2,890 research outputs found
Spectroscopy of nanoscopic semiconductor rings
Making use of self-assembly techniques, we demonstrate the realization of
nanoscopic semiconductor quantum rings in which the electronic states are in
the true quantum limit. We employ two complementary spectroscopic techniques to
investigate both the ground states and the excitations of these rings. Applying
a magnetic field perpendicular to the plane of the rings, we find that when
approximately one flux quantum threads the interior of each ring, a change in
the ground state from angular momentum to takes place.
This ground state transition is revealed both by a drastic modification of the
excitation spectrum and by a change in the magnetic field dispersion of the
single-electron charging energy
Anisotropic magneto-Coulomb effect versus spin accumulation in a ferromagnetic single-electron device
We investigate the magneto-transport characteristics of nanospintronics
single-electron devices. The devices consist of single non-magnetic
nano-objects (nanometer size nanoparticles of Al or Cu) connected to Co
ferromagnetic leads. The comparison with simulations allows us attribute the
observed magnetoresistance to either spin accumulation or anisotropic
magneto-Coulomb effect (AMC), two effects with very different origins. The fact
that the two effects are observed in similar samples demonstrates that a
careful analysis of Coulomb blockade and magnetoresistance behaviors is
necessary in order to discriminate them in magnetic single-electron devices. As
a tool for further studies, we propose a simple way to determine if spin
transport or AMC effect dominates from the Coulomb blockade I-V curves of the
spintronics device
Polarization memory in single Quantum Dots
We measured the polarization memory of excitonic and biexcitonic optical
transitions from single quantum dots at either positive, negative or neutral
charge states. Positive, negative and no circular or linear polarization memory
was observed for various spectral lines, under the same quasi-resonant
excitation below the wetting layer band-gap. We developed a model which
explains both qualitatively and quantitatively the experimentally measured
polarization spectrum for all these optical transitions. We consider quite
generally the loss of spin orientation of the photogenerated electron-hole pair
during their relaxation towards the many-carrier ground states. Our analysis
unambiguously demonstrates that while electrons maintain their initial spin
polarization to a large degree, holes completely dephase.Comment: 6 pages, 4 figure
Coulomb interactions in single, charged self-assembled quantum dots: radiative lifetime and recombination energy
We present results on the charge dependence of the radiative recombination
lifetime, Tau, and the emission energy of excitons confined to single
self-assembled InGaAs quantum dots. There are significant dot-to-dot
fluctuations in the lifetimes for a particular emission energy. To reach
general conclusions, we present the statistical behavior by analyzing data
recorded on a large number of individual quantum dots. Exciton charge is
controlled with extremely high fidelity through an n-type field effect
structure, providing access to the neutral exciton (X0), the biexciton (2X0)
and the positively (X1+) and negatively (X1-) charged excitons. We find
significant differences in the recombination lifetime of each exciton such
that, on average, Tau(X1-) / Tau(X0) = 1.25, Tau(X1+) / Tau(X0) = 1.58 and
Tau(2X0) / Tau(X0) = 0.65. We attribute the change in lifetime to significant
changes in the single particle hole wave function on charging the dot, an
effect more pronounced on charging X0 with a single hole than with a single
electron. We verify this interpretation by recasting the experimental data on
exciton energies in terms of Coulomb energies. We show directly that the
electron-hole Coulomb energy is charge dependent, reducing in value by 5-10% in
the presence of an additional electron, and that the electron-electron and
hole-hole Coulomb energies are almost equal.Comment: 8 pages, 7 figures, submitted to Phys. Rev.
Cognitive-Behavioral Therapy for Adolescents with an Age-Adapted Diagnosis of Binge-Eating Disorder: A Randomized Clinical Trial
Binge-eating disorder (BED) is characterized by recurrent objective binge eating that occurs in the absence of compensatory behaviors to prevent weight gain. As the most common eating disorder emerging in youth, BED co-occurs with increased eating disorder and general psychopathology, impaired quality of life, and
obesity [1]. Despite its clinical significance, there is a dearth of treatment studies in adolescents [1, 2]. Regarding cognitive-behavioral therapy (CBT), the most well-established treatment for adults with BED [2], one pilot randomized-controlled trial (RCT) in 25 adolescent girls with objective binge eating suggested superiority to wait-list (WL) in achieving binge-eating abstinence through 6
months following randomization and in improving eating disorder psychopathology, but not in reducing binge eating or standardized body mass index (BMI; kg/m2) [3]. Other CBT-related RCTs documented efficacy of Internet-based, weight loss-oriented self-help versus WL [4] and no differences in dialectical behavior therapy versus weight management [5]. Based on this preliminary
evidence, the aim of the BEDA (Binge Eating Disorder in Adolescents) study was to provide a confirmatory test of the efficacy of CBT in adolescent BED. It was hypothesized that CBT will be superior to WL in improving binge eating, associated psychopathology, and quality of life, but not BMI, with long-term maintenance
of effects
Complete control of a matter qubit using a single picosecond laser pulse
We demonstrate for the first time that a matter physical two level system, a
qubit, can be fully controlled using one ultrafast step. We show that the spin
state of an optically excited electron, an exciton, confined in a quantum dot,
can be rotated by any desired angle, about any desired axis, during such a
step. For this we use a single, resonantly tuned, picosecond long, polarized
optical pulse. The polarization of the pulse defines the rotation axis, while
the pulse detuning from a non-degenerate absorption resonance, defines the
magnitude of the rotation angle. We thereby achieve a high fidelity, universal
gate operation, applicable to other spin systems, using only one short optical
pulse. The operation duration equals the pulse temporal width, orders of
magnitude shorter than the qubit evolution life and coherence times.Comment: main text: 4 pages, 3 figures Supplemental material: 3 pages, 1
figur
Spin injection in a single metallic nanoparticle: a step towards nanospintronics
We have fabricated nanometer sized magnetic tunnel junctions using a new
nanoindentation technique in order to study the transport properties of a
single metallic nanoparticle. Coulomb blockade effects show clear evidence for
single electron tunneling through a single 2.5 nm Au cluster. The observed
magnetoresistance is the signature of spin conservation during the transport
process through a non magnetic cluster.Comment: 3 page
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