314 research outputs found
Tailoring Chirp in Spin-Lasers
The usefulness of semiconductor lasers is often limited by the undesired
frequency modulation, or chirp, a direct consequence of the intensity
modulation and carrier dependence of the refractive index in the gain medium.
In spin-lasers, realized by injecting, optically or electrically,
spin-polarized carriers, we elucidate paths to tailoring chirp. We provide a
generalized expression for chirp in spin-lasers and introduce modulation
schemes that could simultaneously eliminate chirp and enhance the bandwidth, as
compared to the conventional (spin-unpolarized) lasers.Comment: 4 pages, 3 figure
Piezomagnetic Quantum Dots
We study the influence of deformations on magnetic ordering in quantum dots
doped with magnetic impurities. The reduction of symmetry and the associated
deformation from circular to elliptical quantum confinement lead to the
formation of piezomagnetic quantum dots. The strength of elliptical deformation
can be controlled by the gate voltage to change the magnitude of magnetization,
at a fixed number of carriers and in the absence of applied magnetic field. We
reveal a reentrant magnetic ordering with the increase of elliptical
deformation and suggest that the piezomagnetic quantum dots can be used as
nanoscale magnetic switches.Comment: 4 pages, 3 figure
Equilibrium spin currents: Non-Abelian gauge invariance and color diamagnetism in condensed matter
The spin-orbit (SO) interaction in condensed matter can be described in terms
of a non-Abelian potential known in high-energy physics as a color field. I
show that a magnetic component of this color field inevitably generates
diamagnetic color currents which are just the equilibrium spin currents
discussed in a condensed matter context. These dissipationless spin currents
thus represent a universal property of systems with SO interaction. In
semiconductors with linear SO coupling the spin currents are related to the
effective non-Abelian field via Yang-Mills magnetostatics equation.Comment: RevTeX, 4 page
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-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
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
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
Proposal for a phonon laser utilizing quantum-dot spin states
We propose a nanoscale realization of a phonon laser utilizing phonon-assisted spin flips in quantum dots to amplify sound. Owing to a long spin relaxation time, the device can be operated in a strong pumping regime, in which the population inversion is close to its maximal value allowed under Fermi statistics. In this regime, the threshold for stimulated emission is unaffected by spontaneous spin flips. Considering a nanowire with quantum dots defined along its length, we show that a further improvement arises from confining the phonons to one dimension, and thus reducing the number of phonon modes available for spontaneous emission. Our work calls for the development of nanowire-based, high-finesse phonon resonators. © 2013 American Physical Society.A. K. acknowledges financial support from the SPINMET Project (FP7-PEOPLE-2009-IRSES). The work was supported by the ONR, DOE-BES DESC0004890, and NSF-ECCS. X. Hu also acknowledges support by the U.S. ARO and NSF-PIF. The work of V. N. G. was supported by the Spanish Ministry of Economy and Competitiveness under Project No. FIS2011-28851-C02-02.Peer Reviewe
- …