288 research outputs found
Spin-polarized current amplification and spin injection in magnetic bipolar transistors
The magnetic bipolar transistor (MBT) is a bipolar junction transistor with
an equilibrium and nonequilibrium spin (magnetization) in the emitter, base, or
collector. The low-injection theory of spin-polarized transport through MBTs
and of a more general case of an array of magnetic {\it p-n} junctions is
developed and illustrated on several important cases. Two main physical
phenomena are discussed: electrical spin injection and spin control of current
amplification (magnetoamplification). It is shown that a source spin can be
injected from the emitter to the collector. If the base of an MBT has an
equilibrium magnetization, the spin can be injected from the base to the
collector by intrinsic spin injection. The resulting spin accumulation in the
collector is proportional to , where is the proton
charge, is the bias in the emitter-base junction, and is the
thermal energy. To control the electrical current through MBTs both the
equilibrium and the nonequilibrium spin can be employed. The equilibrium spin
controls the magnitude of the equilibrium electron and hole densities, thereby
controlling the currents. Increasing the equilibrium spin polarization of the
base (emitter) increases (decreases) the current amplification. If there is a
nonequilibrium spin in the emitter, and the base or the emitter has an
equilibrium spin, a spin-valve effect can lead to a giant magnetoamplification
effect, where the current amplifications for the parallel and antiparallel
orientations of the the equilibrium and nonequilibrium spins differ
significantly. The theory is elucidated using qualitative analyses and is
illustrated on an MBT example with generic materials parameters.Comment: 14 PRB-style pages, 10 figure
Spin-polarized transport in inhomogeneous magnetic semiconductors: theory of magnetic/nonmagnetic p-n junctions
A theory of spin-polarized transport in inhomogeneous magnetic semiconductors
is developed and applied to magnetic/nonmagnetic p-n junctions. Several
phenomena with possible spintronic applications are predicted, including
spinvoltaic effect, spin valve effect, and giant magnetoresistance. It is
demonstrated that only nonequilibrium spin can be injected across the
space-charge region of a p-n junction, so that there is no spin injection (or
extraction) at low bias.Comment: Minor Revisions. To appear in Phys. Rev. Let
Has the nonlinear Meissner effect been observed?
We examine recent high-precision experimental data on the magnetic field,
, dependence of the penetration depth in
(YBCO) for several field directions in the
plane. In a new theoretical analysis that incorporates the effects of
orthorhombic symmetry, we show that the data at sufficiently high magnetic
fields and low temperatures are in quantitative agreement with the theoretical
predictions of the nonlinear Meissner effect.Comment: 4 text pages plus 3 postscript figure
Spin transport in inhomogeneous magnetic fields: a proposal for Stern-Gerlach-like experiments with conduction electrons
Spin dynamics in spatially inhomogeneous magnetic fields is studied within
the framework of Boltzmann theory. Stern-Gerlach-like separation of spin up and
spin down electrons occurs in ballistic and diffusive regimes, before spin
relaxation sets in. Transient dynamics and spectral response to time-dependent
inhomogeneous magnetic fields are investigated, and possible experimental
observations of our findings are discussed.Comment: 7 pages, 4 figures; revised and extended version, to appear in PR
Low Frequency Nonlinear Magnetic Response of an Unconventional Superconductor
We consider an unconventional superconductor in a low frequency harmonic
magnetic field. In the Meissner regime at low T a nonlinear magnetic response
arises from quasiparticle excitations near minima in the energy gap. Various
physical quantities then acquire higher harmonics of the frequency of the
applied field. We discuss how examination of the field and angular dependence
of these harmonics allows determination of the structure of the energy gap. We
show how to distinguish nodes from small finite minima ("quasinodes"). Gaps
with nodal lines give rise to universal power law field dependences for the
nonlinear magnetic moment and the nonlinear torque. They both have separable
temporal and angular dependences. In contrast, when there are quasinodes these
quantities have more complicated and nonseparable field, temporal, and angular
dependences. We illustrate this on the example of an s+id gap. We discuss how
to perform measurements so as to maximize the nonlinear signal and how to
investigate in detail the properties of the superconducting minima, thus
determining the gap function symmetry.Comment: To appear in Phys Rev B. Ten figures, 13 text page
Coherent spin transport through a 350-micron-thick Silicon wafer
We use all-electrical methods to inject, transport, and detect spin-polarized
electrons vertically through a 350-micron-thick undoped single-crystal silicon
wafer. Spin precession measurements in a perpendicular magnetic field at
different accelerating electric fields reveal high spin coherence with at least
13pi precession angles. The magnetic-field spacing of precession extrema are
used to determine the injector-to-detector electron transit time. These transit
time values are associated with output magnetocurrent changes (from in-plane
spin-valve measurements), which are proportional to final spin polarization.
Fitting the results to a simple exponential spin-decay model yields a
conduction electron spin lifetime (T1) lower bound in silicon of over 500ns at
60K.Comment: Accepted in PR
Angular dependence of the penetration depth in unconventional superconductors
We examine the Meissner state nonlinear electrodynamic effects on the field
and angular dependence of the low temperature penetration depth, , of
superconductors in several kinds of unconventional pairing states, with nodes
or deep minima (``quasinodes'') in the energy gap. Our calculations are
prompted by the fact that, for typical unconventional superconducting material
parameters, the predicted size of these effects for exceeds the
available experimental precision for this quantity by a much larger factor than
for others. We obtain expressions for the nonlinear component of the
penetration depth, , for different two- and three- dimensional
nodal or quasinodal structures. Each case has a characteristic signature as to
its dependence on the size and orientation of the applied magnetic field. This
shows that measurements can be used to elucidate the nodal or
quasinodal structure of the energy gap. For nodal lines we find that
is linear in the applied field, while the dependence is
quadratic for point nodes. For layered materials with
(YBCO) type anisotropy, our results for the
angular dependence of differ greatly from those for tetragonal
materials and are in agreement with experiment. For the two- and three-
dimensional quasinodal cases, is no longer proportional to a
power of the field and the field and angular dependences are not separable,
with a suppression of the overall signal as the node is filled in.Comment: 16 pages plus nine figure
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