504 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
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
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
Critical voltage of a mesoscopic superconductor
We study the role of the quasiparticle distribution function f on the
properties of a superconducting nanowire. We employ a numerical calculation
based upon the Usadel equation. Going beyond linear response, we find a
non-thermal distribution for f caused by an applied bias voltage. We
demonstrate that the even part of f (the energy mode f_L) drives a first order
transition from the superconducting state to the normal state irrespective of
the current
Coherent back-scattering near the two-dimensional metal-insulator transition
We have studied corrections to conductivity due to the coherent
backscattering in low-disordered two-dimensional electron systems in silicon
for a range of electron densities including the vicinity of the metal-insulator
transition, where the dramatic increase of the spin susceptibility has been
observed earlier. We show that the corrections, which exist deeper in the
metallic phase, weaken upon approaching to the transition and practically
vanish at the critical density, thus suggesting that the localization is
suppressed near and at the transition even in zero field.Comment: to appear in PR
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
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
Quasiparticle relaxation in optically excited high-Q superconducting resonators
The quasiparticle relaxation time in superconducting films has been measured
as a function of temperature using the response of the complex conductivity to
photon flux. For tantalum and aluminium, chosen for their difference in
electron-phonon coupling strength, we find that at high temperatures the
relaxation time increases with decreasing temperature, as expected for
electron-phonon interaction. At low temperatures we find in both
superconducting materials a saturation of the relaxation time, suggesting the
presence of a second relaxation channel not due to electron-phonon interaction.Comment: 4 pages, 3 figure
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