4,775 research outputs found
Measurement of the energy dependence of phase relaxation by single electron tunneling
Single electron tunneling through a single impurity level is used to probe
the fluctuations of the local density of states in the emitter. The energy
dependence of quasi-particle relaxation in the emitter can be extracted from
the damping of the fluctuations of the local density of states (LDOS). At
larger magnetic fields Zeeman splitting is observed.Comment: 2 pages, 4 figures; 25th International Conference on the Physics of
Semiconductors, Osaka, Japan, September 17-22, 200
A large-deviations approach to gelation
A large-deviations principle (LDP) is derived for the state at fixed time, of the multiplicative coalescent in the large particle number limit. The rate function is explicit and describes each of the three parts of the state: microscopic, mesoscopic and macroscopic. In particular, it clearly captures the well known gelation phase transition given by the formation of a particle containing a positive fraction of the system mass. Via a standard map of the multiplicative coalescent onto a time-dependent version of the Erdős-Rényi random graph, our results can also be rephrased as an LDP for the component sizes in that graph. The proofs rely on estimates and asymptotics for the probability that smaller Erdős-Rényi graphs are connected
Interplay of Coulomb interaction and spin-orbit effects in multi-level quantum dots
We study electron transport through a multi-level quantum dot with Rashba
spin-orbit interaction in the presence of local Coulomb repulsion. Motivated by
recent experiments, we compute the level splitting induced by the spin-orbit
interaction at finite Zeeman fields , which provides a measure of the
renormalized spin-orbit energy. This level splitting is responsible for the
suppression of the Kondo ridges at finite characteristic for the
multi-level structure. In addition, the dependence of renormalized -factors
on the relative orientation of the applied field and the spin-orbit
direction following two different protocols used in experiments is
investigated.Comment: 11 pages, 13 figure
Kondo quantum dot coupled to ferromagnetic leads: Numerical renormalization group study
We systematically study the influence of ferromagnetic leads on the Kondo
resonance in a quantum dot tuned to the local moment regime. We employ Wilson's
numerical renormalization group method, extended to handle leads with a spin
asymmetric density of states, to identify the effects of (i) a finite spin
polarization in the leads (at the Fermi-surface), (ii) a Stoner splitting in
the bands (governed by the band edges) and (iii) an arbitrary shape of the
leads density of states. For a generic lead density of states the quantum dot
favors being occupied by a particular spin-species due to exchange interaction
with ferromagnetic leads leading to a suppression and splitting of the Kondo
resonance. The application of a magnetic field can compensate this asymmetry
restoring the Kondo effect. We study both the gate-voltage dependence (for a
fixed band structure in the leads) and the spin polarization dependence (for
fixed gate voltage) of this compensation field for various types of bands.
Interestingly, we find that the full recovery of the Kondo resonance of a
quantum dot in presence of leads with an energy dependent density of states is
not only possible by an appropriately tuned external magnetic field but also
via an appropriately tuned gate voltage. For flat bands simple formulas for the
splitting of the local level as a function of the spin polarization and gate
voltage are given.Comment: 18 pages, 18 figures, accepted for publication in PR
Noisy Preprocessing and the Distillation of Private States
We provide a simple security proof for prepare & measure quantum key
distribution protocols employing noisy processing and one-way postprocessing of
the key. This is achieved by showing that the security of such a protocol is
equivalent to that of an associated key distribution protocol in which, instead
of the usual maximally-entangled states, a more general {\em private state} is
distilled. Besides a more general target state, the usual entanglement
distillation tools are employed (in particular, Calderbank-Shor-Steane
(CSS)-like codes), with the crucial difference that noisy processing allows
some phase errors to be left uncorrected without compromising the privacy of
the key.Comment: 4 pages, to appear in Physical Review Letters. Extensively rewritten,
with a more detailed discussion of coherent --> iid reductio
Progressor: Social navigation support through open social student modeling
The increased volumes of online learning content have produced two problems: how to help students to find the most appropriate resources and how to engage them in using these resources. Personalized and social learning have been suggested as potential ways to address these problems. Our work presented in this paper combines the ideas of personalized and social learning in the context of educational hypermedia. We introduce Progressor, an innovative Web-based tool based on the concepts of social navigation and open student modeling that helps students to find the most relevant resources in a large collection of parameterized self-assessment questions on Java programming. We have evaluated Progressor in a semester-long classroom study, the results of which are presented in this paper. The study confirmed the impact of personalized social navigation support provided by the system in the target context. The interface encouraged students to explore more topics attempting more questions and achieving higher success rates in answering them. A deeper analysis of the social navigation support mechanism revealed that the top students successfully led the way to discovering most relevant resources by creating clear pathways for weaker students. © 2013 Taylor and Francis Group, LLC
Identical Bands in Superdeformed Nuclei: A Relativistic Description
Relativistic Mean Field Theory in the rotating frame is used to describe
superdeformed nuclei. Nuclear currents and the resulting spatial components of
the vector meson fields are fully taken into account. Identical bands in
neighboring Rare Earth nuclei are investigated and excellent agreement with
recent experimental data is observed.Comment: 11 pages (Latex) and 4 figures (available upon request)
TUM-ITP-Ko93/
Kondo Correlations and the Fano Effect in Closed AB-Interferometers
We study the Fano-Kondo effect in a closed Aharonov-Bohm (AB) interferometer
which contains a single-level quantum dot and predict a frequency doubling of
the AB oscillations as a signature of Kondo-correlated states. Using Keldysh
formalism, Friedel sum rule and Numerical Renormalization Group, we calculate
the exact zero-temperature linear conductance as a function of AB phase
and level position . In the unitary limit, reaches
its maximum at . We find a Fano-suppressed Kondo plateau
for similar to recent experiments.Comment: 4 pages, 4 eps 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
Transmission phase lapses in quantum dots: the role of dot-lead coupling asymmetry
Lapses of transmission phase in transport through quantum dots are ubiquitous
already in the absence of interaction, in which case their precise location is
determined by the signs and magnitudes of the tunnelling matrix elements.
However, actual measurements for a quantum dot embedded in an Aharonov-Bohm
interferometer show systematic sequences of phase lapses separated by Coulomb
peaks -- an issue that attracted much attention and generated controversy.
Using a two-level quantum dot as an example we show that this phenomenon can be
accounted for by the combined effect of asymmetric dot-lead couplings (left
lead/right lead asymmetry as well as different level broadening for different
levels) and interaction-induced "population switching" of the levels, rendering
this behaviour generic. We construct and analyse a mean field scheme for an
interacting quantum dot, and investigate the properties of the mean field
solution, paying special attention to the character of its dependence
(continuous vs. discontinuous) on the chemical potential or gate voltage.Comment: 34 LaTeX pages in IOP format, 9 figures; misprints correcte
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