dissipated energy in a system of a colloidal particle
trapped in a double well potential periodically modulated by an external
perturbation. The work done by the external force and the dissipated energy are
measured close to the stochastic resonance where the injected power is maximum.
We show a good agreement between the probability density functions exactly
computed from a Langevin dynamics and the measured ones. The probability
density function of the work done on the particle satisfies the fluctuation
theorem
Effective temperatures of a heated Brownian particle
We investigate various possible definitions of an effective temperature for a
particularly simple nonequilibrium stationary system, namely a heated Brownian
particle suspended in a fluid. The effective temperature based on the
fluctuation dissipation ratio depends on the time scale under consideration, so
that a simple Langevin description of the heated particle is impossible. The
short and long time limits of this effective temperature are shown to be
consistent with the temperatures estimated from the kinetic energy and Einstein
relation, respectively. The fluctuation theorem provides still another
definition of the temperature, which is shown to coincide with the short time
value of the fluctuation dissipation ratio
Irreversible effects of memory
The steady state of a Langevin equation with short ranged memory and coloured
noise is analyzed. When the fluctuation-dissipation theorem of second kind is
not satisfied, the dynamics is irreversible, i.e. detailed balance is violated.
We show that the entropy production rate for this system should include the
power injected by ``memory forces''. With this additional contribution, the
Fluctuation Relation is fairly verified in simulations. Both dynamics with
inertia and overdamped dynamics yield the same expression for this additional
power. The role of ``memory forces'' within the fluctuation-dissipation
relation of first kind is also discussed.Comment: 6 pages, 1 figure, publishe
Minimal Normalization of Wiener–Hopf Operators in Spaces of Bessel Potentials
AbstractA class of operators is investigated which results from certain boundary and transmission problems, the so-called Sommerfeld diffraction problems. In various cases these are of normal type but not normally solvable, and the problem is how to normalize the operators in a physically relevant way, i.e., not loosing the Hilbert space structure of function spaces defined by a locally finite energy norm. The present approach solves this question rigorously for the case where the lifted Fourier symbol matrix function is Hölder continuous on the real line with a jump at infinity. It incorporates the intuitive concept of compatibility conditions which is known from some canonical problems. Further it presents explicit analytical formulas for generalized inverses of the normalized operators in terms of matrix factorization
Current-Induced Spin Polarization in Gallium Nitride
Electrically generated spin polarization is probed directly in bulk GaN using
Kerr rotation spectroscopy. A series of n-type GaN epilayers are grown in the
wurtzite phase both by molecular beam epitaxy (MBE) and metalorganic chemical
vapor deposition (MOCVD) with a variety of doping densities chosen to broadly
modulate the transverse spin lifetime, T2*. The spin polarization is
characterized as a function of electrical excitation energy over a range of
temperatures. Despite weak spin-orbit interactions in GaN, a current-induced
spin polarization (CISP) is observed in the material at temperatures of up to
200 K.Comment: 16 pages, 3 figure
- …