286 research outputs found
Calculation of dephasing times in closed quantum dots
Dephasing of one-particle states in closed quantum dots is analyzed within
the framework of random matrix theory and Master equation. Combination of this
analysis with recent experiments on the magnetoconductance allows for the first
time to evaluate the dephasing times of closed quantum dots. These dephasing
times turn out to depend on the mean level spacing and to be significantly
enhanced as compared with the case of open dots. Moreover, the experimental
data available are consistent with the prediction that the dephasing of
one-particle states in finite closed systems disappears at low enough energies
and temperatures.Comment: 4 pages, 3 figure
High Bias Transport and Magnetometer Design in Open Quantum Dots
We report transport measurements as a function of bias in open semiconductor
quantum dots. These measurements are well described by an effective electron
temperature derived from Joule heating at the point contacts and cooling by
Wiedemann-Franz out-diffusion of thermal electrons. Using this model, we
propose and analyze a quantum dot based sensor which measures absolute magnetic
field at micron scales with a noise floor of
at 300 mK.Comment: 10 pages including 3 figure
«Белая дача» А.П. Чехова – как часть культурного ландшафта Крыма
Данная работа написана в рамках научного проекта кафедры культурологии «Культурные ландшафты Крыма» и посвящена рассмотрению «Белой дачи» А.П. Чехова (ныне Дом-музей А.П. Чехова в Ялте) как культурному ландшафту – результату сотворчества человека и природы
Decoherence in Nearly-Isolated Quantum Dots
Decoherence in nearly-isolated GaAs quantum dots is investigated using the
change in average Coulomb blockade peak height upon breaking time-reversal
symmetry. The normalized change in average peak height approaches the predicted
universal value of 1/4 at temperatures well below the single-particle level
spacing, but is greatly suppressed for temperature greater than the level
spacing, suggesting that inelastic scattering or other dephasing mechanisms
dominate in this regime.Comment: Significant revisions to include comparison to theory. Related papers
available at http://marcuslab.harvard.ed
Photocurrent, Rectification, and Magnetic Field Symmetry of Induced Current Through Quantum Dots
We report mesoscopic dc current generation in an open chaotic quantum dot
with ac excitation applied to one of the shape-defining gates. For excitation
frequencies large compared to the inverse dwell time of electrons in the dot
(i.e., GHz), we find mesoscopic fluctuations of induced current that are fully
asymmetric in the applied perpendicular magnetic field, as predicted by recent
theory. Conductance, measured simultaneously, is found to be symmetric in
field. In the adiabatic (i.e., MHz) regime, in contrast, the induced current is
always symmetric in field, suggesting its origin is mesoscopic rectification.Comment: related papers at http://marcuslab.harvard.ed
Conductance fluctuations and weak localization in chaotic quantum dots
We study the conductance statistical features of ballistic electrons flowing
through a chaotic quantum dot. We show how the temperature affects the
universal conductance fluctuations by analyzing the influence of dephasing and
thermal smearing. This leads us to two main findings. First, we show that the
energy correlations in the transmission, which were overlooked so far, are
important for calculating the variance and higher moments of the conductance.
Second, we show that there is an ambiguity in the method of determination of
the dephasing rate from the size of the of the weak localization. We find that
the dephasing times obtained at low temperatures from quantum dots are
underestimated.Comment: 4 pages, 4 figures, to appear in Phys. Rev. Let
Dephasing in the semiclassical limit is system-dependent
We investigate dephasing in open quantum chaotic systems in the limit of
large system size to Fermi wavelength ratio, . We
semiclassically calculate the weak localization correction to the
conductance for a quantum dot coupled to (i) an external closed dot and (ii) a
dephasing voltage probe. In addition to the universal algebraic suppression
with the dwell time through
the cavity and the dephasing rate , we find an exponential
suppression of weak localization by a factor , with a system-dependent . In the
dephasing probe model, coincides with the Ehrenfest time,
, for both perfectly and partially
transparent dot-lead couplings. In contrast, when dephasing occurs due to the
coupling to an external dot, depends on the
correlation length of the coupling potential instead of .Comment: 4 pages 3 figures (v2 contains numerous cosmetic changes
Distributions of the Conductance and its Parametric Derivatives in Quantum Dots
Full distributions of conductance through quantum dots with single-mode leads
are reported for both broken and unbroken time-reversal symmetry. Distributions
are nongaussian and agree well with random matrix theory calculations that
account for a finite dephasing time, , once broadening due to finite
temperature is also included. Full distributions of the derivatives of
conductance with respect to gate voltage are also investigated.Comment: 4 pages (REVTeX), 4 eps figure
Probe-configuration dependent dephasing in a mesoscopic interferometer
Dephasing in a ballistic four-terminal Aharonov-Bohm geometry due to charge
and voltage fluctuations is investigated. Treating two terminals as voltage
probes, we find a strong dependence of the dephasing rate on the probe
configuration in agreement with a recent experiment by Kobayashi et al. (J.
Phys. Soc. Jpn. 71, 2094 (2002)). Voltage fluctuations in the measurement
circuit are shown to be the source of the configuration dependence.Comment: 4 pages, 3 figure
The clinical effectiveness of evidence-based interventions for depression: A pragmatic trial in routine practice
Abstract BACKGROUND: Controversy persists about how effectively empirically-supported treatments for major depression work in actual clinical practice as well as how patients choose among them. We examined the acute phase effectiveness of cognitive therapy (CT), interpersonal psychotherapy (IPT), and combined psychotherapy-pharmacotherapy (PHT) in a naturalistic setting, allowing patients their choice of treatment. METHODS: The study compared CT (n=63), IPT (n=56), CT-PHT (n=34), and IPT-PHT (n=21) for 174 subjects with major depression in a secondary care mood disorders clinic. Patient preference, rather than randomization, determined treatment selection. The Beck Depression Inventory-II (BDI) was the primary outcome variable. Exclusion criteria were minimal. RESULTS: All treatments were associated with a reduction in depressive symptoms, with a 35% remission rate by week 26. Overall improvement was well within ranges reported in efficacy trials. On average, treatment effects of the different interventions straddled the same range, but moderation analyses revealed that BDI scores dropped faster in the first 16 weeks in patients who received CT alone than patients who received CT and pharmacotherapy, a pattern not found in patients who received IPT (with or without pharmacotherapy). LIMITATIONS: Limitations consist of a modest sample size, choice of treatment was made by participants which may have been influenced by many sources, and the absence of a non-active control group. CONCLUSIONS: This study supports the effectiveness of empirically-supported antidepressant treatments selected by patients in routine settings, and provides an indication that speed of therapeutic response may vary amongst treatments
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