7,763 research outputs found
Penalized maximum likelihood and semiparametric second-order efficiency
We consider the problem of estimation of a shift parameter of an unknown
symmetric function in Gaussian white noise. We introduce a notion of
semiparametric second-order efficiency and propose estimators that are
semiparametrically efficient and second-order efficient in our model. These
estimators are of a penalized maximum likelihood type with an appropriately
chosen penalty. We argue that second-order efficiency is crucial in
semiparametric problems since only the second-order terms in asymptotic
expansion for the risk account for the behavior of the ``nonparametric
component'' of a semiparametric procedure, and they are not dramatically
smaller than the first-order terms.Comment: Published at http://dx.doi.org/10.1214/009053605000000895 in the
Annals of Statistics (http://www.imstat.org/aos/) by the Institute of
Mathematical Statistics (http://www.imstat.org
Strong Electron Tunneling through a Small Metallic Grain
Electron tunneling through mesoscopic metallic grains can be treated
perturbatively only provided the tunnel junction conductances are sufficiently
small. If it is not the case, fluctuations of the grain charge become strong.
As a result (i) contributions of all -- including high energy -- charge states
become important and (ii) excited charge states become broadened and
essentially overlap. At the same time the grain charge remains discrete and the
system conductance -periodically depends on the gate charge. We develop a
nonperturbative approach which accounts for all these features and calculate
the temperature dependent conductance of the system in the strong tunneling
regime at different values of the gate charge.Comment: revtex, 8 pages, 2 .ps figure
Statistics of voltage fluctuations in resistively shunted Josephson junctions
The intrinsic nonlinearity of Josephson junctions converts Gaussian current
noise in the input into non-Gaussian voltage noise in the output. For a
resistively shunted Josephson junction with white input noise we determine
numerically exactly the properties of the few lowest cumulants of the voltage
fluctuations, and we derive analytical expressions for these cumulants in
several important limits. The statistics of the voltage fluctuations is found
to be Gaussian at bias currents well above the Josephson critical current, but
Poissonian at currents below the critical value. In the transition region close
to the critical current the higher-order cumulants oscillate and the voltage
noise is strongly non-Gaussian. For coloured input noise we determine the third
cumulant of the voltage.Comment: 9 pages, 5 figure
Evidence for magnetoplasmon character of the cyclotron resonance response of a two-dimensional electron gas
Experimental results on the absolute magneto-transmission of a series of high
density, high mobility GaAs quantum wells are compared with the predictions of
a recent magnetoplasmon theory for values of the filling factor above 2. We
show that the magnetoplasmon picture can explain the non-linear features
observed in the magnetic field evolution of the cyclotron resonance energies
and of the absorption oscillator strength. This provides experimental evidence
that inter Landau level excitations probed by infrared spectroscopy need to be
considered as many body excitations in terms of magnetoplasmons: this is
especially true when interpreting the oscillator strengths of the cyclotron
transitions
Strong Tunneling and Coulomb Blockade in a Single-Electron Transistor
We have developed a detailed experimental study of a single-electron
transistor in a strong tunneling regime. Although weakened by strong charge
fluctuations, Coulomb effects were found to persist in all samples including
one with the effective conductance 8 times higher than the quantum value (6.45
k). A good agreement between our experimental data and
theoretical results for the strong tunneling limit is found. A reliable
operation of transistors with conductances 3-4 times larger than the quantum
value is demonstrated.Comment: revtex, 4 page
Electron transport through interacting quantum dots
We present a detailed theoretical investigation of the effect of Coulomb
interactions on electron transport through quantum dots and double barrier
structures connected to a voltage source via an arbitrary linear impedance.
Combining real time path integral techniques with the scattering matrix
approach we derive the effective action and evaluate the current-voltage
characteristics of quantum dots at sufficiently large conductances. Our
analysis reveals a reach variety of different regimes which we specify in
details for the case of chaotic quantum dots. At sufficiently low energies the
interaction correction to the current depends logarithmically on temperature
and voltage. We identify two different logarithmic regimes with the crossover
between them occurring at energies of order of the inverse dwell time of
electrons in the dot. We also analyze the frequency-dependent shot noise in
chaotic quantum dots and elucidate its direct relation to interaction effects
in mesoscopic electron transport.Comment: 21 pages, 4 figures. References added, discussion slightly extende
Coulomb Interaction and Quantum Transport through a Coherent Scatterer
An interplay between charge discreteness, coherent scattering and Coulomb
interaction yields nontrivial effects in quantum transport. We derive a real
time effective action and an equivalent quantum Langevin equation for an
arbitrary coherent scatterer and evaluate its current-voltage characteristics
in the presence of interactions. Within our model, at large conductances
and low (but outside the instanton-dominated regime) the interaction
correction to saturates and causes conductance suppression by a universal
factor which depends only on the type of the conductor.Comment: 4 pages, no figure
Quantal Brownian Motion - Dephasing and Dissipation
We analyze quantal Brownian motion in dimensions using the unified model
for diffusion localization and dissipation, and Feynman-Vernon formalism. At
high temperatures the propagator possess a Markovian property and we can write
down an equivalent Master equation. Unlike the case of the
Zwanzig-Caldeira-Leggett model, genuine quantum mechanical effects manifest
themselves due to the disordered nature of the environment. Using Wigner
picture of the dynamics we distinguish between two different mechanisms for
destruction of coherence. The analysis of dephasing is extended to the low
temperature regime by using a semiclassical strategy. Various results are
derived for ballistic, chaotic, diffusive, both ergodic and non-ergodic motion.
We also analyze loss of coherence at the limit of zero temperature and clarify
the limitations of the semiclassical approach. The condition for having
coherent effect due to scattering by low-frequency fluctuations is also pointed
out. It is interesting that the dephasing rate can be either larger or smaller
than the dissipation rate, depending on the physical circumstances.Comment: LaTex, 23 pages, 4 figures, published vesio
Full Counting Statistics for a Single-Electron Transistor, Non-equilibrium Effects at Intermediate Conductance
We evaluate the current distribution for a single-electron transistor with
intermediate strength tunnel conductance. Using the Schwinger-Keldysh approach
and the drone (Majorana) fermion representation we account for the
renormalization of system parameters. Nonequilibrium effects induce a lifetime
broadening of the charge-state levels, which suppress large current
fluctuations.Comment: 4 pages, 1 figur
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