85 research outputs found
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
Discrete quantum dot like emitters in monolayer MoSe2: Spatial mapping, Magneto-optics and Charge tuning
Transition metal dichalcogenide monolayers such as MoSe2,MoS2 and WSe2 are
direct bandgap semiconductors with original optoelectronic and spin-valley
properties. Here we report spectrally sharp, spatially localized emission in
monolayer MoSe2. We find this quantum dot like emission in samples exfoliated
onto gold substrates and also suspended flakes. Spatial mapping shows a
correlation between the location of emitters and the existence of wrinkles
(strained regions) in the flake. We tune the emission properties in magnetic
and electric fields applied perpendicular to the monolayer plane. We extract an
exciton g-factor of the discrete emitters close to -4, as for 2D excitons in
this material. In a charge tunable sample we record discrete jumps on the meV
scale as charges are added to the emitter when changing the applied voltage.
The control of the emission properties of these quantum dot like emitters paves
the way for further engineering of the light matter interaction in these
atomically thin materials.Comment: 5 pages, 2 figure
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
Phonon-assisted Photoluminescence from Dark Excitons in Monolayers of Transition Metal Dichalcogenides
The photoluminescence (PL) spectrum of transition metal dichalcogenides
(TMDs) shows a multitude of emission peaks below the bright exciton line and
not all of them have been explained yet. Here, we study the emission traces of
phonon-assisted recombinations of momentum-dark excitons. To this end, we
develop a microscopic theory describing simultaneous exciton, phonon and photon
interaction and including consistent many-particle dephasing. We explain the
drastically different PL below the bright exciton in tungsten- and
molybdenum-based materials as result of different configurations of bright and
dark states. In good agreement with experiments, we show that WSe exhibits
clearly visible low-temperature PL signals stemming from the phonon-assisted
recombination of momentum-dark excitons
Hyperfine interaction in InAs/GaAs self-assembled quantum dots : dynamical nuclear polarization versus spin relaxation
We report on the influence of hyperfine interaction on the optical
orientation of singly charged excitons X+ and X- in self-assembled InAs/GaAs
quantum dots. All measurements were carried out on individual quantum dots
studied by micro-photoluminescence at low temperature. We show that the
hyperfine interaction leads to an effective partial spin relaxation, under
50kHz modulated excitation polarization, which becomes however strongly
inhibited under steady optical pumping conditions because of dynamical nuclear
polarization. This optically created magnetic-like nuclear field can become
very strong (up to ~4 T) when it is generated in the direction opposite to a
longitudinally applied field, and exhibits then a bistability regime. This
effect is very well described by a theoretical model derived in a perturbative
approach, which reveals the key role played by the energy cost of an electron
spin flip in the total magnetic field. Eventually, we emphasize the
similarities and differences between X+ and X- trions with respect to the
hyperfine interaction, which turn out to be in perfect agreement with the
theoretical description.Comment: 10 pages, 5 figure
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
De-Confinement in high multiplicity proton-proton collisions at LHC energies
Recently, the CMS Collaboration has published identified particle transverse
momentum spectra in high multiplicity events at LHC energies =
0.9-13 TeV. In the present work the transverse momentum spectra have been
analyzed in the framework of the color fields inside the clusters of
overlapping strings, which are produced in high energy hadronic collisions. The
non-Abelian nature is reflected in the coherence sum of the color fields which
as a consequence gives rise to an enhancement of the transverse momentum and a
suppression of the multiplicities relative to the non overlapping strings.
The initial temperature and shear viscosity to entropy density ratio
are obtained. For the higher multiplicity events at =7 and 13 TeV
the initial temperature is above the universal hadronization temperature and is
consistent with the creation of de-confined matter. In these small systems it
can be argued that the thermalization is a consequence of the quantum tunneling
through the event horizon introduced by the confining color fields, in analogy
to the Hawking-Unruh effect. The small shear viscosity to entropy density ratio
near the critical temperature suggests that the matter is a strongly
coupled Quark Gluon Plasma.Comment: 5 pages, 4 figure
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