258 research outputs found
Spin accumulation in forward-biased MnAs/GaAs Schottky diodes
We describe a new means for electrically creating spin polarization in
semiconductors. In contrast to spin injection of electrons by tunneling through
a reverse-biased Schottky barrier, we observe spin accumulation at the
metal/semiconductor interface of forward-biased ferromagnetic Schottky diodes,
which is consistent with a theory of spin-dependent reflection off the
interface. Spatiotemporal Kerr microscopy is used to image the electron spin
and the resulting dynamic nuclear polarization that arises from the non
equilibrium carrier polarization.Comment: 13 pages, 4 figures, submitted for publicatio
Scaling of transverse nuclear magnetic relaxation due to magnetic nanoparticle aggregation
The aggregation of superparamagnetic iron oxide (SPIO) nanoparticles
decreases the transverse nuclear magnetic resonance (NMR) relaxation time T2 of
adjacent water molecules measured by a Carr-Purcell-Meiboom-Gill (CPMG)
pulse-echo sequence. This effect is commonly used to measure the concentrations
of a variety of small molecules. We perform extensive Monte Carlo simulations
of water diffusing around SPIO nanoparticle aggregates to determine the
relationship between T2 and details of the aggregate. We find that in the
motional averaging regime T2 scales as a power law with the number N of
nanoparticles in an aggregate. The specific scaling is dependent on the fractal
dimension d of the aggregates. We find T2 N^{-0.44} for aggregates with d=2.2,
a value typical of diffusion limited aggregation. We also find that in
two-nanoparticle systems, T2 is strongly dependent on the orientation of the
two nanoparticles relative to the external magnetic field, which implies that
it may be possible to sense the orientation of a two-nanoparticle aggregate. To
optimize the sensitivity of SPIO nanoparticle sensors, we propose that it is
best to have aggregates with few nanoparticles, close together, measured with
long pulse-echo times.Comment: 20 pages, 3 figures, submitted to Journal of Magnetism and Magnetic
Material
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Imaging Coherent Transport in Graphene (Part I): Mapping Universal Conductance Fluctuations
Graphene provides a fascinating testbed for new physics and exciting opportunities for future applications based on quantum phenomena. To understand the coherent flow of electrons through a graphene device, we employ a nanoscale probe that can access the relevant length scales—the tip of a liquid-He-cooled scanning probe microscope (SPM) capacitively couples to the graphene device below, creating a movable scatterer for electron waves. At sufficiently low temperatures and small size scales, the diffusive transport of electrons through graphene becomes coherent, leading to universal conductance fluctuations (UCF). By scanning the tip over a device, we map these conductance fluctuations versus scatterer position. We find that the conductance is highly sensitive to the tip position, producing fluctuations when the tip is displaced by a distance comparable to half the Fermi wavelength. These measurements are in good agreement with detailed quantum simulations of the imaging experiment and demonstrate the value of a cooled SPM for probing coherent transport in graphene.Chemistry and Chemical BiologyEngineering and Applied SciencesPhysic
Time-resolved ferromagnetic resonance in epitaxial Fe1-xCox films
Magnetodynamics in epitaxial Fe1-xCox films on GaAs (100) are studied using
time-resolved ferromagnetic resonance, in which the free precession of the
magnetization after an impulsive excitation is measured using the polar Kerr
effect. The sample is rotated with respect to the static and pulsed field
directions, providing a complete mapping of the free energy surface and
characteristic relaxation times. The magnetic response can be simulated with a
simple coherent rotation model except in the immediate vicinity of switching
fields. Bulk and surface anisotropies are identified, and unusual dynamics
associated with the coexistence of cubic and uniaxial anisotropies are
observed.Comment: PDF - 4 figure
Optically-patterned nuclear doughnuts in GaAs/MnAs heterostructures
We demonstrate a scheme for optically patterning nuclear spin polarization in
semiconductor/ferromagnet heterostructures. A scanning time-resolved Kerr
rotation microscope is used to image the nuclear spin polarization that results
when GaAs/MnAs epilayers are illuminated with a focused laser having a Gaussian
profile. Rather than tracking the intensity profile of the laser spot, these
images reveal that the nuclear polarization forms an annular lateral structure
having circular symmetry with a dip rather than a peak at its center.Comment: 11 pages, 3 figure
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