2,207 research outputs found
Theory of coherent acoustic phonons in InGaN/GaN multi-quantum wells
A microscopic theory for the generation and propagation of coherent LA
phonons in pseudomorphically strained wurzite (0001) InGaN/GaN multi-quantum
well (MQW) p-i-n diodes is presented. The generation of coherent LA phonons is
driven by photoexcitation of electron-hole pairs by an ultrafast Gaussian pump
laser and is treated theoretically using the density matrix formalism. We use
realistic wurzite bandstructures taking valence-band mixing and strain-induced
piezo- electric fields into account. In addition, the many-body Coulomb
ineraction is treated in the screened time-dependent Hartree-Fock
approximation. We find that under typical experimental conditions, our
microscopic theory can be simplified and mapped onto a loaded string problem
which can be easily solved.Comment: 20 pages, 17 figure
Neutron scattering study of spin ordering and stripe pinning in superconducting LaSrCuO
The relationships among charge order, spin fluctuations, and
superconductivity in underdoped cuprates remain controversial. We use neutron
scattering techniques to study these phenomena in
LaSrCuO, a superconductor with a transition temperature
of ~K. At , we find incommensurate spin fluctuations with a
quasielastic energy spectrum and no sign of a gap within the energy range from
0.2 to 15 meV. A weak elastic magnetic component grows below ~K,
consistent with results from local probes. Regarding the atomic lattice, we
have discovered unexpectedly strong fluctuations of the CuO octahedra about
Cu-O bonds, which are associated with inequivalent O sites within the CuO
planes. Furthermore, we observed a weak elastic superlattice peak
that implies a reduced lattice symmetry. The presence of inequivalent O sites
rationalizes various pieces of evidence for charge stripe order in underdoped
\lsco. The coexistence of superconductivity with quasi-static spin-stripe order
suggests the presence of intertwined orders; however, the rotation of the
stripe orientation away from the Cu-O bonds might be connected with evidence
for a finite gap at the nodal points of the superconducting gap function.Comment: 13 pages, 11 figures; accepted versio
Electron transport and terahertz gain in quantum-dot cascades
Electron transport through quantum-dot (QD) cascades was investigated using the formalism of nonequilibrium Green's functions within the self-consistent Born approximation. Polar coupling to optical phonons, deformation potential coupling to acoustic phonons, as well as anharmonic decay of longitudinal optical phonons were included in the simulation. A QD cascade laser structure comprising two QDs per period was designed and its characteristics were simulated. Significant values of population inversion enabling lasing in the terahertz frequency range were predicted, with operating current densities being more than an order of magnitude smaller than in existing terahertz quantum-well-based quantum-cascade lasers
Coherent and robust high-fidelity generation of a biexciton in a quantum dot by rapid adiabatic passage
A biexciton in a semiconductor quantum dot is a source of
polarization-entangled photons with high potential for implementation in
scalable systems. Several approaches for non-resonant, resonant and
quasi-resonant biexciton preparation exist, but all have their own
disadvantages, for instance low fidelity, timing jitter, incoherence or
sensitivity to experimental parameters. We demonstrate a coherent and robust
technique to generate a biexciton in an InGaAs quantum dot with a fidelity
close to one. The main concept is the application of rapid adiabatic passage to
the ground state-exciton-biexciton system. We reinforce our experimental
results with simulations which include a microscopic coupling to phonons.Comment: Main manuscript 5 pages and 4 figures, Supplementary Information 5
pages and 3 figures, accepted as a Rapid Communication in PRB. arXiv admin
note: text overlap with arXiv:1701.0130
Engineering and Manipulating Exciton Wave Packets
When a semiconductor absorbs light, the resulting electron-hole superposition
amounts to a uncontrolled quantum ripple that eventually degenerates into
diffusion. If the conformation of these excitonic superpositions could be
engineered, though, they would constitute a new means of transporting
information and energy. We show that properly designed laser pulses can be used
to create such excitonic wave packets. They can be formed with a prescribed
speed, direction and spectral make-up that allows them to be selectively
passed, rejected or even dissociated using superlattices. Their coherence also
provides a handle for manipulation using active, external controls. Energy and
information can be conveniently processed and subsequently removed at a distant
site by reversing the original procedure to produce a stimulated emission. The
ability to create, manage and remove structured excitons comprises the
foundation for opto-excitonic circuits with application to a wide range of
quantum information, energy and light-flow technologies. The paradigm is
demonstrated using both Tight-Binding and Time-Domain Density Functional Theory
simulations.Comment: 16 figure
Ultrafast optical manipulation of atomic arrangements in chalcogenide alloy memory materials
A class of chalcogenide alloy materials that shows significant changes in
optical properties upon an amorphous-to-crystalline phase transition has lead
to development of large data capacities in modern optical data storage. Among
chalcogenide phase-change materials, Ge2Sb2Te5 (GST) is most widely used
because of its reliability. We use a pair of femtosecond light pulses to
demonstrate the ultrafast optical manipulation of atomic arrangements from
tetrahedral (amorphous) to octahedral (crystalline) Ge-coordination in GST
superlattices. Depending on the parameters of the second pump-pulse, ultrafast
nonthermal phase-change occurred within only few-cycles (~ 1 ps) of the
coherent motion corresponding to a GeTe4 local vibration. Using the ultrafast
switch in chalcogenide alloy memory could lead to a major paradigm shift in
memory devices beyond the current generation of silicon-based flash-memory.Comment: 11 pages, 7 figures, accepted for publication in Optics Expres
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