1,496 research outputs found
Nanoengineered Curie Temperature in Laterally-Patterned Ferromagnetic Semiconductor Heterostructures
We demonstrate the manipulation of the Curie temperature of buried layers of
the ferromagnetic semiconductor (Ga,Mn)As using nanolithography to enhance the
effect of annealing. Patterning the GaAs-capped ferromagnetic layers into
nanowires exposes free surfaces at the sidewalls of the patterned (Ga,Mn)As
layers and thus allows the removal of Mn interstitials using annealing. This
leads to an enhanced Curie temperature and reduced resistivity compared to
unpatterned samples. For a fixed annealing time, the enhancement of the Curie
temperature is larger for narrower nanowires.Comment: Submitted to Applied Physics Letters (minor corrections
Operator algebra quantum homogeneous spaces of universal gauge groups
In this paper, we quantize universal gauge groups such as SU(\infty), as well
as their homogeneous spaces, in the sigma-C*-algebra setting. More precisely,
we propose concise definitions of sigma-C*-quantum groups and sigma-C*-quantum
homogeneous spaces and explain these concepts here. At the same time, we put
these definitions in the mathematical context of countably compactly generated
spaces as well as C*-compact quantum groups and homogeneous spaces. We also
study the representable K-theory of these spaces and compute it for the quantum
homogeneous spaces associated to the universal gauge group SU(\infty).Comment: 14 pages. Merged with [arXiv:1011.1073
Flux through a hole from a shaken granular medium
We have measured the flux of grains from a hole in the bottom of a shaken
container of grains. We find that the peak velocity of the vibration, vmax,
controls the flux, i.e., the flux is nearly independent of the frequency and
acceleration amplitude for a given value of vmax. The flux decreases with
increasing peak velocity and then becomes almost constant for the largest
values of vmax. The data at low peak velocity can be quantitatively described
by a simple model, but the crossover to nearly constant flux at larger peak
velocity suggests a regime in which the granular density near the container
bottom is independent of the energy input to the system.Comment: 14 pages, 4 figures. to appear in Physical Review
Impurity Band Conduction in a High Temperature Ferromagnetic Semiconductor
The band structure of a prototypical dilute ferromagnetic semiconductor,
GaMnAs, is studied across the phase diagram via optical
spectroscopy. We prove that the Fermi energy () resides in a Mn induced
impurity band (IB). This conclusion is based upon careful analysis of the
frequency and temperature dependence of the optical conductivity
(). From our analysis of we infer
a large effective mass () of the carriers, supporting the view that
conduction occurs in an IB. Our results also provide useful insights into the
transport properties of Mn-doped GaAs.Comment: 4 pages, 4 figure
Antisite effect on ferromagnetism in (Ga,Mn)As
We study the Curie temperature and hole density of (Ga,Mn)As while
systematically varying the As-antisite density. Hole compensation by
As-antisites limits the Curie temperature and can completely quench long-range
ferromagnetic order in the low doping regime of 1-2% Mn. Samples are grown by
molecular beam epitaxy without substrate rotation in order to smoothly vary the
As to Ga flux ratio across a single wafer. This technique allows for a
systematic study of the effect of As stoichiometry on the structural,
electronic, and magnetic properties of (Ga,Mn)As. For concentrations less than
1.5% Mn, a strong deviation from Tc ~ p^0.33 is observed. Our results emphasize
that proper control of As-antisite compensation is critical for controlling the
Curie temperatures in (Ga,Mn)As at the low doping limit.Comment: 10 pages, 7 figure
Capping-induced suppression of annealing in Ga(1-x)Mn(x)As epilayers
We have studied the effects of capping ferromagnetic Ga(1-x)Mn(x)As epilayers
with a thin layer of undoped GaAs, and we find that even a few monolayers of
GaAs have a significant effect on the ferromagnetic properties. In particular,
the presence of a capping layer only 10 monolayers thick completely suppresses
the enhancement of the ferromagnetism associated with low temperature
annealing. This result, which demonstrates that the surface of a Ga(1-x)Mn(x)As
epilayer strongly affects the defect structure, has important implications for
the incorporation of Ga(1-x)Mn(x)As into device heterostructures.Comment: 13 pages with figures attatche
Magnetoresistance Anomalies in (Ga,Mn)As Epilayers with Perpendicular Magnetic Anisotropy
We report the observation of anomalies in the longitudinal magnetoresistance
of tensile-strained (Ga,Mn)As epilayers with perpendicular magnetic anisotropy.
Magnetoresistance measurements carried out in the planar geometry (magnetic
field parallel to the current density) reveal "spikes" that are antisymmetric
with respect to the direction of the magnetic field. These anomalies always
occur during magnetization reversal, as indicated by a simultaneous change in
sign of the anomalous Hall effect. The data suggest that the antisymmetric
anomalies originate in anomalous Hall effect contributions to the longitudinal
resistance when domain walls are located between the voltage probes. This
interpretation is reinforced by carrying out angular sweeps of ,
revealing an antisymmetric dependence on the helicity of the field sweep.Comment: Submitted to Phys. Rev.
Optimal multistage schemes for Euler equations with residual smoothing
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/76583/1/AIAA-12860-858.pd
Stochastic Ballistic Annihilation and Coalescence
We study a class of stochastic ballistic annihilation and coalescence models
with a binary velocity distribution in one dimension. We obtain an exact
solution for the density which reveals a universal phase diagram for the
asymptotic density decay. By universal we mean that all models in the class are
described by a single phase diagram spanned by two reduced parameters. The
phase diagram reveals four regimes, two of which contain the previously studied
cases of ballistic annihilation. The two new phases are a direct consequence of
the stochasticity. The solution is obtained through a matrix product approach
and builds on properties of a q-deformed harmonic oscillator algebra.Comment: 4 pages RevTeX, 3 figures; revised version with some corrections,
additional discussion and in RevTeX forma
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