1,645 research outputs found
Crossover of magnetoconductance autocorrelation for a ballistic chaotic quantum dot
The autocorrelation function C_{\varphi,\eps}(\Delta\varphi,\,\Delta \eps)=
\langle \delta g(\varphi,\,\eps)\, \delta
g(\varphi+\Delta\varphi,\,\eps+\Delta \eps)\rangle ( and \eps are
rescaled magnetic flux and energy) for the magnetoconductance of a ballistic
chaotic quantum dot is calculated in the framework of the supersymmetric
non-linear -model. The Hamiltonian of the quantum dot is modelled by a
Gaussian random matrix. The particular form of the symmetry breaking matrix is
found to be relevant for the autocorrelation function but not for the average
conductance. Our results are valid for the complete crossover from orthogonal
to unitary symmetry and their relation with semiclassical theory and an
-matrix Brownian motion ensemble is discussed.Comment: 7 pages, no figures, accepted for publication in Europhysics Letter
Rate equations for Coulomb blockade with ferromagnetic leads
We present a density-matrix rate-equation approach to sequential tunneling
through a metal particle weakly coupled to ferromagnetic leads. The
density-matrix description is able to deal with correlations between degenerate
many-electron states that the standard rate equation formalism in terms of
occupation probabilities cannot describe. Our formalism is valid for an
arbitrary number of electrons on the dot, for an arbitrary angle between the
polarization directions of the leads, and with or without spin-orbit scattering
on the metal particle. Interestingly, we find that the density-matrix
description may be necessary even for metal particles with unpolarized leads if
three or more single-electron levels contribute to the transport current and
electron-electron interactions in the metal particle are described by the
`universal interaction Hamiltonian'.Comment: 10 pages, 4 figures, REVTeX
Fine Structure in Energy Spectra of Ultrasmall Au Nanoparticles
We have studied tunneling into individual Au nanoparticles of estimated
diameters 2-5 nm, at dilution refrigerator temperatures. The I-V curves
indicate resonant tunneling via discrete energy levels of the particle. Unlike
previously studied normal metal particles of Au and Al, in these samples we
find that the lowest energy tunneling resonances are split into clusters of
2-10 subresonances. Such effects appear to be increasingly important in smaller
grains, as might be expected from the larger characteristic energies.Comment: 1 pdf fil
Spin and Conductance-Peak-Spacing Distributions in Large Quantum Dots: A Density Functional Theory Study
We use spin-density-functional theory to study the spacing between
conductance peaks and the ground-state spin of 2D model quantum dots with up to
200 electrons. Distributions for different ranges of electron number are
obtained in both symmetric and asymmetric potentials. The even/odd effect is
pronounced for small symmetric dots but vanishes for large asymmetric ones,
suggesting substantially stronger interaction effects than expected. The
fraction of high-spin ground states is remarkably large.Comment: 4 pages, 3 figure
Griffiths effects and quantum critical points in dirty superconductors without spin-rotation invariance: One-dimensional examples
We introduce a strong-disorder renormalization group (RG) approach suitable
for investigating the quasiparticle excitations of disordered superconductors
in which the quasiparticle spin is not conserved. We analyze one-dimensional
models with this RG and with elementary transfer matrix methods. We find that
such models with broken spin rotation invariance {\it generically} lie in one
of two topologically distinct localized phases. Close enough to the critical
point separating the two phases, the system has a power-law divergent
low-energy density of states (with a non-universal continuously varying
power-law) in either phase, due to quantum Griffiths singularities. This
critical point belongs to the same infinite-disorder universality class as the
one dimensional particle-hole symmetric Anderson localization problem, while
the Griffiths phases in the vicinity of the transition are controlled by lines
of strong (but not infinite) disorder fixed points terminating in the critical
point.Comment: 14 pages (two-column PRB format), 9 eps figure
Laparoscopic Sentinel Lymph Node Biopsy for Prostate Cancer: The Relevance of Locations Outside the Extended Dissection Area
Objective. To assess the relevance of sentinel lymph nodes (SNs) outside the extended pelvic lymph node dissection area (e-PLND). Patients and Methods. Evaluation of our laparoscopic SN procedures for prostate cancer patients of intermediate prognosis. Retrospective data collection on the exact location of the excised SNs and the pathology results were analyzed. Results and Limitations. Of the 121 patients, 49 had positive lymph nodes. 37 patients (31%) had SNs outside the e-PLND template. Five of these nodes were tumor bearing but only twice exclusively so. Of the 14 patients considered for salvage treatment, 6 were node positive. 7 of these 14 patients (50%) had SNs outside the extended dissection area, yet none of these nodes were tumor positive. Limitations are those of a retrospective study. Conclusions. Laparoscopic SN biopsy may show SNs outside the e-PLND template in 31% of the patients. However, nodes that are exclusively positive in one of these areas are rare. For the dichotomy positive or negative nodes, the locations outside the e-PLND area are not often relevant. Nevertheless, when all positive nodes are to be treated by resection or radiotherapy, these locations are relevant. When considering salvage treatment for prostate cancer, the method is feasible
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
Interactions and Disorder in Quantum Dots: Instabilities and Phase Transitions
Using a fermionic renormalization group approach we analyse a model where the
electrons diffusing on a quantum dot interact via Fermi-liquid interactions.
Describing the single-particle states by Random Matrix Theory, we find that
interactions can induce phase transitions (or crossovers for finite systems) to
regimes where fluctuations and collective effects dominate at low energies.
Implications for experiments and numerical work on quantum dots are discussed.Comment: 4 pages, 1 figure; version to appear in Phys Rev Letter
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
GENERALIZED CIRCULAR ENSEMBLE OF SCATTERING MATRICES FOR A CHAOTIC CAVITY WITH NON-IDEAL LEADS
We consider the problem of the statistics of the scattering matrix S of a
chaotic cavity (quantum dot), which is coupled to the outside world by
non-ideal leads containing N scattering channels. The Hamiltonian H of the
quantum dot is assumed to be an M x N hermitian matrix with probability
distribution P(H) ~ det[lambda^2 + (H - epsilon)^2]^[-(beta M + 2- beta)/2],
where lambda and epsilon are arbitrary coefficients and beta = 1,2,4 depending
on the presence or absence of time-reversal and spin-rotation symmetry. We show
that this ``Lorentzian ensemble'' agrees with microscopic theory for an
ensemble of disordered metal particles in the limit M -> infinity, and that for
any M >= N it implies P(S) ~ |det(1 - \bar S^{\dagger} S)|^[-(beta M + 2 -
beta)], where \bar S is the ensemble average of S. This ``Poisson kernel''
generalizes Dyson's circular ensemble to the case \bar S \neq 0 and was
previously obtained from a maximum entropy approach. The present work gives a
microscopic justification for the case that the chaotic motion in the quantum
dot is due to impurity scattering.Comment: 15 pages, REVTeX-3.0, 2 figures, submitted to Physical Review B
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