2,697 research outputs found
Predicted band structures of III-V semiconductors in wurtzite phase
While non-nitride III-V semiconductors typically have a zincblende structure,
they may also form wurtzite crystals under pressure or when grown as
nanowhiskers. This makes electronic structure calculation difficult since the
band structures of wurtzite III-V semiconductors are poorly characterized. We
have calculated the electronic band structure for nine III-V semiconductors in
the wurtzite phase using transferable empirical pseudopotentials including
spin-orbit coupling. We find that all the materials have direct gaps. Our
results differ significantly from earlier {\it ab initio} calculations, and
where experimental results are available (InP, InAs and GaAs) our calculated
band gaps are in good agreement. We tabulate energies, effective masses, and
linear and cubic Dresselhaus zero-field spin-splitting coefficients for the
zone-center states. The large zero-field spin-splitting coefficients we find
may lead to new functionalities for designing devices that manipulate spin
degrees of freedom
Spin properties of single electron states in coupled quantum dots
Spin properties of single electron states in laterally coupled quantum dots
in the presence of a perpendicular magnetic field are studied by exact
numerical diagonalization. Dresselhaus (linear and cubic) and Bychkov-Rashba
spin-orbit couplings are included in a realistic model of confined dots based
on GaAs. Group theoretical classification of quantum states with and without
spin orbit coupling is provided. Spin-orbit effects on the g-factor are rather
weak. It is shown that the frequency of coherent oscillations (tunneling
amplitude) in coupled dots is largely unaffected by spin-orbit effects due to
symmetry requirements. The leading contributions to the frequency involves the
cubic term of the Dresselhaus coupling. Spin-orbit coupling in the presence of
magnetic field leads to a spin-dependent tunneling amplitude, and thus to the
possibility of spin to charge conversion, namely spatial separation of spin by
coherent oscillations in a uniform magnetic field. It is also shown that spin
hot spots exist in coupled GaAs dots already at moderate magnetic fields, and
that spin hot spots at zero magnetic field are due to the cubic Dresselhaus
term only.Comment: 16 pages, 12 figure
Generation of spin currents and spin densities in systems with reduced symmetry
We show that the spin-current response of a semiconductor crystal to an
external electric field is considerably more complex than previously assumed.
While in systems of high symmetry only the spin-Hall components are allowed, in
systems of lower symmetry other non-spin-Hall components may be present. We
argue that, when spin-orbit interactions are present only in the band
structure, the distinction between intrinsic and extrinsic contributions to the
spin current is not useful. We show that the generation of spin currents and
that of spin densities in an electric field are closely related, and that our
general theory provides a systematic way to distinguish between them in
experiment. We discuss also the meaning of vertex corrections in systems with
spin-orbit interactions.Comment: 4 page
Steady-state spin densities and currents
This article reviews steady-state spin densities and spin currents in
materials with strong spin-orbit interactions. These phenomena are intimately
related to spin precession due to spin-orbit coupling which has no equivalent
in the steady state of charge distributions. The focus will be initially on
effects originating from the band structure. In this case spin densities arise
in an electric field because a component of each spin is conserved during
precession. Spin currents arise because a component of each spin is continually
precessing. These two phenomena are due to independent contributions to the
steady-state density matrix, and scattering between the conserved and
precessing spin distributions has important consequences for spin dynamics and
spin-related effects in general. In the latter part of the article extrinsic
effects such as skew scattering and side jump will be discussed, and it will be
shown that these effects are also modified considerably by spin precession.
Theoretical and experimental progress in all areas will be reviewed
Quantum Spectra of Triangular Billiards on the Sphere
We study the quantal energy spectrum of triangular billiards on a spherical
surface. Group theory yields analytical results for tiling billiards while the
generic case is treated numerically. We find that the statistical properties of
the spectra do not follow the standard random matrix results and their peculiar
behaviour can be related to the corresponding classical phase space structure.Comment: 18 pages, 5 eps figure
On the validity of the Wigner-Seitz approximation in neutron star crust
The inner crust of neutron stars formed of nuclear clusters immersed in a
neutron sea has been widely studied in the framework of the Wigner-Seitz
approximation since the seminal work of Negele and Vautherin. In this article,
the validity of this approximation is discussed in the framework of the band
theory of solids. For a typical cell of Zr, present in the external
layers of the inner crust, it is shown that the ground state properties of the
neutron gas are rather well reproduced by the Wigner-Seitz approximation, while
its dynamical properties depend on the energy scale of the process of interest
or on the temperature. It is concluded that the Wigner-Seitz approximation is
well suited for describing the inner crust of young neutron stars and the
collapsing core of massive stars during supernovae explosions. However the band
theory is required for low temperature fluid dynamics.Comment: 7 pages, with figures - PTH, version
14 September 1941 BERNALILLO County Specimen Collection Data
Specimen collected 14 September 1941. Original Locality: Albuquerque, Rio Grande below Diversion Dam. Locality: Rio Grande, below a diversion dam, Albuquerque.Catalog number: MSB637; Taxa: Pimephales promelas; Common name: fathead minnow; Count of specimens: 96; Standard length:Catalog number: MSB782; Taxa: Rhinichthys cataractae; Common name: longnose dace; Count of specimens: 5; Standard length:Catalog number: MSB1079; Taxa: Gambusia affinis; Common name: western mosquitofish; Count of specimens: 60; Standard length:Catalog number: MSB1162; Taxa: Hybognathus amarus; Common name: Rio Grande silvery minnow; Count of specimens: 33; Standard length:Catalog number: MSB1377; Taxa: Notropis jemezanus; Common name: Rio Grande shiner; Count of specimens: 1; Standard length:Catalog number: MSB1410; Taxa: Notropis simus; Common name: bluntnose shiner; Count of specimens: 25; Standard length:Catalog number: MSB1586; Taxa: Cyprinus carpio; Common name: common carp; Count of specimens: 9; Standard length:Catalog number: MSB1645; Taxa: Gila pandora; Common name: Rio Grande chub; Count of specimens: 73; Standard length:Catalog number: MSB1757; Taxa: Platygobio gracilis; Common name: flathead chub; Count of specimens: 118; Standard length:Catalog number: MSB1875; Taxa: Macrhybopsis aestivalis; Common name: speckled chub; Count of specimens: 147; Standard length:Catalog number: MSB2012; Taxa: Salmo trutta; Common name: brown trout; Count of specimens: 1; Standard length:Catalog number: MSB2074; Taxa: Oncorhynchus mykiss; Common name: rainbow trout; Count of specimens: 13; Standard length:Catalog number: MSB3218; Taxa: Carpiodes carpio; Common name: river carpsucker; Count of specimens: 27; Standard length
Invariant expansion for the trigonal band structure of graphene
We present a symmetry analysis of the trigonal band structure in graphene,
elucidating the transformational properties of the underlying basis functions
and the crucial role of time-reversal invariance. Group theory is used to
derive an invariant expansion of the Hamiltonian for electron states near the K
points of the graphene Brillouin zone. Besides yielding the characteristic
k-linear dispersion and higher-order corrections to it, this approach enables
the systematic incorporation of all terms arising from external electric and
magnetic fields, strain, and spin-orbit coupling up to any desired order.
Several new contributions are found, in addition to reproducing results
obtained previously within tight-binding calculations. Physical ramifications
of these new terms are discussed.Comment: 10 pages, 1 figure; expanded version with more details and additional
result
Heterovalent interlayers and interface states: an ab initio study of GaAs/Si/GaAs (110) and (100) heterostructures
We have investigated ab initio the existence of localized states and
resonances in abrupt GaAs/Si/GaAs (110)- and (100)-oriented heterostructures
incorporating 1 or 2 monolayers (MLs) of Si, as well as in the fully developed
Si/GaAs (110) heterojunction. In (100)-oriented structures, we find both
valence- and conduction-band related near-band edge states localized at the
Si/GaAs interface. In the (110) systems, instead, interface states occur deeper
in the valence band; the highest valence-related resonances being about 1 eV
below the GaAs valence-band maximum. Using their characteristic bonding
properties and atomic character, we are able to follow the evolution of the
localized states and resonances from the fully developed Si/GaAs binary
junction to the ternary GaAs/Si/GaAs (110) systems incorporating 2 or 1 ML of
Si. This approach also allows us to show the link between the interface states
of the (110) and (100) systems. Finally, the conditions for the existence of
localized states at the Si/GaAs (110) interface are discussed based on a
Koster-Slater model developed for the interface-state problem.Comment: REVTeX 4, 14 pages, 15 EPS figure
Quasiparticle transport equation with collision delay. II. Microscopic Theory
For a system of non-interacting electrons scattered by neutral impurities, we
derive a modified Boltzmann equation that includes quasiparticle and virial
corrections. We start from quasiclassical transport equation for
non-equilibrium Green's functions and apply limit of small scattering rates.
Resulting transport equation for quasiparticles has gradient corrections to
scattering integrals. These gradient corrections are rearranged into a form
characteristic for virial corrections
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