714 research outputs found
Topological Dirac states in asymmetric Pb1-xSnxTe quantum wells
The electronic structure of lead-salt (IV-VI semiconductor) topological
quantum wells (T-QWs) is investigated with analytical solutions of the
effective 4x4 Dimmock k & BULL; p model, which gives an accurate
description of the bands around the fundamental energy gap. Specific
results for three-layer Pb1-xSnxTe nanostructures with varying Sn
composition are presented and the main differences between topological
and normal (N) QWs highlighted. A series of new features are found in
the spectrum of T-QWs, in particular in asymmetric QWs where large
(Rashba spin-orbit) splittings are obtained for the topological Dirac
states inside the gap
Quantum dots based on spin properties of semiconductor heterostructures
The possibility of a novel type of semiconductor quantum dots obtained by
spatially modulating the spin-orbit coupling intensity in III-V
heterostructures is discussed. Using the effective mass model we predict
confined one-electron states having peculiar spin properties. Furthermore, from
mean field calculations (local-spin-density and Hartree-Fock) we find that even
two electrons could form a bound state in these dots.Comment: 9 pages, 3 figures. Accepted in PRB (Brief Report) (2004
Higher order contributions to Rashba and Dresselhaus effects
We have developed a method to systematically compute the form of Rashba- and
Dresselhaus-like contributions to the spin Hamiltonian of heterostructures to
an arbitrary order in the wavevector k. This is achieved by using the double
group representations to construct general symmetry-allowed Hamiltonians with
full spin-orbit effects within the tight-binding formalism. We have computed
full-zone spin Hamiltonians for [001]-, [110]- and [111]-grown zinc blende
heterostructures (D_{2d},C_{4v},C_{2v},C_{3v} point group symmetries), which
are commonly used in spintronics. After an expansion of the Hamiltonian up to
third order in k, we are able to obtain additional terms not found previously.
The present method also provides the matrix elements for bulk zinc blendes
(T_d) in the anion/cation and effective bond orbital model (EBOM) basis sets
with full spin-orbit effects.Comment: v1: 11 pages, 3 figures, 8 table
Rashba spin precession in quantum Hall edge channels
Quasi--one dimensional edge channels are formed at the boundary of a
two-dimensional electron system subject to a strong perpendicular magnetic
field. We consider the effect of Rashba spin--orbit coupling, induced by
structural inversion asymmetry, on their electronic and transport properties.
Both our analytical and numerical results show that spin--split quantum--Hall
edge channels exhibit properties analogous to that of Rashba--split quantum
wires. Suppressed backscattering and a long spin life time render these edge
channels an ideal system for observing voltage--controlled spin precession.
Based on the latter, we propose a magnet--less spin--dependent electron
interferometer.Comment: 7 pages, 6 figure
Anomalous Rashba spin splitting in two-dimensional hole systems
It has long been assumed that the inversion asymmetry-induced Rashba spin
splitting in two-dimensional (2D) systems at zero magnetic field is
proportional to the electric field that characterizes the inversion asymmetry
of the confining potential. Here we demonstrate, both theoretically and
experimentally, that 2D heavy hole systems in accumulation layer-like single
heterostructures show the opposite behavior, i.e., a decreasing, but nonzero
electric field results in an increasing Rashba coefficient.Comment: 4 pages, 3 figure
Viking speech scale
This scale has been developed to classify childrenâs speech production. The ease with which children can make themselves understood using other methods of communication is scored using different scales.
Speech production relies on the control and coordination of several body functions, including breathing and breath control, phonation (vibration of the vocal cords when speaking which creates the voice) and movement of the lips and tongue for articulation. Motor disorders in cerebral palsy can affect individual functions, giving rise to different speech patterns (e.g. reduction in breath control can lead to difficulties controlling the loudness of speech; impaired movements of the vocal cords is associated with breathy or harsh voice; impaired articulation is evident inability to produce some consonants etc). The extent to which each of the individual functions is affected will vary greatly from child to child. We know that the speech functions are adequate if words are perceived correctly by listeners. Although intelligibility is strictly a measure of activity (communicating a message) it relates directly to speech function and can help differentiate levels of impairment
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