140 research outputs found
Optical orientation of electron spins and valence band spectroscopy in germanium
We have investigated optical orientation in the vicinity of the direct gap of
bulk germanium. The electron spin polarization is studied via
polarization-resolved photoluminescence excitation spectroscopy unfolding the
interplay between doping and ultrafast electron transfer from the center of the
Brillouin zone towards its edge. As a result, the direct-gap photoluminescence
circular polarisation can vary from 30% to -60% when the excitation laser
energy increases. This study provides also simultaneous access to the resonant
electronic Raman scattering due to inter-valence band excitations of
spin-polarized holes, yielding a fast and versatile spectroscopic approach for
the determination of the energy spectrum of holes in semiconducting materials
Plant monitoring and fault detection - Synergy between data reconciliation and principal component analysis
Data reconciliation and principal component analysis are tno recognised statistical methods used for plant monitoring and fault detection. We propose to combine them for increased efficiency. Data reconciliation is used in the first step of the determination of the projection matrix for principal component analysis (eigenvectors). principal component analysis can then be applied to raw process data for monitoring purpose. The combined use of these techniques aims at a better efficiency in fault detection. It relies mainly in a lower number of components to monitor. The method is applied to a modelled ammonia synthesis loop. (C) 2001 Elsevier Science Ltd. All rights reserved
Off-equilibrium corrections to energy and conserved charge densities in the relativistic fluid in heavy-ion collisions
Dissipative processes in relativistic fluids are known to be important in the
analyses of the hot QCD matter created in high-energy heavy-ion collisions. In
this work, I consider dissipative corrections to energy and conserved charge
densities, which are conventionally assumed to be vanishing but could be
finite. Causal dissipative hydrodynamics is formulated in the presence of those
dissipative currents. The relation between hydrodynamic stability and transport
coefficients is discussed. I then study their phenomenological consequences on
the observables of heavy-ion collisions in numerical simulations. It is shown
that particle spectra and elliptic flow can be visibly modified.Comment: 10 pages, 5 figures; title changed, references added, conclusions
unchange
Nuclear spin physics in quantum dots: an optical investigation
The mesoscopic spin system formed by the 10E4-10E6 nuclear spins in a
semiconductor quantum dot offers a unique setting for the study of many-body
spin physics in the condensed matter. The dynamics of this system and its
coupling to electron spins is fundamentally different from its bulk
counter-part as well as that of atoms due to increased fluctuations that result
from reduced dimensions. In recent years, the interest in studying quantum dot
nuclear spin systems and their coupling to confined electron spins has been
fueled by its direct implication for possible applications of such systems in
quantum information processing as well as by the fascinating nonlinear
(quantum-)dynamics of the coupled electron-nuclear spin system. In this
article, we review experimental work performed over the last decades in
studying this mesoscopic,coupled electron-nuclear spin system and discuss how
optical addressing of electron spins can be exploited to manipulate and
read-out quantum dot nuclei. We discuss how such techniques have been applied
in quantum dots to efficiently establish a non-zero mean nuclear spin
polarization and, most recently, were used to reduce fluctuations of the
average quantum dot nuclear spin orientation. Both results in turn have
important implications for the preservation of electron spin coherence in
quantum dots, which we discuss. We conclude by speculating how this recently
gained understanding of the quantum dot nuclear spin system could in the future
enable experimental observation of quantum-mechanical signatures or possible
collective behavior of mesoscopic nuclear spin ensembles.Comment: 61 pages, 45 figures, updated reference list, corrected typographical
error
Dynamic nuclear polarization of a single charge-tunable InAs/GaAs quantum dot
We report on the dynamic nuclear polarization of a single charge-tunable
self-assembled InAs/GaAs quantum dot in a longitudinal magnetic field of
0.2T. The hyperfine interaction between the optically oriented electron
and nuclei spins leads to the polarization of the quantum dot nuclei measured
by the Overhauser-shift of the singly-charged excitons ( and ).
When going from to , we observe a reversal of this shift which
reflects the average electron spin optically written down in the quantum dot
either in the state or in the final state of recombination. We
discuss a theoretical model which indicates an efficient depolarization
mechanism for the nuclei limiting their polarization to ~10%.Comment: 4+ pages, 3 figure
Radiative lifetime of localized excitons in transition metal dichalcogenides
Disorder derived from defects or strain in monolayer TMDs can lead to a
dramatic change in the physical behavior of the interband excitations,
producing inhomogeneous spectral broadening and localization; leading to
radiative lifetime increase. In this study, we have modeled the disorder in the
surface of the sample through a randomized potential in monolayer WSe2. We show
that this model allows us to simulate the spectra of localized exciton states
as well as their radiative lifetime. In this context, we give an in depth study
of the influence of the disorder potential parameters on the optical properties
of these defects through energies, density of states, oscillator strengths,
photoluminescence (PL) spectroscopy and radiative lifetime at low temperature
(4K). We demonstrate that localized excitons have a longer emission time than
free excitons, in the range of tens of picoseconds or more, and we show that it
depends strongly on the disorder parameter and dielectric environment. Finally,
in order to prove the validity of our model we compare it to available
experimental results of the literature.Comment: arXiv admin note: text overlap with arXiv:1409.3996 by other author
Optically monitored nuclear spin dynamics in individual GaAs quantum dots grown by droplet epitaxy
We report optical orientation experiments in individual, strain free GaAs
quantum dots in AlGaAs grown by droplet epitaxy. Circularly polarized optical
excitation yields strong circular polarization of the resulting
photoluminescence at 4K. Optical injection of spin polarized electrons into a
dot gives rise to dynamical nuclear polarization that considerably changes the
exciton Zeeman splitting (Overhauser shift). We show that the created nuclear
polarization is bistable and present a direct measurement of the build-up time
of the nuclear polarization in a single GaAs dot in the order of one second.Comment: 7 pages, 3 figure
A model structure for the Goldman-Millson theorem
By a result of Vallette, we put a sensible model structure on the category of
conilpotent Lie coalgebras. This gives us a powerful tool to study the
subcategory of Lie algebras obtained by linear dualization, also known as the
category of pronilpotent Lie algebras. This way, we recover weaker versions of
the celebrated Goldman-Millson theorem and Dolgushev-Rogers theorem by purely
homotopical methods. We explore the relations of this procedure with the
existent literature, namely the works of Lazarev-Markl and
Buijs-F\'elix-Murillo-Tanr\'e.Comment: 20 pages. (v2) fixed formatting of abstract on arXiv, the core text
was not touche
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