397 research outputs found
Optical and electrical spin injection and spin transport in hybrid Fe/GaAs devices
We discuss methods for imaging the nonequilibrium spin polarization of
electrons in Fe/GaAs spin transport devices. Both optically- and
electrically-injected spin distributions are studied by scanning
magneto-optical Kerr rotation microscopy. Related methods are used to
demonstrate electrical spin detection of optically-injected spin polarized
currents. Dynamical properties of spin transport are inferred from studies
based on the Hanle effect, and the influence of strain on spin transport data
in these devices is discussed.Comment: 5 pages, 6 figs. ICPS-28 proceedings (July'06, Vienna) for J. Appl.
Phy
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Intermittent release of transients in the slow solar wind: 2. In situ evidence
In paper 1, we showed that the Heliospheric Imager (HI) instruments on the pair of NASA STEREO spacecraft can be used to image the streamer belt and, in particular, the variability of the slow solar wind which originates near helmet streamers. The observation of intense intermittent transient outflow by HI implies that the corresponding in situ observations of the slow solar wind and corotating interaction regions (CIRs) should contain many signatures of transients. In the present paper, we compare the HI observations with in situ measurements from the STEREO and ACE spacecraft. Analysis of the solar wind ion, magnetic field, and suprathermal electron flux measurements from
the STEREO spacecraft reveals the presence of both closed and partially disconnected interplanetary magnetic field lines permeating the slow solar wind. We predict that one of the transients embedded within the second CIR (CIR‐D in paper 1) should impact the near‐Earth ACE spacecraft. ACE measurements confirm the presence of a transient at the time of CIR passage; the transient signature includes helical magnetic fields and bidirectional suprathermal electrons. On the same day, a strahl electron dropout is observed at STEREO‐B, correlated with the passage of a high plasma beta structure. Unlike ACE, STEREO‐B observes the transient a few hours ahead of the CIR. STEREO‐A, STEREO‐B, and ACE spacecraft observe very different slow solar wind properties ahead of and during the CIR analyzed in this paper, which we associate with the intermittent release of transients
Stability of trions in strongly spin-polarized two-dimensional electron gases
Low-temperature magneto-photoluminescence studies of negatively charged
excitons (X- trions) are reported for n-type modulation-doped ZnSe/Zn(Cd,Mn)Se
quantum wells over a wide range of Fermi energy and spin-splitting. The
magnetic composition is chosen such that these magnetic two-dimensional
electron gases (2DEGs) are highly spin-polarized even at low magnetic fields,
throughout the entire range of electron densities studied (5e10 to 6.5e11
cm^-2). This spin polarization has a pronounced effect on the formation and
energy of X-, with the striking result that the trion ionization energy (the
energy separating X- from the neutral exciton) follows the temperature- and
magnetic field-tunable Fermi energy. The large Zeeman energy destabilizes X- at
the nu=1 quantum limit, beyond which a new PL peak appears and persists to 60
Tesla, suggesting the formation of spin-triplet charged excitons.Comment: 5 pages (RevTex), 4 embedded EPS figs. Submitted to PRB-R
Revealing exciton masses and dielectric properties of monolayer semiconductors with high magnetic fields
In semiconductor physics, many essential optoelectronic material parameters
can be experimentally revealed via optical spectroscopy in sufficiently large
magnetic fields. For monolayer transition-metal dichalcogenide semiconductors,
this field scale is substantial --tens of teslas or more-- due to heavy carrier
masses and huge exciton binding energies. Here we report absorption
spectroscopy of monolayer MoS, MoSe, MoTe, and WS in very high
magnetic fields to 91~T. We follow the diamagnetic shifts and valley Zeeman
splittings of not only the exciton's ground state but also its excited
, , ..., Rydberg states. This provides a direct experimental
measure of the effective (reduced) exciton masses and dielectric properties.
Exciton binding energies, exciton radii, and free-particle bandgaps are also
determined. The measured exciton masses are heavier than theoretically
predicted, especially for Mo-based monolayers. These results provide essential
and quantitative parameters for the rational design of opto-electronic van der
Waals heterostructures incorporating 2D semiconductors.Comment: updated; now also including data on MoTe2. Accepted & in press,
Nature Commu
Systematically Asymmetric Heliospheric Magnetic Field: Evidence for a Quadrupole Mode and Non-axisymmetry with Polarity Flip-flops
Recent studies of the heliospheric magnetic field (HMF) have detected
interesting, systematic hemispherical and longitudinal asymmetries which have a
profound significance for the understanding of solar magnetic fields. The in
situ HMF measurements since 1960s show that the heliospheric current sheet
(HCS) is systematically shifted (coned) southward during solar minimum times,
leading to the concept of a bashful ballerina. While temporary shifts can be
considerably larger, the average HCS shift (coning) angle is a few degrees,
less than the tilt of the solar rotation axis. Recent solar
observations during the last two solar cycles verify these results and show
that the magnetic areas in the northern solar hemisphere are larger and their
intensity weaker than in the south during long intervals in the late declining
to minimum phase. The multipole expansion reveals a strong quadrupole term
which is oppositely directed to the dipole term. These results imply that the
Sun has a symmetric quadrupole S0 dynamo mode that oscillates in phase with the
dominant dipole A0 mode. Moreover, the heliospheric magnetic field has a strong
tendency to produce solar tilts that are roughly opposite in longitudinal
phase. This implies is a systematic longitudinal asymmetry and leads to a
"flip-flop" type behaviour in the dominant HMF sector whose period is about 3.2
years. This agrees very well with the similar flip-flop period found recently
in sunspots, as well as with the observed ratio of three between the activity
cycle period and the flip-flop period of sun-like stars. Accordingly, these
results require that the solar dynamo includes three modes, A0, S0 and a
non-axisymmetric mode. Obviously, these results have a great impact on solar
modelling.Comment: 13 pages, 4 figures, Solar Physics, Topical Issue of Space Climate
Symposium, in pres
The density dependence of the transition temperature in a homogenous Bose flui
Transition temperature data obtained as a function of particle density in the
He-Vycor system are compared with recent theoretical calculations for 3D
Bose condensed systems. In the low density dilute Bose gas regime we find, in
agreement with theory, a positive shift in the transition temperature of the
form . At higher densities a maximum is
found in the ratio of for a value of the interaction parameter,
na, that is in agreement with path-integral Monte Carlo calculations.Comment: 4 pages, 3 figure
"Quasi two-dimensional" spin distributions in II-VI magnetic semiconductor heterostructures: Clustering and dimensionality
Spin clustering in diluted magnetic semiconductors (DMS) arises from
antiferromagnetic exchange between neighboring magnetic cations and is a strong
function of reduced dimensionality. Epitaxially-grown single monolayers and
abrupt interfaces of DMS are, however, never perfectly two-dimensional (2D) due
to the unavoidable inter-monolayer mixing of atoms during growth. Thus the
magnetization of DMS heterostructures, which is strongly modified by spin
clustering, is intermediate between that of 2D and 3D spin distributions. We
present an exact calculation of spin clustering applicable to arbitrary
distributions of magnetic spins in the growth direction. The results reveal a
surprising insensitivity of the magnetization to the form of the intermixing
profile, and identify important limits on the maximum possible magnetization.
High-field optical studies of heterostructures containing "quasi-2D" spin
distributions are compared with calculation.Comment: 5 pages (RevTeX), 5 embedded EPS figs, published in PRB v61 p1736
(2000
The effect of disorder on the critical temperature of a dilute hard sphere gas
We have performed Path Integral Monte Carlo (PIMC) calculations to determine
the effect of quenched disorder on the superfluid density of a dilute 3D hard
sphere gas. The disorder was introduced by locating set of hard cylinders
randomly inside the simulation cell. Our results indicate that the disorder
leaves the superfluid critical temperature basically unchanged. Comparison to
experiments of helium in Vycor is made.Comment: 4 pages, 4 figure
Ultrafast spin dynamics and critical behavior in half-metallic ferromagnet : Sr_2FeMoO_6
Ultrafast spin dynamics in ferromagnetic half-metallic compound Sr_2FeMoO_6
is investigated by pump-probe measurements of magneto-optical Kerr effect.
Half-metallic nature of this material gives rise to anomalous thermal
insulation between spins and electrons, and allows us to pursue the spin
dynamics from a few to several hundred picoseconds after the optical
excitation. The optically detected magnetization dynamics clearly shows the
crossover from microscopic photoinduced demagnetization to macroscopic critical
behavior with universal power law divergence of relaxation time for wide
dynamical critical region.Comment: 14 pages, 4 figures. Abstract and Figures 1 & 3 are correcte
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