505 research outputs found
Laterally defined freely suspended quantum dots in GaAs/AlGaAs heterostructures
Free standing beams containing a two-dimensional electron system are shaped
from a GaAs/AlGaAs heterostructure. Quantum point contacts and (double) quantum
dots are laterally defined using metal top gates. We investigate the electronic
properties of these nanostructures by transport spectroscopy. Tunable localized
electron states in freely suspended nanostructures are a promising tool to
investigate the electron-phonon-interaction
Dynamic photoconductive gain effect in shallow-etched AlGaAs/GaAs quantum wires
We report on a dynamic photoconductive gain effect in quantum wires which are
lithographically fabricated in an AlGaAs/GaAs quantum well via a shallow-etch
technique. The effect allows resolving the one-dimensional subbands of the
quantum wires as maxima in the photoresponse across the quantum wires. We
interpret the results by optically induced holes in the valence band of the
quantum well which shift the chemical potential of the quantum wire. The
non-linear current-voltage characteristics of the quantum wires also allow
detecting the photoresponse effect of excess charge carriers in the conduction
band of the quantum well. The dynamics of the photoconductive gain are limited
by the recombination time of both electrons and holes
Random Field and Random Anisotropy Effects in Defect-Free Three-Dimensional XY Models
Monte Carlo simulations have been used to study a vortex-free XY ferromagnet
with a random field or a random anisotropy on simple cubic lattices. In the
random field case, which can be related to a charge-density wave pinned by
random point defects, it is found that long-range order is destroyed even for
weak randomness. In the random anisotropy case, which can be related to a
randomly pinned spin-density wave, the long-range order is not destroyed and
the correlation length is finite. In both cases there are many local minima of
the free energy separated by high entropy barriers. Our results for the random
field case are consistent with the existence of a Bragg glass phase of the type
discussed by Emig, Bogner and Nattermann.Comment: 10 pages, including 2 figures, extensively revise
First principles study of strain/electronic interplay in ZnO; Stress and temperature dependence of the piezoelectric constants
We present a first-principles study of the relationship between stress,
temperature and electronic properties in piezoelectric ZnO. Our method is a
plane wave pseudopotential implementation of density functional theory and
density functional linear response within the local density approximation. We
observe marked changes in the piezoelectric and dielectric constants when the
material is distorted. This stress dependence is the result of strong, bond
length dependent, hybridization between the O and Zn electrons. Our
results indicate that fine tuning of the piezoelectric properties for specific
device applications can be achieved by control of the ZnO lattice constant, for
example by epitaxial growth on an appropriate substrate.Comment: accepted for publication in Phys. Rev.
Structural and optical properties of high quality zinc-blende/wurtzite GaAs hetero-nanowires
The structural and optical properties of 3 different kinds of GaAs nanowires
with 100% zinc-blende structure and with an average of 30% and 70% wurtzite are
presented. A variety of shorter and longer segments of zinc-blende or wurtzite
crystal phases are observed by transmission electron microscopy in the
nanowires. Sharp photoluminescence lines are observed with emission energies
tuned from 1.515 eV down to 1.43 eV when the percentage of wurtzite is
increased. The downward shift of the emission peaks can be understood by
carrier confinement at the interfaces, in quantum wells and in random short
period superlattices existent in these nanowires, assuming a staggered
band-offset between wurtzite and zinc-blende GaAs. The latter is confirmed also
by time resolved measurements. The extremely local nature of these optical
transitions is evidenced also by cathodoluminescence measurements. Raman
spectroscopy on single wires shows different strain conditions, depending on
the wurtzite content which affects also the band alignments. Finally, the
occurrence of the two crystallographic phases is discussed in thermodynamic
terms.Comment: 24 page
Diffusion and viscosity in a supercooled polydisperse system
We have carried out extensive molecular dynamics simulations of a supercooled
polydisperse Lennard-Jones liquid with large variations in temperature at a
fixed pressure. The particles in the system are considered to be polydisperse
both in size and mass. The temperature dependence of the dynamical properties
such as the viscosity () and the self-diffusion coefficients () of
different size particles is studied. Both viscosity and diffusion coefficients
show super-Arrhenius temperature dependence and fit well to the well-known
Vogel-Fulcher-Tammann (VFT) equation. Within the temperature range
investigated, the value of the Angell's fragility parameter (D )
classifies the present system into a strongly fragile liquid. The critical
temperature for diffusion () increases with the size of the
particles. The critical temperature for viscosity () is larger than
that for the diffusion and a sizeable deviations appear for the smaller size
particles implying a decoupling of translational diffusion from viscosity in
deeply supercooled liquid. Indeed, the diffusion shows markedly non-Stokesian
behavior at low temperatures where a highly nonlinear dependence on size is
observed. An inspection of the trajectories of the particles shows that at low
temperatures the motions of both the smallest and largest size particles are
discontinuous (jump-type). However, the crossover from continuous Brownian to
large length hopping motion takes place at shorter time scales for the smaller
size particles.Comment: Revtex4, 7 pages, 8 figure
Operations of and Future Plans for the Pierre Auger Observatory
Technical reports on operations and features of the Pierre Auger Observatory,
including ongoing and planned enhancements and the status of the future
northern hemisphere portion of the Observatory. Contributions to the 31st
International Cosmic Ray Conference, Lodz, Poland, July 2009.Comment: Contributions to the 31st ICRC, Lodz, Poland, July 200
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