183 research outputs found
Proof of the cases of the Lieb-Seiringer formulation of the Bessis-Moussa-Villani conjecture
It is shown that the polynomial has
nonnegative coefficients when and A and B are any two complex
positive semidefinite matrices with arbitrary . This proofs a
general nontrivial case of the Lieb-Seiringer formulation of the
Bessis-Moussa-Villani conjecture which is a long standing problem in
theoretical physics.Comment: 5 pages; typos corrected; accepted for publication in Journal of
Statistical Physic
Transient terahertz spectroscopy of excitons and unbound carriers in quasi two-dimensional electron-hole gases
We report a comprehensive experimental study and detailed model analysis of
the terahertz dielectric response and density kinetics of excitons and unbound
electron-hole pairs in GaAs quantum wells. A compact expression is given, in
absolute units, for the complex-valued terahertz dielectric function of
intra-excitonic transitions between the 1s and higher-energy exciton and
continuum levels. It closely describes the terahertz spectra of resonantly
generated excitons. Exciton ionization and formation are further explored,
where the terahertz response exhibits both intra-excitonic and Drude features.
Utilizing a two-component dielectric function, we derive the underlying exciton
and unbound pair densities. In the ionized state, excellent agreement is found
with the Saha thermodynamic equilibrium, which provides experimental
verification of the two-component analysis and density scaling. During exciton
formation, in turn, the pair kinetics is quantitatively described by a Saha
equilibrium that follows the carrier cooling dynamics. The terahertz-derived
kinetics is, moreover, consistent with time-resolved luminescence measured for
comparison. Our study establishes a basis for tracking pair densities via
transient terahertz spectroscopy of photoexcited quasi-two-dimensional
electron-hole gases.Comment: 14 pages, 8 figures, final versio
Electron spin orientation under in-plane optical excitation in GaAs quantum wells
We study the optical orientation of electron spins in GaAs/AlGaAs quantum
wells for excitation in the growth direction and for in-plane excitation. Time-
and polarization-resolved photoluminescence excitation measurements show, for
resonant excitation of the heavy-hole conduction band transition, a negligible
degree of electron spin polarization for in-plane excitation and nearly 100%
for excitation in the growth direction. For resonant excitation of the
light-hole conduction band transition, the excited electron spin polarization
has the same (opposite) direction for in-plane excitation (in the growth
direction) as for excitation into the continuum. The experimental results are
well explained by an accurate multiband theory of excitonic absorption taking
fully into account electron-hole Coulomb correlations and heavy-hole light-hole
coupling.Comment: 10 pages, 4 figures, final versio
Ultrafast pump-probe dynamics in ZnSe-based semiconductor quantum-wells
Pump-probe experiments are used as a controllable way to investigate the
properties of photoexcited semiconductors, in particular, the absorption
saturation. We present an experiment-theory comparison for ZnSe quantum wells,
investigating the energy renormalization and bleaching of the excitonic
resonances. Experiments were performed with spin-selective excitation and
above-bandgap pumping. The model, based on the semiconductor Bloch equations in
the screened Hartree-Fock approximation, takes various scattering processes
into account phenomenologically. Comparing numerical results with available
experimental data, we explain the experimental results and find that the
electron spin-flip occurs on a time scale of 30 ps.Comment: 10 pages, 9 figures. Key words: nonlinear and ultrafast optics,
modeling of femtosecond pump-probe experiments, electron spin-flip tim
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
Basic obstacle for electrical spin-injection from a ferromagnetic metal into a diffusive semiconductor
We have calculated the spin-polarization effects of a current in a two
dimensional electron gas which is contacted by two ferromagnetic metals. In the
purely diffusive regime, the current may indeed be spin-polarized. However, for
a typical device geometry the degree of spin-polarization of the current is
limited to less than 0.1%, only. The change in device resistance for parallel
and antiparallel magnetization of the contacts is up to quadratically smaller,
and will thus be difficult to detect.Comment: Revtex, 4 pages, 3 figures (eps), Definition of spin pilarization
changed to standard definition in GMR, some straight forward algebra removed.
To appear as PRB Rap. Comm. August 15t
Rashba precession in quantum wires with interaction
Rashba precession of spins moving along a one-dimensional quantum channel is
calculated, accounting for Coulomb interactions. The Tomonaga--Luttinger model
is formulated in the presence of spin-orbit scattering and solved by
Bosonization. Increasing interaction strength at decreasing carrier density is
found to {\sl enhance} spin precession and the nominal Rashba parameter due to
the decreasing spin velocity compared with the Fermi velocity. This result can
elucidate the observed pronounced changes of the spin splitting on applied gate
voltages which are estimated to influence the interface electric field in
heterostructures only little.Comment: now replaced by published versio
Spin-polarized transport and Andreev reflection in semiconductor/superconductor hybrid structures
We show that spin-polarized electron transmission across
semiconductor/superconductor (Sm/S) hybrid structures depends sensitively on
the degree of spin polarization as well as the strengths of potential and
spin-flip scattering at the interface. We demonstrate that increasing the Fermi
velocity mismatch in the Sm and S regions can lead to enhanced junction
transparency in the presence of spin polarization. We find that the Andreev
reflection amplitude at the superconducting gap energy is a robust measure of
the spin polarization magnitude, being independent of the strengths of
potential and spin-flip scattering and the Fermi velocity of the
superconductor.Comment: 4 pages, 2 figure
Eliminating Error in the Chemical Abundance Scale for Extragalactic HII Regions
In an attempt to remove the systematic errors which have plagued the
calibration of the HII region abundance sequence, we have theoretically modeled
the extragalactic HII region sequence. We then used the theoretical spectra so
generated in a double blind experiment to recover the chemical abundances using
both the classical electron temperature + ionization correction factor
technique, and the technique which depends on the use of strong emission lines
(SELs) in the nebular spectrum to estimate the abundance of oxygen. We find a
number of systematic trends, and we provide correction formulae which should
remove systematic errors in the electron temperature + ionization correction
factor technique. We also provide a critical evaluation of the various
semi-empirical SEL techniques. Finally, we offer a scheme which should help to
eliminate systematic errors in the SEL-derived chemical abundance scale for
extragalactic HII regions.Comment: 24 pages, 9 Tables, 13 figures, accepted for publication in MNRAS.
Updated considering minor changes during the final edition process and some
few missing reference
Photo-Induced Spin Dynamics in Semiconductor Quantum Wells
We experimentally investigate the dynamics of spins in GaAs quantum wells under applied electric bias by photoluminescence (PL) measurements excited with circularly polarized light. The bias-dependent circular polarization of PL (PPL) with and without magnetic field is studied. ThePPLwithout magnetic field is found to be decayed with an enhancement of increasing the strength of the negative bias. However,PPLin a transverse magnetic field shows oscillations under an electric bias, indicating that the precession of electron spin occurs in quantum wells. The results are discussed based on the electronâhole exchange interaction in the electric field
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