18 research outputs found
Shot noise measurements in NS junctions and the semiclassical theory
We present a new analysis of shot noise measurements in normal
metal-superconductor (NS) junctions [X. Jehl et al., Nature 405, 50 (2000)],
based on a recent semiclassical theory. The first calculations at zero
temperature assuming quantum coherence predicted shot noise in NS contacts to
be doubled with respect to normal contacts. The semiclassical approach gives
the first opportunity to compare data and theory quantitatively at finite
voltage and temperature. The doubling of shot noise is predicted up to the
superconducting gap, as already observed, confirming that phase coherence is
not necessary. An excellent agreement is also found above the gap where the
noise follows the normal case.Comment: 2 pages, revtex, 2 eps figures, to appear in Phys. Rev.
Andreev reflection in Au/La_{2-x}Sr_{x}CuO_{4} point-contact junctions: separation between pseudogap and phase-coherence gap
We made point-contact measurements with Au tips on La_{2-x}Sr_{x}CuO_{4}
samples with 0.08 < x < 0.20 to investigate the relationship between
superconducting gap and pseudogap. We obtained junctions whose conductance
curves presented typical Andreev reflection features at all temperatures from
4.2 K up to T_c^A close to the bulk T_c. Their fit with the
BTK-Tanaka-Kashiwaya model gives good results if a (s+d)-wave gap symmetry is
used. The doping dependence of the low temperature dominant isotropic gap
component Delta_{s} follows very well the T_{c} vs. x curve. These results
support the separation between the superconducting (Andreev) gap and the
pseudogap measured by angle-resolved photoemission spectroscopy (ARPES) and
tunneling.Comment: 4 pages, 5 eps figures, 1 table. SNS 2001 Conferenc
Shot noise in tunneling transport through molecules and quantum dots
We consider electrical transport through single molecules coupled to metal
electrodes via tunneling barriers. Approximating the molecule by the Anderson
impurity model as the simplest model which includes Coulomb charging effects,
we extend the ``orthodox'' theory to expand current and shot noise
systematically order by order in the tunnel couplings. In particular, we show
that a combined measurement of current and shot noise reveals detailed
information of the system even in the weak-coupling limit, such as the ratio of
the tunnel-coupling strengths of the molecule to the left and right electrode,
and the presence of the Coulomb charging energy. Our analysis holds for
single-level quantum dots as well.Comment: 8 page
Electron-electron scattering effects on the Full Counting Statistics of Mesoscopic Conductors
In the hot electron regime, electron-electron scattering strongly modifies
not only the shot noise but also the full counting statistics. We employ a
method based on a stochastic path integral to calculate the counting statistics
of two systems in which noise in the hot electron regime has been
experimentally measured. We give an analytical expression for the counting
statistics of a chaotic cavity and find that heating due to electron-electron
scattering renders the distribution of transmitted charge symmetric in the shot
noise limit. We also discuss the frequency dispersion of the third order
correlation function and present numerical calculations for the statistics of
diffusive wires in the hot electron regime
Influence of a Random Telegraph Process on the Transport through a Point Contact
We describe the transport properties of a point contact under the influence
of a classical two-level fluctuator. We employ a transfer matrix formalism
allowing us to calculate arbitrary correlation functions of the stochastic
process by mapping them on matrix products. The result is used to obtain the
generating function of the full counting statistics of a classical point
contact subject to a classical fluctuator, including extensions to a pair of
two-level fluctuators as well as to a quantum point contact. We show that the
noise in the quantum point contact is a sum of the (quantum) partitioning noise
and the (classical) noise due to the two-level fluctuator. As a side result, we
obtain the full counting statistics of a quantum point contact with
time-dependent transmission probabilities.Comment: 8 pages, 2 figure; a new section about experiments and a figure
showing the crossover from sub- to superpoissonian noise have been adde
Spin-Flip Noise in a Multi-Terminal Spin-Valve
We study shot noise and cross correlations in a four terminal spin-valve
geometry using a Boltzmann-Langevin approach. The Fano factor (shot noise to
current ratio) depends on the magnetic configuration of the leads and the
spin-flip processes in the normal metal. In a four-terminal geometry, spin-flip
processes are particular prominent in the cross correlations between terminals
with opposite magnetization.Comment: 4 pages, 3 figure
Current noise in long diffusive SNS junctions in the incoherent MAR regime
Spectral density of current fluctuations at zero frequency is calculated for
a long diffusive SNS junction with low-resistive interfaces. At low
temperature, T << Delta, the subgap shot noise approaches linear voltage
dependence, S=(2/ 3R)(eV + 2Delta), which is the sum of the shot noise of the
normal conductor and voltage independent excess noise. This result can also be
interpreted as the 1/3-suppressed Poisson noise for the effective charge q =
e(1+2Delta/eV) transferred by incoherent multiple Andreev reflections (MAR). At
higher temperatures, anomalies of the current noise develop at the gap
subharmonics, eV = 2Delta/n. The crossover to the hot electron regime from the
MAR regime is analyzed in the limit of small applied voltages.Comment: improved version, to be published in Phys. Rev.
Interface effects on the shot noise in normal metal- d-wave superconductor Junctions
The current fluctuation in normal metal / d-wave superconductor junctions are
studied for various orientation of the crystal by taking account of the spatial
variation of the pair potentials. Not only the zero-energy Andreev bound states
(ZES) but also the non-zero energy Andreev bound states influence on the
properties of differential shot noise. At the tunneling limit, the noise power
to current ratio at zero voltage becomes 0, once the ZES are formed at the
interface. Under the presence of a subdominant s-wave component at the
interface which breaks time-reversal symmetry, the ratio becomes 4eComment: 13 pages, 3 figure
Stochastic resonance in an RF SQUID with shunted ScS junction
Using a point (superconductor–constriction–superconductor, ScS) contact in a single-Josephson-junction superconducting quantum interference device (RF SQUID) provides stochastic resonance conditions at any arbitrary small value of loop inductance and contact critical current, unlike SQUIDs with more traditional tunnel (superconductor–insulator–superconductor, SIS) junctions. This is due to the unusual potential energy of the ScS RF SQUID which always has a barrier between two wells, thus making the device bistable. This paper presents the results of a numerical simulation of the stochastic dynamics of the magnetic flux in an ScS RF SQUID loop affected by band-limited white Gaussian noise and low-frequency sine signals of small and moderate amplitudes. The difference in stochastic amplification of RF SQUID loops incorporating ScS and SIS junctions is discussed
Shot noise in normal metal-d-wave superconducting junctions
We present theoretical calculations and predictions for the shot noise in
voltage biased junctions of superconductors and normal metal
counter-electrodes. In the clean limit for the d-wave superconductor the shot
noise vanishes at zero voltage because of resonant Andreev reflection by
zero-energy surface bound states. We examine the sensitivity of this resonance
to impurity scattering. We report theoretical results for the magnetic field
dependence of the shot noise, as well the fingerprints of subdominant - and
pairing channels.Comment: 15 pages, 8 figures and 3 tables embedde