2,215 research outputs found
Interference Phenomena in Electronic Transport Through Chaotic Cavities: An Information-Theoretic Approach
We develop a statistical theory describing quantum-mechanical scattering of a
particle by a cavity when the geometry is such that the classical dynamics is
chaotic. This picture is relevant to a variety of systems, ranging from atomic
nuclei to microwave cavities; the main application here is to electronic
transport through ballistic microstructures. The theory describes the regime in
which there are two distinct time scales, associated with a prompt and an
equilibrated response, and is cast in terms of the matrix of scattering
amplitudes S. The prompt response is related to the energy average of S which,
through ergodicity, is expressed as the average over an ensemble of systems. We
use an information-theoretic approach: the ensemble of S-matrices is determined
by (1) general physical features-- symmetry, causality, and ergodicity, (2) the
specific energy average of S, and (3) the notion of minimum information in the
ensemble. This ensemble, known as Poisson's kernel, is meant to describe those
situations in which any other information is irrelevant. Thus, one constructs
the one-energy statistical distribution of S using only information expressible
in terms of S itself without ever invoking the underlying Hamiltonian. This
formulation has a remarkable predictive power: from the distribution of S we
derive properties of the quantum conductance of cavities, including its
average, its fluctuations, and its full distribution in certain cases, both in
the absence and presence prompt response. We obtain good agreement with the
results of the numerical solution of the Schrodinger equation for cavities in
which either prompt response is absent or there are two widely separated time
scales. Good agreement with experimental data is obtained once temperature
smearing and dephasing effects are taken into account.Comment: 38 pages, 11 ps files included, uses IOP style files and epsf.st
Quantum Dissipation due to the Interaction with Chaos
We discuss the possibility of having "quantum dissipation" due to the
interaction with chaotic degrees of freedom. We define the conditions that
should be satisfied in order to have a dissipative effect similar to the one
due to an interaction with a (many body) bath. We also compare with the case
where the environment is modeled by a random matrix model. In case of
interaction with "chaos" we observe a regime where the relaxation process is
non-universal, and reflects the underlaying semiclassical dynamics. As an
example we consider a two level system (spin) that interacts with a two
dimensional anharmonic oscillator.Comment: 5 pages, 1 figure, final improved version, to be published as Phys
Rev. E Rapid Communicatio
Mesoscopic Transport Through Ballistic Cavities: A Random S-Matrix Theory Approach
We deduce the effects of quantum interference on the conductance of chaotic
cavities by using a statistical ansatz for the S matrix. Assuming that the
circular ensembles describe the S matrix of a chaotic cavity, we find that the
conductance fluctuation and weak-localization magnitudes are universal: they
are independent of the size and shape of the cavity if the number of incoming
modes, N, is large. The limit of small N is more relevant experimentally; here
we calculate the full distribution of the conductance and find striking
differences as N changes or a magnetic field is applied.Comment: 4 pages revtex 3.0 (2-column) plus 2 postscript figures (appended),
hub.pam.94.
Wave Scattering through Classically Chaotic Cavities in the Presence of Absorption: An Information-Theoretic Model
We propose an information-theoretic model for the transport of waves through
a chaotic cavity in the presence of absorption. The entropy of the S-matrix
statistical distribution is maximized, with the constraint : n is the dimensionality of S, and meaning complete (no) absorption. For strong absorption our result
agrees with a number of analytical calculations already given in the
literature. In that limit, the distribution of the individual (angular)
transmission and reflection coefficients becomes exponential -Rayleigh
statistics- even for n=1. For Rayleigh statistics is attained even
with no absorption; here we extend the study to . The model is
compared with random-matrix-theory numerical simulations: it describes the
problem very well for strong absorption, but fails for moderate and weak
absorptions. Thus, in the latter regime, some important physical constraint is
missing in the construction of the model.Comment: 4 pages, latex, 3 ps figure
Distribution of the S-matrix in chaotic microwave cavities with direct processes and absorption
We quantify the presence of direct processes in the S-matrix of chaotic
microwave cavities with absorption in the one-channel case. To this end the
full distribution P_S(S) of the S-matrix, i.e. S=\sqrt{R}e^{i\theta}, is
studied in cavities with time-reversal symmetry for different antenna coupling
strengths T_a or direct processes. The experimental results are compared with
random-matrix calculations and with numerical simulations based on the
Heidelberg approach including absorption. The theoretical result is a
generalization of the Poisson kernel. The experimental and the numerical
distributions are in excellent agreement with random-matrix predictions for all
cases.Comment: 4 pages, 4 figure
Buprenorphine added on brief cognitive behavioral therapy for treatment of methamphetamine use disorder
Background: Methamphetamine (MA) use remains a major public health concern around the world. Recent findings suggest that buprenorphine may be helpful for cocaine use reduction. Moreover, animal studies described reduced dopamine peak effect following MA use, due to the administration of low dose buprenorphine.
Objectives: This study examined the effectiveness of buprenorphine with brief cognitive behavioral therapy on MA use disorder.
Methods: The study was conducted in an outpatient substance abuse treatment center in Qazvin, Iran. Nineteen MA users received buprenorphine for 24 weeks combined with brief cognitive behavioral therapy in an outpatient substance abuse treatment program, three times per week, as a before and after non - randomization study. Clinical outcomes included treatment retention, MA use, degree of MA dependency and craving, quality of life, cognitive abilities questionnaire, addiction severity and also adverse events. Data was analyzed by performing repeated measures analysis and the Friedman test for nonparametric variables.
Results: Fifteen participants completed the study during six months and frequency of MA use was significantly decreased at 24 weeks (P < 0.001). There were also significant reductions in craving (P < 0.001), degree of MA dependence (P < 0.001), and improvements in quality of life, cognitive ability, and some subscales of addiction severity.
Conclusions: The results of this preliminary clinical study demonstrated that buprenorphine could potentially attenuate MA craving and alternate rewarding effects of MA and had promising effects on cognitive impairment. Furthermore, buprenorphine can be considered as a harm reduction intervention in some communities, in which the people, as a result of cultural beliefs, do not accept a therapy, which only consists of counseling and no medications
Vacuum Polarization of a Massless Scalar Field in the Background of a Global Monopole with Finite Core
In this paper we analyze the vacuum polarization effects of a massless scalar
field in the background of a global monopole considering a inner structure to
it. Specifically we investigate the effect of its structure on the vacuum
expectation value of the square of the field operator, ,
admitting a non-minimal coupling between the field with the geometry: $\xi
{\cal{R}}\hat{\Phi}^2$. Also we calculate the corrections on the vacuum
expectation value of the energy-momentum tensor, , due to
the inner structure of the monopole. In order to develop these analysis, we
calculate the Euclidean Green function associated with the system for points in
the region outside the core. As we shall see, for specific value of the
coupling parameter , the corrections caused by the inner structure of the
monopole can provide relevant contributions on these vacuum polarizations.Comment: Accepted for publication in Classical and Quantum Gravity. Added
references. 22 pages, 1 figur
Ballistic Transport Through Chaotic Cavities: Can Parametric Correlations and the Weak Localization Peak be Described by a Brownian Motion Model?
A Brownian motion model is devised on the manifold of S-matrices, and applied
to the calculation of conductance-conductance correlations and of the weak
localization peak. The model predicts that (i) the correlation function in
has the same shape and width as the weak localization peak; (ii) the functions
behave as , thus excluding a linear line shape; and
(iii) their width increases as the square root of the number of channels in the
leads. Some of these predictions agree with experiment and with other
calculations only in the limit of small and a large number of channels.Comment: 5 pages revtex (twocolumn
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