1,218 research outputs found
Detecting Spin-Polarized Currents in Ballistic Nanostructures
We demonstrate a mesoscopic spin polarizer/analyzer system that allows the
spin polarization of current from a quantum point contact in an in-plane
magnetic field to be measured. A transverse focusing geometry is used to couple
current from an emitter point contact into a collector point contact. At large
in-plane fields, with the point contacts biased to transmit only a single spin
(g < e^2/h), the voltage across the collector depends on the spin polarization
of the current incident on it. Spin polarizations of greater than 80% are found
for both emitter and collector at 300mK and 7T in-plane field.Comment: related papers at http://marcuslab.harvard.ed
How branching can change the conductance of ballistic semiconductor devices
We demonstrate that branching of the electron flow in semiconductor
nanostructures can strongly affect macroscopic transport quantities and can
significantly change their dependence on external parameters compared to the
ideal ballistic case even when the system size is much smaller than the mean
free path. In a corner-shaped ballistic device based on a GaAs/AlGaAs
two-dimensional electron gas we observe a splitting of the commensurability
peaks in the magnetoresistance curve. We show that a model which includes a
random disorder potential of the two-dimensional electron gas can account for
the random splitting of the peaks that result from the collimation of the
electron beam. The shape of the splitting depends on the particular realization
of the disorder potential. At the same time magnetic focusing peaks are largely
unaffected by the disorder potential.Comment: accepted for publication in Phys. Rev.
Spin separation in cyclotron motion
Charged carriers with different spin states are spatially separated in a
two-dimensional hole gas. Due to strong spin-orbit interaction holes at the
Fermi energy have different momenta for two possible spin states travelling in
the same direction and, correspondingly, different cyclotron orbits in a weak
magnetic field. Two point contacts, acting as a monochromatic source of
ballistic holes and a narrow detector in the magnetic focusing geometry are
demonstrated to work as a tunable spin filter.Comment: 4 pages, 2 figure
Hemispheric Asymmetry in New Neurons in Adulthood Is Associated with Vocal Learning and Auditory Memory
Many brain regions exhibit lateral differences in structure and function, and also incorporate new neurons in adulthood, thought to function in learning and in the formation of new memories. However, the contribution of new neurons to hemispheric differences in processing is unknown. The present study combines cellular, behavioral, and physiological methods to address whether 1) new neuron incorporation differs between the brain hemispheres, and 2) the degree to which hemispheric lateralization of new neurons correlates with behavioral and physiological measures of learning and memory. The songbird provides a model system for assessing the contribution of new neurons to hemispheric specialization because songbird brain areas for vocal processing are functionally lateralized and receive a continuous influx of new neurons in adulthood. In adult male zebra finches, we quantified new neurons in the caudomedial nidopallium (NCM), a forebrain area involved in discrimination and memory for the complex vocalizations of individual conspecifics. We assessed song learning and recorded neural responses to song in NCM. We found significantly more new neurons labeled in left than in right NCM; moreover, the degree of asymmetry in new neuron numbers was correlated with the quality of song learning and strength of neuronal memory for recently heard songs. In birds with experimentally impaired song quality, the hemispheric difference in new neurons was diminished. These results suggest that new neurons may contribute to an allocation of function between the hemispheres that underlies the learning and processing of complex signals
Electron focusing, mode spectroscopy and mass enhancement in small GaAs/AlGaAs rings
A new electron focusing effect has been discovered in small single and
coupled GaAs/AlGaAs rings. The focusing in the single ring is attributed solely
to internal orbits. The focusing effect allows the ring to be used as a small
mass spectrometer. The focusing causes peaks in the magnetoresistance at low
fields, and the peak positions were used to study the dispersion relation of
the one-dimensional magnetoelectric subbands. The electron effective mass
increases with the applied magnetic field by a factor of , at a magnetic
field of . This is the first time this increase has been measured
directly. General agreement obtains between the experiment and the subband
calculations for straight channels.Comment: 13 pages figures are available by reques
``Smoke Rings'' in Ferromagnets
It is shown that bulk ferromagnets support propagating non-linear modes that
are analogous to the vortex rings, or ``smoke rings'', of fluid dynamics. These
are circular loops of {\it magnetic} vorticity which travel at constant
velocity parallel to their axis of symmetry. The topological structure of the
continuum theory has important consequences for the properties of these
magnetic vortex rings. One finds that there exists a sequence of magnetic
vortex rings that are distinguished by a topological invariant (the Hopf
invariant). We present analytical and numerical results for the energies,
velocities and structures of propagating magnetic vortex rings in ferromagnetic
materials.Comment: 4 pages, 3 eps-figures, revtex with epsf.tex and multicol.sty. To
appear in Physical Review Letters. (Postscript problem fixed.
Growth conditions, structure, and superconductivity of pure and metal-doped FeTe1-xSex single crystals
Superconducting single crystals of pure FeTe1 xSex and FeTe0.65Se0.35 doped
with Co, Ni, Cu, Mn, Zn, Mo, Cd, In, Pb, Hg, V, Ga, Mg, Al, Ti, Cr, Sr or Nd
into Fe ions site have been grown applying Bridgman's method. It has been found
that the sharpness of transition to the superconducting state in FeTe1 xSex is
evidently inversely correlated with crystallographic quality of the crystals.
Among all of the studied dopants only Co, Ni and Cu substitute Fe ions in
FeTe0.65Se0.35 crystals. The remaining examined ions do not incorporate into
the crystal structure. Nevertheless, they form inclusions together with
selenium, tellurium and/or iron, what changes the chemical composition of host
matrix and therefore influences Tc value. Small disorder introduced into
magnetic sublattice, by partial replacement of Fe ions by slight amount of
nonmagnetic ions of Cu (~ 1.5 at%) or by magnetic ions of Ni (~ 2 at%) and Co
(~5 at%) with spin value different than that of Fe ion, completely suppresses
superconductivity in FeTe1 xSex system. This indicates that even if
superconductivity is observed in the system containing magnetic ions it can not
survive when the disorder in magnetic ions sublattice is introduced, most
likely because of magnetic scattering of Cooper pairs.Comment: 18 pages, 12 figures, 3 table
Magnetic vortex oscillator driven by dc spin-polarized current
Transfer of angular momentum from a spin-polarized current to a ferromagnet
provides an efficient means to control the dynamics of nanomagnets. A peculiar
consequence of this spin-torque, the ability to induce persistent oscillations
of a nanomagnet by applying a dc current, has previously been reported only for
spatially uniform nanomagnets. Here we demonstrate that a quintessentially
nonuniform magnetic structure, a magnetic vortex, isolated within a nanoscale
spin valve structure, can be excited into persistent microwave-frequency
oscillations by a spin-polarized dc current. Comparison to micromagnetic
simulations leads to identification of the oscillations with a precession of
the vortex core. The oscillations, which can be obtained in essentially zero
magnetic field, exhibit linewidths that can be narrower than 300 kHz, making
these highly compact spin-torque vortex oscillator devices potential candidates
for microwave signal-processing applications, and a powerful new tool for
fundamental studies of vortex dynamics in magnetic nanostructures.Comment: 14 pages, 4 figure
Localized effect of PbI2 excess in perovskite solar cells probed by high-resolution chemical–optoelectronic mapping
We report the laser irradiation of CH3NH3PbI3 solar cells to generate and control localized PbI2 degradation product. We show that by tuning the laser power and illumination time, we can controllably form a local excess of PbI2. High-resolution advanced multi-mapping techniques are used to highlight the effect of PbI2 on the photophysical and photoelectrical properties in a complete perovskite device. Whereas a thick PbI2 film at the perovskite/hole transport layer interface has a detrimental effect on the photocurrent and photoluminescence, a thin PbI2 film (<20 nm) leads to a significant photocurrent increase, which is ascribed to the passivation of non-radiative defects and reduced charge recombination at the interface. Our findings reveal that laser irradiation is a new approach to understand the effect of PbI2 surface layers and potentially offers a means to passivate trap states and improve PV properties of perovskite devices
Field dependence of magnetization reversal by spin transfer
We analyse the effect of the applied field (Happl) on the current-driven
magnetization reversal in pillar-shaped Co/Cu/Co trilayers, where we observe
two different types of transition between the parallel (P) and antiparallel
(AP) magnetic configurations of the Co layers. If Happl is weaker than a rather
small threshold value, the transitions between P and AP are irreversible and
relatively sharp. For Happl exceding the threshold value, the same transitions
are progressive and reversible. We show that the criteria for the stability of
the P and AP states and the experimentally observed behavior can be precisely
accounted for by introducing the current-induced torque of the spin transfer
models in a Landau-Lifschitz-Gilbert equation. This approach also provides a
good description for the field dependence of the critical currents
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