62 research outputs found
Geometric and compositional influences on spin-orbit induced circulating currents in nanostructures
Circulating orbital currents, originating from the spin-orbit interaction,
are calculated for semiconductor nanostructures in the shape of spheres, disks,
spherical shells and rings for the electron ground state with spin oriented
along a symmetry axis. The currents and resulting orbital and spin magnetic
moments, which combine to yield the effective electron g factor, are calculated
using a recently introduced formalism that allows the relative contributions of
different regions of the nanostructure to be identified. For all these
spherically or cylindrically symmetric hollow or solid nanostructures,
independent of material composition and whether the boundary conditions are
hard or soft, the dominant orbital current originates from intermixing of
valence band states in the electron ground state, circulates within the
nanostructure, and peaks approximately halfway between the center and edge of
the nanostructure in the plane perpendicular to the spin orientation. For a
specific material composition and confinement character, the confinement energy
and orbital moment are determined by a single size-dependent parameter for
spherically symmetrical nanostructures, whereas they can be independently tuned
for cylindrically symmetric nanostructures.Comment: 22 pages, 20 figure
Spin-orbit-induced circulating currents in a semiconductor nanostructure
Circulating orbital currents produced by the spin-orbit interaction for a
single electron spin in a quantum dot are explicitly evaluated at zero magnetic
field, along with their effect on the total magnetic moment (spin and orbital)
of the electron spin. The currents are dominated by coherent superpositions of
the conduction and valence envelope functions of the electronic state, are
smoothly varying within the quantum dot, and are peaked roughly halfway between
the dot center and edge. Thus the spatial structure of the spin contribution to
the magnetic moment (which is peaked at the dot center) differs greatly from
the spatial structure of the orbital contribution. Even when the spin and
orbital magnetic moments cancel (for ) the spin can interact strongly with
local magnetic fields, e.g. from other spins, which has implications for spin
lifetimes and spin manipulation.Comment: 6 pages, 3 figure
g-Factors and diamagnetic coefficients of electrons, holes and excitons in InAs/InP quantum dots
The electron, hole, and exciton g-factors and diamagnetic coefficients have
been calculated using envelope-function theory for cylindrical InAs/InP quantum
dots in the presence of a magnetic field parallel to the dot symmetry axis. A
clear connection is established between the electron g-factor and the amplitude
of the those valence-state envelope functions which possess non-zero orbital
momentum associated with the envelope function. The dependence of the exciton
diamagnetic coefficients on the quantum dot height is found to correlate with
the energy dependence of the effective mass. Calculated exciton g-factor and
diamagnetic coefficients, constructed from the values associated with the
electron and hole constituents of the exciton, match experimental data well,
however including the Coulomb interaction between the electron and hole states
improves the agreement. Remote-band contributions to the valence-band
electronic structure, included perturbatively, reduce the agreement between
theory and experiment.Comment: 12 pages, 7 figure
Size dependent exciton g-factor in self-assembled InAs/InP quantum dots
We have studied the size dependence of the exciton g-factor in self-assembled
InAs/InP quantum dots. Photoluminescence measurements on a large ensemble of
these dots indicate a multimodal height distribution. Cross-sectional Scanning
Tunneling Microscopy measurements have been performed and support the
interpretation of the macro photoluminescence spectra. More than 160 individual
quantum dots have systematically been investigated by analyzing single dot
magneto-luminescence between 1200nm and 1600 nm. We demonstrate a strong
dependence of the exciton g-factor on the height and diameter of the quantum
dots, which eventually gives rise to a sign change of the g-factor. The
observed correlation between exciton g-factor and the size of the dots is in
good agreement with calculations. Moreover, we find a size dependent anisotropy
splitting of the exciton emission in zero magnetic field.Comment: 15 pages, 7 figure
Single InAs quantum dot arrays and directed self-organization on patterned GaAs (311)B substrates
Formation of laterally ordered single InAs quantum dot (QD) arrays by self-organized anisotropic strain engineering of InGaAs/GaAs superlattice templates on GaAs (311)B by molecular beam epitaxy is achieved through optimization of growth temperature, InAs amount, and annealing. Directed self-organization of these QD arrays is accomplished by coarse substrate patterns providing absolute QD position control over large areas. Due to the absence of one-to-one pattern definition the site-controlled QD arrays exhibit excellent optical properties revealed by resolution limited (80 µeV) linewidth of the low-temperature photoluminescence from individual QDs. © 2009 American Institute of Physics
Spatial structure of Mn-Mn acceptor pairs in GaAs
The local density of states of Mn-Mn pairs in GaAs is mapped with
cross-sectional scanning tunneling microscopy and compared with theoretical
calculations based on envelope-function and tight-binding models. These
measurements and calculations show that the crosslike shape of the Mn-acceptor
wavefunction in GaAs persists even at very short Mn-Mn spatial separations. The
resilience of the Mn-acceptor wave-function to high doping levels suggests that
ferromagnetism in GaMnAs is strongly influenced by impurity-band formation. The
envelope-function and tight-binding models predict similarly anisotropic
overlaps of the Mn wave-functions for Mn-Mn pairs. This anisotropy implies
differing Curie temperatures for Mn -doped layers grown on differently
oriented substrates.Comment: 4 pages, 4 figure
Magneto-gyrotropic effects in semiconductor quantum wells (review)
Magneto-gyrotropic photogalvanic effects in quantum wells are reviewed. We
discuss experimental data, results of phenomenological analysis and microscopic
models of these effects. The current flow is driven by spin-dependent
scattering in low-dimensional structures gyrotropic media resulted in asymmetry
of photoexcitation and relaxation processes. Several applications of the
effects are also considered.Comment: 28 pages, 13 figure
Spatial structure of an individual Mn acceptor in GaAs
The wave function of a hole bound to an individual Mn acceptor in GaAs is
spatially mapped by scanning tunneling microscopy at room temperature and an
anisotropic, cross-like shape is observed. The spatial structure is compared
with that from an envelope-function, effective mass model, and from a
tight-binding model. This demonstrates that anisotropy arising from the cubic
symmetry of the GaAs crystal produces the cross-like shape for the hole
wave-function. Thus the coupling between Mn dopants in GaMnAs mediated by such
holes will be highly anisotropic.Comment: 3 figures, submitted to PR
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
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