424 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
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
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
Mechanisms of change in psychotherapy for depression:An empirical update and evaluation of research aimed at identifying psychological mediators
We present a systematic empirical update and critical evaluation of the current status of research aimed at identifying a variety of psychological mediators in various forms of psychotherapy for depression. We summarize study characteristics and results of 35 relevant studies, and discuss the extent to which these studies meet several important requirements for mechanism research. Our review indicates that in spite of increased attention for the topic, advances in theoretical consensus about necessities for mechanism research, and sophistication of study designs, research in this field is still heterogeneous and unsatisfactory in methodological respect. Probably the biggest challenge in the field is demonstrating the causal relation between change in the mediator and change in depressive symptoms. The field would benefit from a further refinement of research methods to identify processes of therapeutic change. Recommendations for future research are discussed. However, even in the most optimal research designs, explaining psychotherapeutic change remains a challenge. Psychotherapy is a multi-dimensional phenomenon that might work through interplay of multiple mechanisms at several levels. As a result, it might be too complex to be explained in relatively simple causal models of psychological change
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
Spin orbit effects in a GaAs quantum dot in a parallel magnetic field
We analyze the effects of spin-orbit coupling on fluctuations of the
conductance of a quantum dot fabricated in a GaAs heterostructure. We argue
that spin-orbit effects may become important in the presence of a large
parallel magnetic field B_{||}, even if they are negligble for B_{||}=0. This
should be manifest in the level repulsion of a closed dot, and in reduced
conductance fluctuations in dots with a small number of open channels in each
lead, for large B_{||}. Our picture is consistent with the experimental
observations of Folk et al.Comment: 5 page
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
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
Treatment of Intrusive Suicidal Imagery Using Eye Movements
Suicide and suicidal behavior are major public health concerns, and affect 3–9% of the population worldwide. Despite increased efforts for national suicide prevention strategies, there are still few effective interventions available for reducing suicide risk. In this article, we describe various theoretical approaches for suicide ideation and behavior, and propose to examine the possible effectiveness of a new and innovative preventive strategy. A model of suicidal intrusion (mental imagery related to suicide, also referred to as suicidal flash-forwards) is presented describing one of the assumed mechanisms in the etiology of suicide and the mechanism of therapeutic change. We provide a brief rationale for an Eye Movement Dual Task (EMDT) treatment for suicidal intrusions, describing techniques that can be used to target these suicidal mental images and thoughts to reduce overall behavior. Based on the available empirical evidence for the mechanisms of suicidal intrusions, this approach appears to be a promising new treatment to prevent suicidal behavior as it potentially targets one of the linking pins between suicidal ideation and suicidal actions
Spin Degeneracy and Conductance Fluctuations in Open Quantum Dots
The dependence of mesoscopic conductance fluctuations on parallel magnetic
field is used as a probe of spin degeneracy in open GaAs quantum dots. The
variance of fluctuations at high parallel field is reduced from the low-field
variance (with broken time-reversal symmetry) by factors ranging from roughly
two in a 1 square-micron dot at low temperature, to four or greater in 8
square-micron dots. The factor of two is expected for simple Zeeman splitting
of spin degenerate channels. A possible explanation for the unexpected larger
factors in terms of field-dependent spin orbit scattering is proposed.Comment: Includes new reference to related theoretical work, cond-mat/0010064.
Other minor changes. Related papers at http://marcuslab.harvard.ed
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