1,223 research outputs found
Coexistence of Coulomb blockade and zero bias anomaly in a strongly coupled quantum dot
The current-voltage characteristics through a metallic quantum dot which is
well coupled to a metallic lead are measured. It is shown that the I-V curves
are composed of two contributions. One is a suppression of the tunneling
conductivity at the Fermi level and the second is an oscillating feature which
shifts with gate voltage. The results indicate that Zero-Bias-Anomaly and
Coulomb Blockade phenomena coexist in an asymmetric strongly coupled quantum
dot.Comment: 4 pages, 4 figure
Ulta-slow relaxation in discontinuous-film based electron glasses
We present field effect measurements on discontinuous 2D thin films which are
composed of a sub monolayer of nano-grains of Au, Ni, Ag or Al. Like other
electron glasses these systems exhibit slow conductance relaxation and memory
effects. However, unlike other systems, the discontinuous films exhibit a
dramatic slowing down of the dynamics below a characteristic temperature .
is typically between 10-50K and is sample dependent. For the
sample exhibits a few other peculiar features such as repeatable conductance
fluctuations in millimeter size samples. We suggest that the enhanced system
sluggishness is related to the current carrying network becoming very dilute in
discontinuous films so that the system contains many parts which are
electrically very weakly connected and the transport is dominated by very few
weak links. This enables studying the glassy properties of the sample as it
transitions from a macroscopic sample to a mesocopic sample, hence, the results
provide new insight on the underlying physics of electron glasses.Comment: 4 pages, 4 figure
Zero bias anomaly in a two dimensional granular insulator
We compare tunneling density of states (TDOS) into two ultrathin Ag films,
one uniform and one granular, for different degrees of disorder. The uniform
film shows a crossover from Altshuler-Aronov (AA) zero bias anomaly to Efros
Shklovskii (ES) like Coulomb gap as the disorder is increased. The granular
film, on the other hand, exhibits AA behavior even deeply in the insulating
regime. We analyze the data and find that granularity introduces a new regime
for the TDOS. While the conductivity is dominated by hopping between clusters
of grains and is thus insulating, the TDOS probes the properties of an
individual cluster which is "metallic".Comment: 4 pages, 4 figure
Absence of weak antilocalization in ferromagnetic films
We present magnetoresistance measurements performed on ultrathin films of
amorphous Ni and Fe. In these films the Curie temperature drops to zero at
small thickness, making it possible to study the effect of ferromagnetism on
localization. We find that non-ferromagnetic films are characterized by
positive magnetoresistance. This is interpreted as resulting from weak
antilocalization due to strong Bychkov-Rashba spin orbit scattering. As the
films become ferromagnetic the magnetoresistance changes sign and becomes
negative. We analyze our data to identify the individual contributions of weak
localization, weak antilocalization and anisotropic magnetoresistance and
conclude that the magnetic order suppresses the influence of spin-orbit effects
on localization phenomena in agreement with theoretical predictions.Comment: 6 pages, 6 figure
Is a multiple excitation of a single atom equivalent to a single excitation of an ensemble of atoms?
Recent technological advances have enabled to isolate, control and measure
the properties of a single atom, leading to the possibility to perform
statistics on the behavior of single quantum systems. These experiments have
enabled to check a question which was out of reach previously: Is the
statistics of a repeatedly excitation of an atom N times equivalent to a single
excitation of an ensemble of N atoms? We present a new method to analyze
quantum measurements which leads to the postulation that the answer is most
probably no. We discuss the merits of the analysis and its conclusion.Comment: 3 pages, 3 figure
Magnetization of small lead particles
The magnetization of an ensemble of isolated lead grains of sizes ranging
from below 6 nm to 1000 nm is measured. A sharp disappearance of Meissner
effect with lowering of the grain size is observed for the smaller grains. This
is a direct observation by magnetization measurement of the occurrence of a
critical particle size for superconductivity, which is consistent with
Anderson's criterion.Comment: 7 pages, 5 figures, Submitted to PR
Percolation model for the superconductor-insulator transition in granular films
We study the temperature dependence of the superconductor-insulator
transition in granular superconductors. Empirically, these systems are
characterized by very broad resistance tails, which depend exponentially on the
temperature, and the normal state resistance. We model these systems by a
two-dimensional random resistor percolation networks in which the resistance
between two grains is governed either by Josephson junction coupling or by
quasi particle tunneling. Our numerical simulations as well as an effective
medium evaluation explain the experimental results over a wide range of
temperatures and resistances. Using effective medium approximation we find an
analytical expression for the effective resistance of the system and the value
of the critical resistance separating conducting from insulating branches.Comment: 4 pages, 2 figure
Subgap anomaly and above-energy-gap structure in chains of diffusive SNS junctions
We present the results of low-temperature transport measurements on chains of
superconductor--normal-constriction--superconductor (SNS) junctions fabricated
on the basis of superconducting PtSi film. A comparative study of the
properties of the chains, consisting of 3 and 20 SNS junctions in series, and
single SNS junctions reveals essential distinctions in the behavior of the
current-voltage characteristics of the systems: (i) the gradual decrease of the
effective suppression voltage for the excess conductivity observed at zero bias
as the quantity of the SNS junctions increases, (ii) a rich fine structure on
the dependences dV/dI-V at dc bias voltages higher than the superconducting gap
and corresponding to some multiples of 2\Delta/e. A model to explain this
above-energy-gap structure based on energy relaxation of electron via
Cooper-pair-breaking in superconducting island connecting normal metal
electrods is proposed.Comment: RevTex, 5 pages, 4 figure
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