388 research outputs found
Superconducting transport through a vibrating molecule
Nonequilibrium electronic transport through a molecular level weakly coupled
to a single coherent phonon/vibration mode has been studied for superconducting
leads. The Keldysh Green function formalism is used to compute the current for
the entire bias voltage range. In the subgap regime, Multiple Andreev
Reflection (MAR) processes accompanied by phonon emission cause rich structure
near the onset of MAR channels, including an even-odd parity effect that can be
interpreted in terms of an inelastic MAR ladder picture. Thereby we establish a
connection between the Keldysh formalism and the Landauer scattering approach
for inelastic MAR.Comment: 5 pages, 5 figures, version contains now more details, accepted by
PR
Transport and Noise Characteristics of Submicron High-Temperature Superconductor Grain-Boundary Junctions
We have investigated the transport and noise properties of submicron YBCO
bicrystal grain-boundary junctions prepared using electron beam lithography.
The junctions show an increased conductance for low voltages reminiscent of
Josephson junctions having a barrier with high transmissivity. The voltage
noise spectra are dominated by a few Lorentzian components. At low temperatures
clear two-level random telegraph switching (RTS) signals are observable in the
voltage vs time traces. We have investigated the temperature and voltage
dependence of individual fluctuators both from statistical analysis of voltage
vs time traces and from fits to noise spectra. A transition from tunneling to
thermally activated behavior of individual fluctuators was clearly observed.
The experimental results support the model of charge carrier traps in the
barrier region.Comment: 4 pages, 4 figures, to be published in Appl. Phys. Let
Field-Effect Transistors on Tetracene Single Crystals
We report on the fabrication and electrical characterization of field-effect
transistors at the surface of tetracene single crystals. We find that the
mobility of these transistors reaches the room-temperature value of $0.4 \
cm^2/Vs$. The non-monotonous temperature dependence of the mobility, its weak
gate voltage dependence, as well as the sharpness of the subthreshold slope
confirm the high quality of single-crystal devices. This is due to the
fabrication process that does not substantially affect the crystal quality.Comment: Accepted by Appl. Phys. Lett, tentatively scheduled for publication
in the November 24, 2003 issu
Enhancement of quasiparticle recombination in Ta and Al superconductors by implantation of magnetic and nonmagnetic atoms
The quasiparticle recombination time in superconducting films, consisting of
the standard electron-phonon interaction and a yet to be identified low
temperature process, is studied for different densities of magnetic and
nonmagnetic atoms. For both Ta and Al, implanted with Mn, Ta and Al, we observe
an increase of the recombination rate. We conclude that the enhancement of
recombination is not due to the magnetic moment, but arises from an enhancement
of disorder.Comment: 4 pages, 4 figure
Influence of the gate leakage current on the stability of organic single-crystal field-effect transistors
We investigate the effect of a small leakage current through the gate
insulator on the stability of organic single-crystal field-effect transistors
(FETs). We find that, irrespective of the specific organic molecule and
dielectric used, leakage current flowing through the gate insulator results in
an irreversible degradation of the single-crystal FET performance. This
degradation occurs even when the leakage current is several orders of magnitude
smaller than the source-drain current. The experimental data indicate that a
stable operation requires the leakage current to be smaller than $10^{-9} \
\mathrm{A/cm}^2$. Our results also suggest that gate leakage currents may
determine the lifetime of thin-film transistors used in applications.Comment: submitted to Appl. Phys. Let
Energy gap measurement of nanostructured thin aluminium films for use in single Cooper-pair devices
Within the context of superconducting gap engineering, Al-\alox-Al tunnel
junctions have been used to study the variation in superconducting gap,
, with film thickness. Films of thickness 5, 7, 10 and 30 nm were used
to form the small area superconductor-insulator-superconductor (SIS) tunnel
junctions. In agreement with previous measurements we have observed an increase
in the superconducting energy gap of aluminium with a decrease in film
thickness. In addition, we find grain size in small area films with thickness
\textbf{} 10 nm has no appreciable effect on energy gap. Finally, we
utilize 7 and 30 nm films in a single Cooper-pair transistor, and observe the
modification of the finite bias transport processes due to the engineered gap
profile
Reduced frequency noise in superconducting resonators
We report a reduction of the frequency noise in coplanar waveguide
superconducting resonators. The reduction of 7 dB is achieved by removing the
exposed dielectric substrate surface from the region with high electric fields
and by using NbTiN. In a model-analysis the surface of NbTiN is found to be a
negligible source of noise, experimentally supported by a comparison with NbTiN
on SiOx resonators. The reduction is additive to decreasing the noise by
widening the resonators.Comment: 4 pages, 4 figure
Dynamical Coulomb blockade of multiple Andreev reflections
We analyze the dynamical Coulomb blockade of multiple Andreev reflections
(MAR) in a superconducting quantum point contact coupled to a macroscopic
impedance. We find that at very low transmission the blockade scales as
with , where is the bias voltage and is the
superconducting gap, as it would correspond to the occurrence of "shots" of
charge . For higher transmission the blockade is reduced both due to Pauli
principle and to elastic renormalization of the MAR probability, and for
certain voltage regions it may even become an "antiblockade", i.e. the current
is enhanced due to the coupling with the electromagnetic environment.Comment: 5 pages, 4 figures, submitted to Phys. Rev. Let
- …