419 research outputs found
Transport and Boundary Scattering in Confined Geometries: Analytical Results
We utilize a geometric argument to determine the effects of boundary
scattering on the carrier mean-free path in samples of various cross sections.
Analytic expressions for samples with rectangular and circular cross sections
are obtained. We also outline a method for incorporating these results into
calculations of the thermal conductivity.Comment: 35 pages, Late
Electron-Phonon Interaction in Embedded Semiconductor Nanostructures
The modification of acoustic phonons in semiconductor nanostructures embedded
in a host crystal is investigated including corrections due to strain within
continuum elasticity theory. Effective elastic constants are calculated
employing {\em ab initio} density functional theory. For a spherical InAs
quantum dot embedded in GaAs barrier material, the electron-phonon coupling is
calculated. Its strength is shown to be suppressed compared to the assumption
of bulk phonons
Cerenkov generation of high-frequency confined acoustic phonons in quantum wells
We analyze the Cerenkov emission of high-frequency confined acoustic phonons
by drifting electrons in a quantum well. We find that the electron drift can
cause strong phonon amplification (generation). A general formula for the gain
coefficient, alpha, is obtained as a function of the phonon frequency and the
structure parameters. The gain coefficient increases sharply in the short-wave
region. For the example of a Si/SiGe/Si device it is shown that the
amplification coefficients of the order of hundreds of 1/cm can be achieved in
the sub-THz frequency range.Comment: 4 pages, 2 figures. Submitted to AP
Quantum theory of intersubband polarons
We present a microscopic quantum theory of intersubband polarons,
quasiparticles originated from the coupling between intersubband transitions
and longitudinal optical phonons. To this aim we develop a second quantized
theory taking into account both the Fr\"ohlich interaction between phonons and
intersubband transitions and the Coulomb interaction between the intersubband
transitions themselves. Our results show that the coupling between the phonons
and the intersubband transitions is extremely intense, thanks both to the
collective nature of the intersubband excitations and to the natural tight
confinement of optical phonons. Not only the coupling is strong enough to
spectroscopically resolve the resonant splitting between the modes (strong
coupling regime), but it can become comparable to the bare frequency of the
excitations (ultrastrong coupling regime). We thus predict the possibility to
exploit intersubband polarons both for applied optoelectronic research, where a
precise control of the phonon resonances is needed, and also to observe
fundamental quantum vacuum physics, typical of the ultrastrong coupling regime
Grain Boundary Loops in Graphene
Topological defects can affect the physical properties of graphene in
unexpected ways. Harnessing their influence may lead to enhanced control of
both material strength and electrical properties. Here we present a new class
of topological defects in graphene composed of a rotating sequence of
dislocations that close on themselves, forming grain boundary loops that either
conserve the number of atoms in the hexagonal lattice or accommodate
vacancy/interstitial reconstruction, while leaving no unsatisfied bonds. One
grain boundary loop is observed as a "flower" pattern in scanning tunneling
microscopy (STM) studies of epitaxial graphene grown on SiC(0001). We show that
the flower defect has the lowest energy per dislocation core of any known
topological defect in graphene, providing a natural explanation for its growth
via the coalescence of mobile dislocations.Comment: 23 pages, 7 figures. Revised title; expanded; updated reference
Determination of step--edge barriers to interlayer transport from surface morphology during the initial stages of homoepitaxial growth
We use analytic formulae obtained from a simple model of crystal growth by
molecular--beam epitaxy to determine step--edge barriers to interlayer
transport. The method is based on information about the surface morphology at
the onset of nucleation on top of first--layer islands in the submonolayer
coverage regime of homoepitaxial growth. The formulae are tested using kinetic
Monte Carlo simulations of a solid--on--solid model and applied to estimate
step--edge barriers from scanning--tunneling microscopy data on initial stages
of Fe(001), Pt(111), and Ag(111) homoepitaxy.Comment: 4 pages, a Postscript file, uuencoded and compressed. Physical Review
B, Rapid Communications, in press
Variational Derivation of Relativistic Fermion-Antifermion Wave Equations in QED
We present a variational method for deriving relativistic two-fermion wave
equations in a Hamiltonian formulation of QED. A reformulation of QED is
performed, in which covariant Green functions are used to solve for the
electromagnetic field in terms of the fermion fields. The resulting modified
Hamiltonian contains the photon propagator directly. The reformulation permits
one to use a simple Fock-space variational trial state to derive relativistic
fermion-antifermion wave equations from the corresponding quantum field theory.
We verify that the energy eigenvalues obtained from the wave equation agree
with known results for positronium.Comment: 25 pages, accepted in Journal of Mathematical Physics (2004
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