523 research outputs found
Mesoscopic Fluctuations in Quantum Dots in the Kondo Regime
Properties of the Kondo effect in quantum dots depend sensitively on the
coupling parameters and so on the realization of the quantum dot -- the Kondo
temperature itself becomes a mesoscopic quantity. Assuming chaotic dynamics in
the dot, we use random matrix theory to calculate the distribution of both the
Kondo temperature and the conductance in the Coulomb blockade regime. We study
two experimentally relevant cases: leads with single channels and leads with
many channels. In the single-channel case, the distribution of the conductance
is very wide as fluctuates on a logarithmic scale. As the number of
channels increases, there is a slow crossover to a self-averaging regime.Comment: 4 pages, 3 figure
“Medical high risk” designation is not associated with survival after carotid artery stenting
BackgroundWhile medical high risk (MHR) has been proposed as an indication for carotid artery stenting (CAS), the impact of MHR on long-term survival and stroke after CAS has not been described.MethodsA retrospective chart review of CAS procedures at our institution was performed. One hundred seventy-nine consecutive patients who underwent 196 CAS procedures were classified by MHR status based on cardiac, pulmonary, and renal criteria routinely used in high-risk clinical trials. Survival and stroke rates were compared after 90 CAS procedures in MHR patients vs 106 CAS procedures in normal risk patients. Survival results were also compared with 365 contemporaneous carotid endarterectomy (CEA) procedures in 346 patients.ResultsThe mean age of CAS patients was 72 years, with 87% having a smoking history, 85% hypertension, 38% diabetes, 39% symptomatic, and 74% documented coronary artery disease. Mean follow-up was 23 months. Recurrent stenosis after CEA comprised 21% of all CAS procedures. During the 30-day post-procedure period, there were five minor strokes, one major stroke, and one death, for a combined stroke/death rate of 3.6%. Kaplan-Meier analysis demonstrated mortality of 5% at 1 year and 21% at 3 years for the entire cohort. Cox regression analysis found that MHR designation was not associated with increased mortality or an increase in a composite end point of death or stroke. MHR patients had mortality of 4% at 1 year and 22% at 3 years. Normal risk patients had mortality of 6% at 1 year and 20% at 3 years. Preoperative age over 80 years old, low density lipoprotein (LDL) ≥160 mg/dL, and serum creatinine ≥1.5 mg/dL conferred statistically significant risk for death (Hazard ratios: 2.9, 4.3, and 2.4, respectively). As a point of comparison, a contemporaneous group of CEA patients were analyzed similarly. After adjusting for age over 80 years old and serum creatinine ≥1.5 mg/dL, there was no survival difference between MHR patients undergoing CAS or CEA.ConclusionsThe presence of MHR did not impact long-term survival or stroke rate after CAS, and overall survival of MHR patients in our series was comparable with risk-adjusted controls undergoing CEA. These results suggest the need for more refined predictors of medical risk to optimally guide patients in selecting carotid revascularization strategies
Electron Cotunneling in a Semiconductor Quantum Dot
We report transport measurements on a semiconductor quantum dot with a small
number of confined electrons. In the Coulomb blockade regime, conduction is
dominated by cotunneling processes. These can be either elastic or inelastic,
depending on whether they leave the dot in its ground state or drive it into an
excited state, respectively. We are able to discriminate between these two
contributions and show that inelastic events can occur only if the applied bias
exceeds the lowest excitation energy. Implications to energy-level spectroscopy
are discussed.Comment: To be published in Phys. Rev. Let
Structure and transport in multi-orbital Kondo systems
We consider Kondo impurity systems with multiple local orbitals, such as rare
earth ions in a metallic host or multi--level quantum dots coupled to metallic
leads. It is shown that the multiplet structure of the local orbitals leads to
multiple Kondo peaks above the Fermi energy , and to ``shadow'' peaks
below . We use a slave boson mean field theory, which recovers the strong
coupling Fermi liquid fixed point, to calculate the Kondo peak positions,
widths, and heights analytically at T=0, and NCA calculations to fit the
temperature dependence of high--resolution photoemission spectra of Ce
compounds. In addition, an approximate conductance quantization for transport
through multi--level quantum dots or single--atom transistors in the Kondo
regime due to a generalized Friedel sum rule is demonstrated.Comment: 4 pages, 3 figures. Invited article, 23rd International Conference on
Low Temperature Physics LT23, Hiroshima, Japan 200
Mesoscopic Coulomb Blockade in One-channel Quantum Dots
Signatures of "mesoscopic Coulomb blockade" are reported for quantum dots
with one fully transmitting point-contact lead, T1 = 1, T2 << 1. Unlike Coulomb
blockade (CB) in weak-tunneling devices (T1, T2 << 1), one-channel CB is a
mesoscopic effect requiring quantum coherence. Several distinctive features of
mesoscopic CB are observed, including a reduction in CB upon breaking
time-reversal symmetry with a magnetic field, relatively large fluctuations of
peak position as a function of magnetic field, and strong temperature
dependence on the scale of the quantum level spacing.Comment: 12 pages, including 4 figure
Coulomb Blockade Fluctuations in Strongly Coupled Quantum Dots
Quantum fluctuations of Coulomb blockade are investigated as a function of
the coupling to reservoirs in semiconductor quantum dots. We use fluctuations
in the distance between peaks apart to characterize both the
amplitude and correlation of peak motion. For strong coupling, peak motion is
greatly enhanced at low temperature, but does not show an increase in
peak-to-peak correlation. These effects can lead to anomalous temperature
dependence in the Coulomb valleys, similar to behavior ascribed to Kondo
physics.Comment: figures made smaller so download works. Revised, including new data.
Related papers at http://www.stanford.edu/group/MarcusLab/grouppubs.htm
On the perturbative expansion of the magnetization in the out-of-equilibrium Kondo model
This paper is concerned with the out-of-equilibrium two-lead Kondo model,
considered as a model of a quantum dot in the Kondo regime. We revisit the
perturbative expansion of the dot's magnetization, and conclude that, even at
order 0 in the Kondo interactions, the magnetization is not given by the usual
equilibrium result. We use the Schwinger-Keldysh method to derive a Dyson
equation describing the steady state induced by the voltage between the two
leads, and thus present the correct procedure for calculating perturbative
expansions of steady-state properties of the system.Comment: Minor corrections forgotten in v
Fano Resonances in Electronic Transport through a Single Electron Transistor
We have observed asymmetric Fano resonances in the conductance of a single
electron transistor resulting from interference between a resonant and a
nonresonant path through the system. The resonant component shows all the
features typical of quantum dots, but the origin of the non-resonant path is
unclear. A unique feature of this experimental system, compared to others that
show Fano line shapes, is that changing the voltages on various gates allows
one to alter the interference between the two paths.Comment: 8 pages, 6 figures. Submitted to PR
Kondo effects in a C_60 single-molecule transistor
We have used the electromigration technique to fabricate a
single-molecule transistor (SMT). We present a full
experimental study as a function of temperature, down to 35 mK, and as a
function of magnetic field up to 8 T in a SMT with odd number of electrons,
where the usual spin-1/2 Kondo effect occurs, with good agreement with theory.
In the case of even number of electrons, a low temperature magneto-transport
study is provided, which demonstrates a Zeeman splitting of the zero-bias
anomaly at energies well below the Kondo scale.Comment: 4 pages, 3 figure
Conduction through a quantum dot near a singlet-triplet transition
Kondo effect in the vicinity of a singlet-triplet transition in a vertical
quantum dot is considered. This system is shown to map onto a special version
of the two-impurity Kondo model. At any value of the control parameter, the
system has a Fermi-liquid ground state. Explicit expressions for the linear
conductance as a function of the control parameter and temperature are
obtained. At T=0, the conductance reaches the unitary limit at
the triplet side of the transition, and decreases with the increasing distance
to the transition at the singlet side. At finite temperature, the conductance
exhibits a peak near the transition point
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