978 research outputs found
Non-orthogonal Theory of Polarons and Application to Pyramidal Quantum Dots
We present a general theory for semiconductor polarons in the framework of
the Froehlich interaction between electrons and phonons. The latter is
investigated using non-commuting phonon creation/annihilation operators
associated with a natural set of non-orthogonal modes. This setting proves
effective for mathematical simplification and physical interpretation and
reveals a nested coupling structure of the Froehlich interaction. The theory is
non-perturbative and well adapted for strong electron-phonon coupling, such as
found in quantum dot (QD) structures. For those particular structures we
introduce a minimal model that allows the computation and qualitative
prediction of the spectrum and geometry of polarons. The model uses a generic
non-orthogonal polaron basis, baptized the "natural basis". Accidental and
symmetry-related electronic degeneracies are studied in detail and are shown to
generate unentangled zero-shift polarons, which we consistently eliminate. As a
practical example, these developments are applied to realistic pyramidal GaAs
QDs. The energy spectrum and the 3D-geometry of polarons are computed and
analyzed, and prove that realistic pyramidal QDs clearly fall in the regime of
strong coupling. Further investigation reveals an unexpected substructure of
"weakly coupled strong coupling regimes", a concept originating from overlap
considerations. Using Bennett's entanglement measure, we finally propose a
heuristic quantification of the coupling strength in QDs.Comment: 17 pages, 11 figures, 3 table
Enhancement of the Binding Energy of Charged Excitons in Disordered Quantum Wires
Negatively and positively charged excitons are identified in the
spatially-resolved photoluminescence spectra of quantum wires. We demonstrate
that charged excitons are weakly localized in disordered quantum wires. As a
consequence, the enhancement of the "binding energy" of a charged exciton is
caused, for a significant part, by the recoil energy transferred to the
remaining charged carrier during its radiative recombination. We discover that
the Coulomb correlation energy is not the sole origin of the "binding energy",
in contrast to charged excitons confined in quantum dots.Comment: 4 Fig
Inheritance flows in Switzerland, 1911–2011
We estimate the size of inheritance flows in Switzerland over a long span of data, in close analogy to the study for France by Piketty (2011). We find that inheritance flows had been growing more slowly than national income up until the 1970s, but have been outpacing income growth since. According to our central estimates, the annual flow of inheritance amounted to 13.2% of national income in 2011. The share of total wealth that is attributable to inheritance has remained relatively stable over time, fluctuating between 45% and 60%
Dynamics of optical injection of charge and spin currents in quantum wells
We develop a dynamical theory for the optical injection of charge and spin current originating from the interferences between two coherent laser pulses of frequencies omega and 2omega. Multiband Bloch equations which include one- and two-photon interband transitions are derived. They also account for ac Stark shifts and intersubband two-photon transitions. The model is used to describe the case of time-dependent charge and spin current injection in a symmetric GaAs/AlGaAs quantum well. A comparison to the bulk case is also made. The separate contributions of the populations and intersubband coherences to the charge current and THz emission are identified. The influence of the Stark shifts and the inter-valence-band two-photon transitions are calculated and discussed
Minimum decoherence cat-like states in Gaussian noisy channels
We address the evolution of cat-like states in general Gaussian noisy
channels, by considering superpositions of coherent and squeezed-coherent
states coupled to an arbitrarily squeezed bath. The phase space dynamics is
solved and decoherence is studied keeping track of the purity of the evolving
state. The influence of the choice of the state and channel parameters on
purity is discussed and optimal working regimes that minimize the decoherence
rate are determined. In particular, we show that squeezing the bath to protect
a non squeezed cat state against decoherence is equivalent to orthogonally
squeezing the initial cat state while letting the bath be phase insensitive.Comment: 10 pages, 2 figures, references added, submitted to J. Opt.
Optical injection of charge current in quantum wires: oscillations induced by excitonic effects
We investigate the charge current that is optically injected by interference between one- and two-photon excitation in the presence of excitonic effects. We consider a realistic V-shaped quantum wire excited slightly below the band gap by two simultaneous femtosecond laser pulses of frequency 2 and that interact, respectively, with the lowest B1 and A1 excitons. Using effective multiband Bloch equations for two-photon transitions, including the Coulomb interaction within the Hartree-Fock approximation, we show that, because of the different symmetry properties of the involved excitons, the generated charge current displays oscillations due to the quantum interference between the excitonic coherences
Entanglement and purity of two-mode Gaussian states in noisy channels
We study the evolution of purity, entanglement and total correlations of
general two--mode Gaussian states of continuous variable systems in arbitrary
uncorrelated Gaussian environments. The time evolution of purity, Von Neumann
entropy, logarithmic negativity and mutual information is analyzed for a wide
range of initial conditions. In general, we find that a local squeezing of the
bath leads to a faster degradation of purity and entanglement, while it can
help to preserve the mutual information between the modes.Comment: 10 pages, 8 figure
Competition and bistability of longitudinal modes in a Raman laser
A model for Raman lasers including several longitudinal modes is analyzed. Depending on the choice of the parameters the system can exhibit single-mode emission, wide bistability domains, and self-pulsing. The latter is often characterized by two frequencies, which are clearly related to single-mode and multimode instabilities, in agreement with the interpretation of earlier experimental results
Maximal symetrization and reduction of fields: application to wavefunctions in solid state nanostructures
A novel general formalism for the maximal symetrization and reduction of
fields (MSRF) is proposed and applied to wavefunctions in solid state
nanostructures. Its primary target is to provide an essential tool for the
study and analysis of the electronic and optical properties of semiconductor
quantum heterostructures with relatively high point-group symmetry, and studied
with the formalism. Nevertheless the approach is valid in a much
larger framework than theory, it is applicable to arbitrary systems
of coupled partial differential equations (e.g. strain equations or Maxwell
equations). For spinless problems (scalar equations), one can use a systematic
Spatial Domain Reduction (SDR) technique which allows, for every irreducible
representation, to reduce the set of equations on a minimal domain with
automatic incorporation of the boundary conditions at the border, which are
shown to be non-trivial in general. For a vectorial or spinorial set of
functions, the SDR technique must be completed by the use of an optimal basis
in vectorial or spinorial space (in a crystal we call it the Optimal Bloch
function Basis - OBB). The advantages are numerous: sharper insights on the
symmetry properties of every eigenstate, minimal coupling schemes, analytically
and computationally exploitable at the component function level, minimal
computing domains. The formalism can be applied also as a postprocessing
operation, offering all subsequent analytical and computationnal advantages of
symmetrization. The specific case of a quantum wire (QWRs) with point
group symmetry is used as a concrete illustration of the application of MSRF.Comment: 33 pages, 13 figures, Many small changes in equations, which use more
standard conventions in the passive point of view, and corrections of a
number of minor mistake
Quantifying decoherence in continuous variable systems
We present a detailed report on the decoherence of quantum states of
continuous variable systems under the action of a quantum optical master
equation resulting from the interaction with general Gaussian uncorrelated
environments. The rate of decoherence is quantified by relating it to the decay
rates of various, complementary measures of the quantum nature of a state, such
as the purity, some nonclassicality indicators in phase space and, for two-mode
states, entanglement measures and total correlations between the modes.
Different sets of physically relevant initial configurations are considered,
including one- and two-mode Gaussian states, number states, and coherent
superpositions. Our analysis shows that, generally, the use of initially
squeezed configurations does not help to preserve the coherence of Gaussian
states, whereas it can be effective in protecting coherent superpositions of
both number states and Gaussian wave packets.Comment: Review article; 36 pages, 19 figures; typos corrected, references
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