79 research outputs found
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
Checkerboard local density of states in striped domains pinned by vortices
Within a Green's function formalism we calculate the electronic structure
around static extended magnetic and non-magnetic perturbations in a d-wave
superconductor. In partucular, we discuss recent elastic neutron scattering and
scanning tunneling experiments on High-T_c cuprates exposed to an applied
magnetic field. A physical picture consisting of antiferromagnetic vortex cores
operating as pinning centers for surrounding stripes is qualitatively
consistent with the neutron data provided the stripes have the usual antiphase
modulation. The low energy electronic structure in such a region reveals a
checkerboard interference pattern consistent with recent scanning tunneling
experiments.Comment: 5 pages, 4 figure
Local density of states around a magnetic impurity in high-Tc superconductors based on the t-J model
The local density of states (LDOS) around a magnetic impurity in high-Tc
superconductors is studied using the two-dimensional t-J model with a realistic
band structure. The order parameters are determined in a self-consistent way
within the Gutzwiller approximation and the Bogoliubov-de Gennes theory. In
sharp contrast with the nonmagnetic impurity case, the LDOS near the magnetic
impurity shows two resonance peaks reflecting the presence of spin-dependent
resonance states. It is also shown that these resonance states are
approximately localized around the impurity. The present results have an large
implication on the scanning tunneling spectroscopy observation of
Bi_{2}Sr_{2}Ca(Cu_{1-x}Ni[Zn]_{x})_{2}O_{8+delta}.Comment: 4 pages, 3 figures, to appear in Phys. Rev. Let
Spin injection through the depletion layer: a theory of spin-polarized p-n junctions and solar cells
A drift-diffusion model for spin-charge transport in spin-polarized {\it p-n}
junctions is developed and solved numerically for a realistic set of material
parameters based on GaAs. It is demonstrated that spin polarization can be
injected through the depletion layer by both minority and majority carriers,
making all-semiconductor devices such as spin-polarized solar cells and bipolar
transistors feasible. Spin-polarized {\it p-n} junctions allow for
spin-polarized current generation, spin amplification, voltage control of spin
polarization, and a significant extension of spin diffusion range.Comment: 4 pages, 3 figure
Theory of optical spectra of polar quantum wells: Temperature effects
Theoretical and numerical calculations of the optical absorption spectra of
excitons interacting with longitudinal-optical phonons in quasi-2D polar
semiconductors are presented. In II-VI semiconductor quantum wells, exciton
binding energy can be tuned on- and off-resonance with the longitudinal-optical
phonon energy by varying the quantum well width. A comprehensive picture of
this tunning effect on the temperature-dependent exciton absorption spectrum is
derived, using the exciton Green's function formalism at finite temperature.
The effective exciton-phonon interaction is included in the Bethe-Salpeter
equation. Numerical results are illustrated for ZnSe-based quantum wells. At
low temperatures, both a single exciton peak as well as a continuum resonance
state are found in the optical absorption spectra. By contrast, at high enough
temperatures, a splitting of the exciton line due to the real phonon absorption
processes is predicted. Possible previous experimental observations of this
splitting are discussed.Comment: 10 pages, 9 figures, to appear in Phys. Rev. B. Permanent address:
[email protected]
Spin-drift transport and its applications
We study the generation of non-equilibrium spin currents in systems with
spatially-inhomogeneous magnetic potentials. For sufficiently high current
densities, the spin polarization can be transported over distances
significantly exceeding the intrinsic spin-diffusion length. This enables
applications that are impossible within the conventional spin-diffusion regime.
Specifically, we propose dc measurement schemes for the carrier spin relaxation
times, and , as well as demonstrate the possibility of spin species
separation by driving current through a region with an inhomogeneous magnetic
potential.Comment: 4 pages, 2 eps figure
Magnetic Anisotropy of Single Mn Acceptors in GaAs in an External Magnetic Field
We investigate the effect of an external magnetic field on the physical
properties of the acceptor hole states associated with single Mn acceptors
placed near the (110) surface of GaAs. Crosssectional scanning tunneling
microscopy images of the acceptor local density of states (LDOS) show that the
strongly anisotropic hole wavefunction is not significantly affected by a
magnetic field up to 6 T. These experimental results are supported by
theoretical calculations based on a tightbinding model of Mn acceptors in GaAs.
For Mn acceptors on the (110) surface and the subsurfaces immediately
underneath, we find that an applied magnetic field modifies significantly the
magnetic anisotropy landscape. However the acceptor hole wavefunction is
strongly localized around the Mn and the LDOS is quite independent of the
direction of the Mn magnetic moment. On the other hand, for Mn acceptors placed
on deeper layers below the surface, the acceptor hole wavefunction is more
delocalized and the corresponding LDOS is much more sensitive on the direction
of the Mn magnetic moment. However the magnetic anisotropy energy for these
magnetic impurities is large (up to 15 meV), and a magnetic field of 10 T can
hardly change the landscape and rotate the direction of the Mn magnetic moment
away from its easy axis. We predict that substantially larger magnetic fields
are required to observe a significant field-dependence of the tunneling current
for impurities located several layers below the GaAs surface.Comment: Non
Localized surface states in HTSC: Alternative mechanism of zero-bias conductance peaks
It is shown that the quasiparticle states localized in the vicinity of
surface imperfections of atomic size can be responsible for the zero-bias
tunneling conductance peaks in high-Tc superconductors. The contribution from
these states can be easily separated from other mechanisms using their
qualitatively different response on an external magnetic field.Comment: REVTeX, 4 pages, 2 figs; to be published in PR
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
Evidence of Doping-Dependent Pairing Symmetry in Cuprate Superconductors
Scanning tunneling spectroscopy (STS) studies reveal long-range spatial
homogeneity and predominantly -pairing spectral characteristics in
under- and optimally doped superconductors, whereas
STS on exhibits {\it
microscopic} spatial modulations and strong scattering near the Zn or Mg
impurity sites, together with global suppression of the pairing potential. In
contrast, in overdoped ,
-pairing symmetry is found, suggesting significant changes in
the superconducting ground-state at a critical doping value.Comment: 4 pages, 4 figures. Published in Physical Review Letters.
Corresponding author: Nai-Chang Yeh (e-mail address: [email protected]
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