431 research outputs found
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
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
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
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
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
Electron spin relaxation in carbon nanotubes
The long standing problem of inexplicably short spin relaxation in carbon
nanotubes (CNTs) is examined. The curvature-mediated spin-orbital interaction
is shown to induce fluctuating electron spin precession causing efficient
relaxation in a manner analogous to the Dyakonov-Perel mechanism. Our
calculation estimates longitudinal (spin-flip) and transversal (decoherence)
relaxation times as short as 150 ps and 110 ps at room temperature,
respectively, along with a pronounced anisotropic dependence. Interference of
electrons originating from different valleys can lead to even faster dephasing.
The results can help clarify the measured data, resolving discrepancies in the
literature.Comment: 9 pages, 3 figure
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