1,539 research outputs found
Exciton condensates in semiconductor quantum wells emit coherent light
We show that a quasi-two dimensional condensate of optically active excitons
emits coherent light even in the absence of population inversion. This allows
an unambiguous and clear experimental detection of the condensed phase. We
prove that, due to the exciton-photon coupling, quantum and thermal
fluctuations do not destroy condensation at finite temperature. Suitable
conditions to achieve condensation are temperatures of a few K for typical
exciton densities, and the use of a pulsed, and preferably circularly
polarized, laser.Comment: 5 pages, no figure
Ultrafast Coherent Spectroscopy of the Fermi Edge Singularity
In this work we present a theoretical description of the transient response
of the Fermi Edge Singularity (FES). We study the linear and the nonlinear
response of an n-doped QW to laser pulses in the Coherent Control (CC) and Four
Wave Mixing (FWM) Configurations. By means of a bosonization formalism we
calculate the FWM signal emitted by the sample when it is excited by pulses
spectrally peaked around the FES and we show that the long time behavior of the
nonlinear signal is very similar to the linear case.Comment: Conference paper (13 EP2DS
Exciton Beats in GaAs Quantum Wells: Bosonic Representation and Collective Effects
We discuss light-heavy hole beats observed in transient optical experiments
in GaAs quantum wells in terms of a free-boson coherent state model. This
approach is compared with descriptions based on few-level representations.
Results lead to an interpretation of the beats as due to classical
electromagnetic interference. The boson picture correctly describes photon
excitation of extended states and accounts for experiments involving coherent
control of the exciton density and Rayleigh scattering beating.Comment: 4 pages, no figures. Accepted for publication in Solid State
Communication
Spin dynamics of current driven single magnetic adatoms and molecules
A scanning tunneling microscope can probe the inelastic spin excitations of a
single magnetic atom in a surface via spin-flip assisted tunneling in which
transport electrons exchange spin and energy with the atomic spin. If the
inelastic transport time, defined as the average time elapsed between two
inelastic spin flip events, is shorter than the atom spin relaxation time, the
STM current can drive the spin out of equilibrium. Here we model this process
using rate equations and a model Hamiltonian that describes successfully spin
flip assisted tunneling experiments, including a single Mn atom, a Mn dimer and
Fe Phthalocyanine molecules. When the STM current is not spin polarized, the
non-equilibrium spin dynamics of the magnetic atom results in non-monotonic
curves. In the case of spin polarized STM current, the spin orientation
of the magnetic atom can be controlled parallel or anti-parallel to the
magnetic moment of the tip. Thus, spin polarized STM tips can be used both to
probe and to control the magnetic moment of a single atom.Comment: 15 pages, 12 figure
COLT: A New Weapon to Disseminate Knowledge
Too few researchers receive adequate pre- or postgraduate training to conduct a rigorous scientific study. In the digital age, new tools are emerging, and the development of distance education could improve this worrying situation. In this context, Health Science e-Training (HSeT), a nonprofit Swiss foundation, has developed new pedagogical concepts and tools under customized online training (COLT). For the ADMIRE Cost network, we have used an article-based e-learning (ABL) tool that allowed the students to learn how to read in depth and critically a scientific article and to rigorously address the problem of scientific reproducibility. The evaluation of the program by the students and the tutors has been quite positive. In conclusion COLT was well adapted to the needs of the ADMIRE Cost Action, a European network in which students from countries separated by thousands of miles can work at distance under the online supervision of tutors and then meet in a face-to-face session to maximize their learning experience and the interactions between peers and tutors
Van der Waals spin valves
We propose spin valves where a 2D non-magnetic conductor is intercalated
between two ferromagnetic insulating layers. In this setup, the relative
orientation of the magnetizations of the insulating layers can have a strong
impact on the in-plane conductivity of the 2D conductor. We first show this for
a graphene bilayer, described with a tight-binding model, placed between two
ferromagnetic insulators. In the anti-parallel configuration, a band gap opens
at the Dirac point, whereas in the parallel configuration, the graphene bilayer
remains conducting. We then compute the electronic structure of graphene
bilayer placed between two monolayers of the ferromagnetic insulator CrI,
using density functional theory. Consistent with the model, we find that a gap
opens at the Dirac point only in the antiparallel configuration.Comment: 5 pages, 4 figure
Optical control of the spin state of two Mn atoms in a quantum dot
We report on the optical spectroscopy of the spin of two magnetic atoms (Mn)
embedded in an individual quantum dot interacting with either a single
electron, a single exciton and single trion. As a result of their interaction
to a common entity, the Mn spins become correlated. The dynamics of this
process is probed by time resolved spectroscopy, that permits to determine the
optical orientation time in the range of a few tens of . In addition, we
show that the energy of the collective spin states of the two Mn atoms can be
tuned through the optical Stark effect induced by a resonant laser field
Spin-phonon coupling in single Mn doped CdTe quantum dot
The spin dynamics of a single Mn atom in a laser driven CdTe quantum dot is
addressed theoretically. Recent experimental
results\cite{Le-Gall_PRL_2009,Goryca_PRL_2009,Le-Gall_PRB_2010}show that it is
possible to induce Mn spin polarization by means of circularly polarized
optical pumping. Pumping is made possible by the faster Mn spin relaxation in
the presence of the exciton. Here we discuss different Mn spin relaxation
mechanisms. First, Mn-phonon coupling, which is enhanced in the presence of the
exciton. Second, phonon-induced hole spin relaxation combined with carrier-Mn
spin flip coupling and photon emission results in Mn spin relaxation. We model
the Mn spin dynamics under the influence of a pumping laser that injects
excitons into the dot, taking into account exciton-Mn exchange and phonon
induced spin relaxation of both Mn and holes. Our simulations account for the
optically induced Mn spin pumping.Comment: 17 pages, 11 figures, submitted to PR
Storage of classical information in quantum spins
Digital magnetic recording is based on the storage of a bit of information in
the orientation of a magnetic system with two stable ground states. Here we
address two fundamental problems that arise when this is done on a quantized
spin: quantum spin tunneling and back-action of the readout process. We show
that fundamental differences exist between integer and semi-integer spins when
it comes to both, read and record classical information in a quantized spin.
Our findings imply fundamental limits to the miniaturization of magnetic bits
and are relevant to recent experiments where spin polarized scanning tunneling
microscope reads and records a classical bit in the spin orientation of a
single magnetic atom
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