7,218 research outputs found
Rashba spin splitting in biased semiconductor quantum wells
Rashba spin splitting (RSS) in biased semiconductor quantum wells is
investigated theoretically based on the eight-band envelope function model. We
find that at large wave vectors, RSS is both nonmonotonic and anisotropic as a
function of in-plane wave vector, in contrast to the widely used linear and
isotropic model. We derive an analytical expression for RSS, which can
correctly reproduce such nonmonotonic behavior at large wave vectors. We also
investigate numerically the dependence of RSS on the various band parameters
and find that RSS increases with decreasing band gap and subband index,
increasing valence band offset, external electric field, and well width. Our
analytical expression for RSS provides a satisfactory explanation to all these
features.Comment: 5 pages, 4 figures, author names corrected, submitted to Phys. Rev.
Lateral spin-orbit interaction and spin polarization in quantum point contacts
We study ballistic transport through semiconductor quantum point contact
systems under different confinement geometries and applied fields. In
particular, we investigate how the {\em lateral} spin-orbit coupling,
introduced by asymmetric lateral confinement potentials, affects the spin
polarization of the current. We find that even in the absence of external
magnetic fields, a variable {\em non-zero spin polarization} can be obtained by
controlling the asymmetric shape of the confinement potential. These results
suggest a new approach to produce spin polarized electron sources and we study
the dependence of this phenomenon on structural parameters and applied magnetic
fields. This asymmetry-induced polarization provides also a plausible
explanation of our recent observations of a 0.5 conductance plateau (in units
of ) in quantum point contacts made on InAs quantum-well structures.
Although our estimates of the required spin-orbit interaction strength in these
systems do not support this explanation, they likely play a role in the effects
enhanced by electron-electron interactions.Comment: Summited to PRB (2009
Real space first-principles derived semiempirical pseudopotentials applied to tunneling magnetoresistance
In this letter we present a real space density functional theory (DFT)
localized basis set semi-empirical pseudopotential (SEP) approach. The method
is applied to iron and magnesium oxide, where bulk SEP and local spin density
approximation (LSDA) band structure calculations are shown to agree within
approximately 0.1 eV. Subsequently we investigate the qualitative
transferability of bulk derived SEPs to Fe/MgO/Fe tunnel junctions. We find
that the SEP method is particularly well suited to address the tight binding
transferability problem because the transferability error at the interface can
be characterized not only in orbital space (via the interface local density of
states) but also in real space (via the system potential). To achieve a
quantitative parameterization, we introduce the notion of ghost semi-empirical
pseudopotentials extracted from the first-principles calculated Fe/MgO bonding
interface. Such interface corrections are shown to be particularly necessary
for barrier widths in the range of 1 nm, where interface states on opposite
sides of the barrier couple effectively and play a important role in the
transmission characteristics. In general the results underscore the need for
separate tight binding interface and bulk parameter sets when modeling
conduction through thin heterojunctions on the nanoscale.Comment: Submitted to Journal of Applied Physic
Spin-Injection Spectroscopy of a Spin-Orbit Coupled Fermi Gas
The coupling of the spin of electrons to their motional state lies at the
heart of recently discovered topological phases of matter. Here we create and
detect spin-orbit coupling in an atomic Fermi gas, a highly controllable form
of quantum degenerate matter. We reveal the spin-orbit gap via spin-injection
spectroscopy, which characterizes the energy-momentum dispersion and spin
composition of the quantum states. For energies within the spin-orbit gap, the
system acts as a spin diode. To fully inhibit transport, we open an additional
spin gap, thereby creating a spin-orbit coupled lattice whose spinful band
structure we probe. In the presence of s-wave interactions, such systems should
display induced p-wave pairing, topological superfluidity, and Majorana edge
states
Spin interference in silicon three-terminal one-dimensional rings
We present the first findings of the spin transistor effect in the Rashba
gate-controlled ring embedded in the p-type self-assembled silicon quantum well
that is prepared on the n-type Si (100) surface. The coherence and phase
sensitivity of the spin-dependent transport of holes are studied by varying the
value of the external magnetic field and the bias voltage that are applied
perpendicularly to the plane of the double-slit ring. Firstly, the amplitude
and phase sensitivity of the 0.7(2e^2/h) feature of the hole quantum
conductance staircase revealed by the quantum point contact inserted in the one
of the arms of the double-slit ring are found to result from the interplay of
the spontaneous spin polarization and the Rashba spin-orbit interaction.
Secondly, the quantum scatterers connected to two one-dimensional leads and the
quantum point contact inserted are shown to define the amplitude and the phase
of the Aharonov-Bohm and the Aharonov-Casher conductance oscillations.Comment: 8 pages, 5 figure
The Host Galaxies of Narrow-Line Seyfert 1s: Evidence for Bar-Driven Fueling
We present a study of the host-galaxy morphologies of narrow- and broad-line
Seyfert 1 galaxies (NLS1s and BLS1s) based on broad-band optical images from
the Hubble Space Telescope archives. We find that large-scale stellar bars,
starting at ~1 kpc from the nucleus, are much more common in NLS1s than BLS1s.
Furthermore, the fraction of NLS1 spirals that have bars increases with
decreasing full-width at half-maximum (FWHM) of the broad component of H-beta.
These results suggest a link between the large-scale bars, which can support
high fueling rates to the inner kpc, and the high mass-accretion rates
associated with the supermassive black holes in NLS1s.Comment: 19 pages, 4 figures (1a, 1b, 2, and 3), Accepted for publication in
the Astronomical Journa
Absence of the Rashba effect in undoped asymmetric quantum wells
To an electron moving in free space an electric field appears as a magnetic
field which interacts with and can reorient the electron spin. In semiconductor
quantum wells this spin-orbit interaction seems to offer the possibility of
gate-voltage control in spintronic devices but, as the electrons are subject to
both ion-core and macroscopic structural potentials, this over-simple picture
has lead to intense debate. For example, an externally applied field acting on
the envelope of the electron wavefunction determined by the macroscopic
potential, underestimates the experimentally observed spin-orbit field by many
orders of magnitude while the Ehrenfest theorem suggests that it should
actually be zero. Here we challenge, both experimentally and theoretically, the
widely held belief that any inversion asymmetry of the macroscopic potential,
not only electric field, will produce a significant spin-orbit field for
electrons. This conclusion has far-reaching consequences for the design of
spintronic devices while illuminating important fundamental physics.Comment: 7 pages, 5 fig
A multiband envelope function model for quantum transport in a tunneling diode
We present a simple model for electron transport in semiconductor devices
that exhibit tunneling between the conduction and valence bands. The model is
derived within the usual Bloch-Wannier formalism by a k-expansion, and is
formulated in terms of a set of coupled equations for the electron envelope
functions. Its connection with other models present in literature is discussed.
As an application we consider the case of a Resonant Interband Tunneling Diode,
demonstrating the ability of the model to reproduce the expected behaviour of
the current as a function of the applied voltageComment: 8 pages, 4 figure
Greenhouse gas scenarios for Austria: a comparison of different approaches to emission trends
In the present paper, national and externally organized projections of greenhouse gas emissions for Austria were compared to gain insight on the underlying scenario data assumptions. National greenhouse gas emission trends extend until 2030, an assessment of European Union (EU) countries to 2050. In addition, data for 2000-2100 was extracted from the global emission database described by the Representative Concentration Pathways (RCP). By identifying trends in these projections, it was possible to produce (a) a long-term assessment of national scenarios until 2100, (b) an assessment of the ambition level toward national climate strategies, and (c) a standardized method to compare trends across countries. By extracting RCP data, Austrian's methane, nitrous oxide,and carbon dioxide emissions up to 2100 could be projected for all sources as well as specific sectors. With respect to the RCP scenario emission data, national projectons did not seem to employ the mitigation potentials available for the most stringent RCP scenario, RCP2.6. Comparing projections that supported the EU Climate Strategy 2030 with national projections revealed similar trends. Because RCP2.6 is the only scenario consistent with a 2 degrees C global warming target, and it is much more ambitious than any of the national or European projections, further measures will be required if Austria is to adequately contribute to this widely accepted policy goal
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