17,691 research outputs found
Supercurrent on a vortex core in 2H-NbSe: current driven scanning tunneling spectroscopy
We report current driven scanning tunneling spectroscopy (CDSTS) measurements
at very low temperatures on vortices in 2H-NbSe2. We find that a current
produces an increase of the density of states at the Fermi level in between
vortices, and a reduction of the zero bias peak at the vortex center. This
occurs well below the de-pairing current. We conclude that a supercurrent
affects the low energy part of the superconducting gap structure of 2H-NbSe2.Comment: 5 pages, 5 figure
Superconducting nanobridges under magnetic fields
We report on the study of superconducting nanotips and nanobridges of lead
with a Scanning Tunnelling Microscope in tunnel and point contact regimes. We
deal with three different structures. A nanotip that remains superconducting
under a field of 2 T. For this case we present model calculations of the order
parameter, which are in good agreement with the experiments. An asymmetric
nanobridge of lead showing a two steps loss of the Andreev excess current due
to different heating and dissipation phenomena in each side of the structure. A
study of the effect of the thermal fluctuations on the Josephson coupling
between the two sides of a superconducting nanobridge submitted to magnetic
fields. The different experiments were made under magnetic fields up to twenty
five times the volume critical field of lead, and in a temperature range
between 0.6 K and 7.2 K.Comment: 17 pages, 7 figure
A Growth model for DNA evolution
A simple growth model for DNA evolution is introduced which is analytically
solvable and reproduces the observed statistical behavior of real sequences.Comment: To be published in Europhysics Letter
Experimental demonstration of digital predistortion for orthogonal frequency-division multiplexing-radio over fibre links near laser resonance
Radio over fibre (RoF), an enabling technology for distribution of wireless broadband service signals through analogue optical links, suffers from non-linear distortion. Digital predistortion has been demonstrated as an effective approach to overcome the RoF non-linearity. However, questions remain as to how the approach performs close to laser resonance, a region of significant dynamic non-linearity, and how resilient the approach is to changes in input signal and link operating conditions. In this work, the performance of a digital predistortion approach is studied for directly modulated orthogonal frequency-division multiplexing RoF links operating from 2.47 to 3.7 GHz. It extends previous works to higher frequencies, and to higher quadrature amplitude modulation (QAM) levels. In addition, the resilience of the predistortion approach to changes in modulation level of QAM schemes, and average power levels are investigated, and a novel predistortion training approach is proposed and demonstrated. Both memoryless and memory polynomial predistorter models, and a simple off-line least-squares-based identification method, are used, with excellent performance improvements demonstrated up to 3.0 GHz
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