4,142 research outputs found
Study of Magnetic Properties of A_2B^'NbO_6 (A=Ba,Sr, (BaSr): and B^'=Fe and Mn) double perovskites
We have studied the magnetic properties of Ba_2FeNbO_6 and Ba_2MnNbO_6. it is
seen that Ba_2FeNbO_6 is an antiferromagnet with a weak ferromagnetic behaviour
at 5K while Ba_2MnNbO_6 shows two magnetic transitions one at 45 K and the
other at 12K. Electron spin resonance (ESR) measurements at room temperature
show that the Mn compound does not show any Jahn-Teller distortion. It is also
seen that the Neel temperature of the A_2FeNbO_6 (A=Ba,Sr, BaSr) compounds do
not vary significantly. However variations in the average A-site ionic radius
influence the formation of short range correlations that persist above T_N.Comment: 10 oages, 5 figures, MMM, to appear in J.Appl.Phy
Double quantum dot with tunable coupling in an enhancement-mode silicon metal-oxide semiconductor device with lateral geometry
We present transport measurements of a tunable silicon
metal-oxide-semiconductor double quantum dot device with lateral geometry.
Experimentally extracted gate-to-dot capacitances show that the device is
largely symmetric under the gate voltages applied. Intriguingly, these gate
voltages themselves are not symmetric. Comparison with numerical simulations
indicates that the applied gate voltages serve to offset an intrinsic asymmetry
in the physical device. We also show a transition from a large single dot to
two well isolated coupled dots, where the central gate of the device is used to
controllably tune the interdot coupling.Comment: 4 pages, 3 figures, to be published in Applied Physics Letter
Valley degeneracy in biaxially strained aluminum arsenide quantum wells
This paper details a complete formalism for calculating electron subband
energy and degeneracy in strained multi-valley quantum wells grown along any
orientation with explicit results for the AlAs quantum well case. A
standardized rotation matrix is defined to transform from the conventional-
cubic-cell basis to the quantum-well-transport basis whereby effective mass
tensors, valley vectors, strain matrices, anisotropic strain ratios, and
scattering vectors are all defined in their respective bases. The specific
cases of (001)-, (110)-, and (111)-oriented aluminum arsenide (AlAs) quantum
wells are examined, as is the unconventional (411) facet, which is of
particular importance in AlAs literature. Calculations of electron confinement
and strain in the (001), (110), and (411) facets determine the critical well
width for crossover from double- to single-valley degeneracy in each system.
The notation is generalized to include miscut angles, and can be adapted to
other multi-valley systems. To help classify anisotropic inter-valley
scattering events, a new primitive unit cell is defined in momentum space which
allows one to distinguish purely in-plane inter-valley scattering events from
those that requires an out-of-plane momentum scattering component.Comment: 17 pages, 4 figures, 2 table
Enhancement mode double top gated MOS nanostructures with tunable lateral geometry
We present measurements of silicon (Si) metal-oxide-semiconductor (MOS)
nanostructures that are fabricated using a process that facilitates essentially
arbitrary gate geometries. Stable Coulomb blockade behavior free from the
effects of parasitic dot formation is exhibited in several MOS quantum dots
with an open lateral quantum dot geometry. Decreases in mobility and increases
in charge defect densities (i.e. interface traps and fixed oxide charge) are
measured for critical process steps, and we correlate low disorder behavior
with a quantitative defect density. This work provides quantitative guidance
that has not been previously established about defect densities for which Si
quantum dots do not exhibit parasitic dot formation. These devices make use of
a double-layer gate stack in which many regions, including the critical gate
oxide, were fabricated in a fully-qualified CMOS facility.Comment: 11 pages, 6 figures, 3 tables, accepted for publication in Phys. Rev.
Modeling of mode-locking in a laser with spatially separate gain media
We present a novel laser mode-locking scheme and discuss its unusual
properties and feasibility using a theoretical model. A large set of
single-frequency continuous-wave lasers oscillate by amplification in spatially
separated gain media. They are mutually phase-locked by nonlinear feedback from
a common saturable absorber. As a result, ultra short pulses are generated. The
new scheme offers three significant benefits: the light that is amplified in
each medium is continuous wave, thereby avoiding issues related to group
velocity dispersion and nonlinear effects that can perturb the pulse shape. The
set of frequencies on which the laser oscillates, and therefore the pulse
repetition rate, is controlled by the geometry of resonator-internal optical
elements, not by the cavity length. Finally, the bandwidth of the laser can be
controlled by switching gain modules on and off. This scheme offers a route to
mode-locked lasers with high average output power, repetition rates that can be
scaled into the THz range, and a bandwidth that can be dynamically controlled.
The approach is particularly suited for implementation using semiconductor
diode laser arrays.Comment: 13 pages, 5 figures, submitted to Optics Expres
Nuclear Inelastic X-Ray Scattering of FeO to 48 GPa
The partial density of vibrational states has been measured for Fe in
compressed FeO (w\"ustite) using nuclear resonant inelastic x-ray scattering.
Substantial changes have been observed in the overall shape of the density of
states close to the magnetic transiton around 20 GPa from the paramagnetic (low
pressure) to the antiferromagnetic (high pressure) state. Our data indicate a
substantial softening of the aggregate sound velocities far below the
transition, starting between 5 and 10 GPa. This is consistent with recent
radial x-ray diffraction measurements of the elastic constants in FeO. The
results indicate that strong magnetoelastic coupling in FeO is the driving
force behind the changes in the phonon spectrum of FeO.Comment: 4 pages, 4 figure
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