3,649 research outputs found
InN dielectric function from the midinfrared to the visible range
The dispersion of the dielectric function for wurtzite InN is analytically
evaluated in the region near the fundamental energy gap. The real part of the
dielectric function has a logarithmic singularity at the absorption edge. This
results in the large contribution into the optical dielectric constant. For
samples with degenerate carriers, the real part of the dielectric function is
divergent at the absorption edge. The divergence is smeared with temperatures
or relaxation rate. The imaginary part of the dielectric function has a plateau
far away from the absorption onset.Comment: 5 pages, 2 figure
What Does Certification Tell Us About Teacher Effectiveness? Evidence from New York City
We use six years of data on student test performance to evaluate the effectiveness of certified, uncertified, and alternatively certified teachers in the New York City public schools. On average, the certification status of a teacher has at most small impacts on student test performance. However, among those with the same certification status, there are large and persistent differences in teacher effectiveness. This evidence suggests that classroom performance during the first two years, rather than certification status, is a more reliable indicator of a teacher's future effectiveness. We also evaluate turnover among teachers with different certification status, and the impact on student achievement of hiring teachers with predictably high turnover. Given relatively modest estimates of experience differentials, even high turnover groups (such as Teach for America participants) would have to be only slightly more effective in their first year to offset the negative effects of their high exit rates.
Electromagnetic field induced suppression of transport through - junctions in graphene
We study quasi-particle transmission through an - junction in a
graphene irradiated by an electromagnetic field (EF). In the absence of EF the
electronic spectrum of undoped graphene is gapless, and one may expect the
perfect transmission of quasi-particles flowing perpendicular to the junction.
We demonstrate that the resonant interaction of propagating quasi-particles
with the component of EF parallel to the junction induces a
\textit{non-equilibrium dynamic gap} between electron and hole
bands in the quasi-particle spectrum of graphene. In this case the strongly
suppressed quasi-particle transmission is only possible due to interband
tunnelling. The effect may be used for controlling transport properties of
diverse structures in graphene, like, e.g., -- transistors, single
electron transistors, quantum dots, etc., by variation of the intensity and
frequency of the external radiation.Comment: 5 pages, 3 figure
Nonlinear screening and ballistic transport in a graphene p-n junction
We study the charge density distribution, the electric field profile, and the
resistance of an electrostatically created lateral p-n junction in graphene. We
show that the electric field at the interface of the electron and hole regions
is strongly enhanced due to limited screening capacity of Dirac quasiparticles.
Accordingly, the junction resistance is lower than estimated in previous
literature.Comment: 4 pages, 2 figures. (v1) Original version (v2) Introduction largely
rewritten, minor typos fixed throughou
Choosing a basis that eliminates spurious solutions in k.p theory
A small change of basis in k.p theory yields a Kane-like Hamiltonian for the
conduction and valence bands of narrow-gap semiconductors that has no spurious
solutions, yet provides an accurate fit to all effective masses. The theory is
shown to work in superlattices by direct comparison with first-principles
density-functional calculations of the valence subband structure. A
reinterpretation of the standard data-fitting procedures used in k.p theory is
also proposed.Comment: 15 pages, 2 figures; v3: expanded with much new materia
Tunable quantum spin Hall effect in double quantum wells
The field of topological insulators (TIs) is rapidly growing. Concerning
possible applications, the search for materials with an easily controllable TI
phase is a key issue. The quantum spin Hall effect, characterized by a single
pair of helical edge modes protected by time-reversal symmetry, has been
demonstrated in HgTe-based quantum wells (QWs) with an inverted bandgap. We
analyze the topological properties of a generically coupled HgTe-based double
QW (DQW) and show how in such a system a TI phase can be driven by an
inter-layer bias voltage, even when the individual layers are non-inverted. We
argue, that this system allows for similar (layer-)pseudospin based physics as
in bilayer graphene but with the crucial absence of a valley degeneracy.Comment: 9 pages, 8 figures, extended version (accepted Phys. Rev. B
Optimal rotations of deformable bodies and orbits in magnetic fields
Deformations can induce rotation with zero angular momentum where dissipation
is a natural ``cost function''. This gives rise to an optimization problem of
finding the most effective rotation with zero angular momentum. For certain
plastic and viscous media in two dimensions the optimal path is the orbit of a
charged particle on a surface of constant negative curvature with magnetic
field whose total flux is half a quantum unit.Comment: 4 pages revtex, 4 figures + animation in multiframe GIF forma
Josephson Current and Noise at a Superconductor-Quantum Spin Hall Insulator-Superconductor Junction
We study junctions between superconductors mediated by the edge states of a
quantum spin Hall insulator. We show that such junctions exhibit a fractional
Josephson effect, in which the current phase relation has a 4\pi, rather than a
2\pi periodicity. This effect is a consequence of the conservation of fermion
parity - the number of electrons modulo 2 - in a superconducting junction, and
is closely related to the Z_2 topological structure of the quantum spin Hall
insulator. Inelastic processes, which violate the conservation of fermion
parity, lead to telegraph noise in the equilibrium supercurrent. We predict
that the low frequency noise due these processes diverges exponentially with
temperature T as T -> 0. Possible experiments on HgCdTe quantum wells will be
discussed.Comment: 4 pages, 2 figure
Gate-Controlled Electron Spin Resonance in a GaAs/AlGaAs Heterostructure
The electron spin resonance (ESR) of two-dimensional electrons is
investigated in a gated GaAs/AlGaAs heterostructure. We found that the ESR
resonance frequency can be turned by means of a gate voltage. The front and
back gates of the heterostructure produce opposite g-factor shift, suggesting
that electron g-factor is being electrostatically controlled by shifting the
equilibrium position of the electron wave function from one epitaxial layer to
another with different g-factors
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