351 research outputs found
Geometrical effects on the optical properties of quantum dots doped with a single magnetic atom
The emission spectra of individual self-assembled quantum dots containing a
single magnetic Mn atom differ strongly from dot to dot. The differences are
explained by the influence of the system geometry, specifically the in-plane
asymmetry of the quantum dot and the position of the Mn atom. Depending on both
these parameters, one has different characteristic emission features which
either reveal or hide the spin state of the magnetic atom. The observed
behavior in both zero field and under magnetic field can be explained
quantitatively by the interplay between the exciton-manganese exchange
interaction (dependent on the Mn position) and the anisotropic part of the
electron-hole exchange interaction (related to the asymmetry of the quantum
dot).Comment: 5 pages, 5 figures, to be published in Phys. Rev. Let
Spin and transport effects in quantum microcavities with polarization splitting
Transport properties of exciton-polaritons in anisotropic quantum
microcavities are considered theoretically. Microscopic symmetry of the
structure is taken into account by allowing for both the
longitudinal-transverse (TE-TM) and anisotropic splitting of polariton states.
The splitting is equivalent to an effective magnetic field acting on polariton
pseudospin, and polarization conversion in microcavities is shown to be caused
by an interplay of exciton-polariton spin precession and elastic scattering. In
addition, we considered the spin-dependent interference of polaritons leading
to weak localization and calculated coherent backscattering intensities in
different polarizations. Our findings are in a very good agreement with the
recent experimental data.Comment: 8 pages, 6 figure
Theory of excitons in cubic III-V semiconductor GaAs, InAs and GaN quantum dots: fine structure and spin relaxation
Exciton fine structures in cubic III-V semiconductor GaAs, InAs and GaN
quantum dots are investigated systematically and the exciton spin relaxation in
GaN quantum dots is calculated by first setting up the effective exciton
Hamiltonian. The electron-hole exchange interaction Hamiltonian, which consists
of the long- and short-range parts, is derived within the effective-mass
approximation by taking into account the conduction, heavy- and light-hole
bands, and especially the split-off band. The scheme applied in this work
allows the description of excitons in both the strong and weak confinement
regimes. The importance of treating the direct electron-hole Coulomb
interaction unperturbatively is demonstrated. We show in our calculation that
the light-hole and split-off bands are negligible when considering the exciton
fine structure, even for GaN quantum dots, and the short-range exchange
interaction is irrelevant when considering the optically active doublet
splitting. We point out that the long-range exchange interaction, which is
neglected in many previous works, contributes to the energy splitting between
the bright and dark states, together with the short-range exchange interaction.
Strong dependence of the optically active doublet splitting on the anisotropy
of dot shape is reported. Large doublet splittings up to 600 eV, and even
up to several meV for small dot size with large anisotropy, is shown in GaN
quantum dots. The spin relaxation between the lowest two optically active
exciton states in GaN quantum dots is calculated, showing a strong dependence
on the dot anisotropy. Long exciton spin relaxation time is reported in GaN
quantum dots. These findings are in good agreement with the experimental
results.Comment: 22+ pages, 16 figures, several typos in the published paper are
corrected in re
Imaging spin flows in semiconductors subject to electric, magnetic, and strain fields
Using scanning Kerr microscopy, we directly acquire two-dimensional images of
spin-polarized electrons flowing laterally in bulk epilayers of n:GaAs. Optical
injection provides a local dc source of polarized electrons, whose subsequent
drift and/or diffusion is controlled with electric, magnetic, and - in
particular - strain fields. Spin precession induced by controlled uniaxial
stress along the axes demonstrates the direct k-linear spin-orbit
coupling of electron spin to the shear (off-diagonal) components of the strain
tensor.Comment: 5 pages, 5 color figure
Magnetotransport in disordered delta-doped heterostructures
We discuss theoretically how electrons confined to two dimensions in a
delta-doped heterostructure can arrange themselves in a droplet-like spatial
distribution due to disorder and screening effects when their density is low.
We apply this droplet picture to magnetotransport and derive the expected
dependence on electron density of several quantities relevant to this
transport, in the regimes of weak and moderate magnetic fields. We find good
qualitative and quantitative agreement between our calculations and recent
experiments on delta-doped heterostructures.Comment: 10 pages RevTeX, 2 figures, uses psfrag; published versio
Slow imbalance relaxation and thermoelectric transport in graphene
We compute the electronic component of the thermal conductivity (TC) and the
thermoelectric power (TEP) of monolayer graphene, within the hydrodynamic
regime, taking into account the slow rate of carrier population imbalance
relaxation. Interband electron-hole generation and recombination processes are
inefficient due to the non-decaying nature of the relativistic energy spectrum.
As a result, a population imbalance of the conduction and valence bands is
generically induced upon the application of a thermal gradient. We show that
the thermoelectric response of a graphene monolayer depends upon the ratio of
the sample length to an intrinsic length scale l_Q, set by the imbalance
relaxation rate. At the same time, we incorporate the crucial influence of the
metallic contacts required for the thermopower measurement (under open circuit
boundary conditions), since carrier exchange with the contacts also relaxes the
imbalance. These effects are especially pronounced for clean graphene, where
the thermoelectric transport is limited exclusively by intercarrier collisions.
For specimens shorter than l_Q, the population imbalance extends throughout the
sample; the TC and TEP asymptote toward their zero imbalance relaxation limits.
In the opposite limit of a graphene slab longer than l_Q, at non-zero doping
the TC and TEP approach intrinsic values characteristic of the infinite
imbalance relaxation limit. Samples of intermediate (long) length in the doped
(undoped) case are predicted to exhibit an inhomogeneous temperature profile,
whilst the TC and TEP grow linearly with the system size. In all cases except
for the shortest devices, we develop a picture of bulk electron and hole number
currents that flow between thermally conductive leads, where steady-state
recombination and generation processes relax the accumulating imbalance.Comment: 14 pages, 4 figure
Spin tunneling through an indirect barrier
Spin-dependent tunneling through an indirect bandgap barrier like the
GaAs/AlAs/GaAs heterostructure along [001] direction is studied by the
tight-binding method. The tunneling is characterized by the proportionality of
the Dresselhaus Hamiltonians at and points in the barrier and by
Fano resonances. The present results suggest that large spin polarization can
be obtained for energy windows that exceed significantly the spin splitting. We
also formulate two conditions that are necessary for the existence of energy
windows with large polarization.Comment: 19 pages, 7 figure
Spin precession and alternating spin polarization in spin-3/2 hole systems
The spin density matrix for spin-3/2 hole systems can be decomposed into a
sequence of multipoles which has important higher-order contributions beyond
the ones known for electron systems [R. Winkler, Phys. Rev. B \textbf{70},
125301 (2004)]. We show here that the hole spin polarization and the
higher-order multipoles can precess due to the spin-orbit coupling in the
valence band, yet in the absence of external or effective magnetic fields. Hole
spin precession is important in the context of spin relaxation and offers the
possibility of new device applications. We discuss this precession in the
context of recent experiments and suggest a related experimental setup in which
hole spin precession gives rise to an alternating spin polarization.Comment: 4 pages, 2 figures, to appear in Physical Review Letter
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