9,973 research outputs found
Quantized circular photogalvanic effect in Weyl semimetals
The circular photogalvanic effect (CPGE) is the part of a photocurrent that
switches depending on the sense of circular polarization of the incident light.
It has been consistently observed in systems without inversion symmetry and
depends on non-universal material details. Here we find that in a class of Weyl
semimetals (e.g. SrSi) and three-dimensional Rashba materials (e.g. doped
Te) without inversion and mirror symmetries, the injection contribution to the
CPGE trace is effectively quantized in terms of the fundamental constants and with no material-dependent parameters. This is so
because the CPGE directly measures the topological charge of Weyl points, and
non-quantized corrections from disorder and additional bands can be small over
a significant range of incident frequencies. Moreover, the magnitude of the
CPGE induced by a Weyl node is relatively large, which enables the direct
detection of the monopole charge with current techniques.Comment: 4+5 pages, 3 figures, published versio
Universal Rashba Spin Precession of Two-Dimensional Electrons and Holes
We study spin precession due to Rashba spin splitting of electrons and holes
in semiconductor quantum wells. Based on a simple analytical expression that we
derive for the current modulation in a broad class of experimental situations
of ferromagnet/nonmagnetic semiconductor/ferromagnet hybrid structures, we
conclude that the Datta-Das spin transistor (i) is feasible with holes and (ii)
its functionality is not affected by integration over injection angles. The
current modulation shows a universal oscillation period, irrespective of the
different forms of the Rashba Hamiltonian for electrons and holes. The analytic
formulas approximate extremely well exact numerical calculations of a more
elaborate Kohn--Luttinger model.Comment: 7 pages, 2 eps figures included, minor change
Energy relaxation dynamics and universal scaling laws in organic light emitting diodes
Electron-hole (e-h) capture in luminescent conjugated polymers (LCPs) is
modeled by the dissipative dynamics of a multilevel electronic system coupled
to a phonon bath. Electroinjected e-h pairs are simulated by a mixed quantum
state, which relaxes via phonon-driven internal conversions to low-lying
charge-transfer (CT) and excitonic (XT) states. The underlying two-band polymer
model reflects PPV and spans monoexcited configuration interaction singlets (S)
and triplets (T), coupled to Franck-Condon active C=C stretches and
ring-torsions. Focusing entirely upon long PPV chains, we consider the
recombination kinetics of an initially separated CT pair. Our model
calculations indicated that S and T recombination proceeds according to a
branched, two-step mechanism dictated by near e-h symmetry. The initial
relaxation occurs rapidly with nearly half of the population going into
excitons ( or ), while the remaining portion remains locked in
metastable CT states. While formation rates of and are nearly
equal, is formed about twice as fast in concurrence with
experimental observations of these systems. Furthermore, breaking e-h symmetry
suppresses the XT to CT branching ratio for triplets and opens a slow CT
XT conversion channel exclusively for singlets due to dipole-dipole
interactions between geminate and non-geminate configurations. Finally, our
calculations yield a remarkable linear relation between chain length and
singlet/triplet branching ratio which can be explained in terms of the binding
energies of the respective final excitonic states and the scaling of
singlet-triplet energy gap with chain length.Comment: For IJQC-Sanibel Quantum Chemistry Symposium, 200
Truncated Levy statistics for transport in disordered semiconductors
Probabilistic interpretation of transition from the dispersive transport
regime to the quasi-Gaussian one in disordered semiconductors is given in terms
of truncated Levy distributions. Corresponding transport equations with
fractional order derivatives are derived. We discuss physical causes leading to
truncated waiting time distributions in the process and describe influence of
truncation on carrier packet form, transient current curves and frequency
dependence of conductivity. Theoretical results are in a good agreement with
experimental facts.Comment: 6 pages, 4 figures, presented in "Nonlinear Science and Complexity -
2010" (Turkey, Ankara
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