58 research outputs found
Measurements of flux dependent screening in Aharonov-Bohm rings
In order to investigate the effect of electronic phase coherence on screening
we have measured the flux dependent polarizability of isolated mesoscopic rings
at 350 MHz. At low temperature (below 100 mK) both non-dissipative and
dissipative parts of the polarizability exhibit flux oscillations with a period
of half a flux quantum in a ring. The sign and amplitude of the effect are in
good agreement with recent theoretical predictions. The observed positive
magneto-polarizability corresponds to an enhancement of screening when time
reversal symmetry is broken. The effect of electronic density and temperature
are also measured.Comment: 4 pages, revtex, 4 figures, to appear in Phys. Rev. Let
Magneto-polarisability of mesoscopic rings
We calculate the average polarisability of two dimensional mesoscopic rings
in the presence of an Aharonov-Bohm flux. The screening is taken into account
self-consistently within a mean-field approximation. We investigate the effects
of statistical ensemble, finite frequency and disorder. We emphasize
geometrical effects which make the observation of field dependent
polarisability much more favourable on rings than on disks or spheres of
comparable radius. The ratio of the flux dependent to the flux independent part
is estimated for typical GaAs rings.Comment: pages, Revtex, 1 eps figur
Effect of disorder on the conductance of a Cu atomic point contact
We present a systematic study of the effect of the disorder in copper point
contacts. We show that peaks in the conductance histogram of copper point
contacts shift upon addition of nickel impurities. The shift increases
initially linerarly with the nickel concentration, thus confirming that it is
due to disorder in the nanowire, in accordance with predictions. In general,
this shift is modelled as a resistance R_s which is placed in series with the
contact resistance R_c. However, we obtain different R_s values for the two
peaks in the histogram, R_s being larger for the peak at higher conductance.Comment: 6 pages, 4 figure
Magneto-polarisability of mesoscopic systems
In order to understand how screening is modified by electronic interferences
in a mesoscopic isolated system, we have computed both analytically and
numerically the average thermodynamic and time dependent polarisabilities of
two dimensional mesoscopic samples in the presence of an Aharonov-Bohm flux.
Two geometries have been considered: rings and squares. Mesoscopic correction
to screening are taken into account in a self consistent way, using the
response function formalism. The role of the statistical ensemble (canonical
and grand canonical), disorder and frequency have been investigated. We have
also computed first order corrections to the polarisability due to
electron-electron interactions. Our main results concern the diffusive regime.
In the canonical ensemble, there is no flux dependence polarisability when the
frequency is smaller than the level spacing. On the other hand, in the grand
canonical ensemble for frequencies larger than the mean broadening of the
energy levels (but still small compared to the level spacing), the
polarisability oscillates with flux, with the periodicity . The order of
magnitude of the effect is given by , where is the Thomas Fermi screening length, the
width of the rings or the size of the squares and their average
dimensionless conductance. This magnetopolarisability of Aharonov-Bohm rings
has been recently measured experimentally \cite{PRL_deblock00} and is in good
agreement with our grand canonical result.Comment: 12 pages, 10 figures, revte
Frequency Dependence of Magnetopolarizability of Mesoscopic Grains
We calculate average magnetopolarizability of an isolated metallic sample at
frequency comparable to the mean level spacing . The frequency
dependence of the magnetopolarizability is described by a universal function of
.Comment: 3 pages, 1 figur
Local tunneling spectroscopy of the electron-doped cuprate Sm1.85Ce0.15CuO4
We present local tunneling spectroscopy in the optimally electron-doped
cuprate Sm2-xCexCuO4 x=0.15. A clear signature of the superconducting gap is
observed with an amplitude ranging from place to place and from sample to
sample (Delta~3.5-6meV). Another spectroscopic feature is simultaneously
observed at high energy above \pm 50meV. Its energy scale and temperature
evolution is found to be compatible with previous photoemission and optical
experiments. If interpreted as the signature of antiferromagnetic order in the
samples, these results could suggest the coexistence on the local scale of
antiferromagnetism and superconductivity on the electron-doped side of cuprate
superconductors
Probing the superconducting condensate on a nanometer scale
Superconductivity is a rare example of a quantum system in which the
wavefunction has a macroscopic quantum effect, due to the unique condensate of
electron pairs. The amplitude of the wavefunction is directly related to the
pair density, but both amplitude and phase enter the Josephson current : the
coherent tunneling of pairs between superconductors. Very sensitive devices
exploit the superconducting state, however properties of the {\it condensate}
on the {\it local scale} are largely unknown, for instance, in unconventional
high-T cuprate, multiple gap, and gapless superconductors.
The technique of choice would be Josephson STS, based on Scanning Tunneling
Spectroscopy (STS), where the condensate is {\it directly} probed by measuring
the local Josephson current (JC) between a superconducting tip and sample.
However, Josephson STS is an experimental challenge since it requires stable
superconducting tips, and tunneling conditions close to atomic contact. We
demonstrate how these difficulties can be overcome and present the first
spatial mapping of the JC on the nanometer scale. The case of an MgB film,
subject to a normal magnetic field, is considered.Comment: 7 pages, 6 figure
Probing the superfluid velocity with a superconducting tip: the Doppler shift effect
We address the question of probing the supercurrents in superconducting (SC)
samples on a local scale by performing Scanning Tunneling Spectroscopy (STS)
experiments with a SC tip. In this configuration, we show that the tunneling
conductance is highly sensitive to the Doppler shift term in the SC
quasiparticle spectrum of the sample, thus allowing the local study of the
superfluid velocity. Intrinsic screening currents, such as those surrounding
the vortex cores in a type II SC in a magnetic field, are directly probed. With
Nb tips, the STS mapping of the vortices, in single crystal 2H-NbSe_2, reveals
both the vortex cores, on the scale of the SC coherence length , and the
supercurrents, on the scale of the London penetration length . A
subtle interplay between the SC pair potential and the supercurrents at the
vortex edge is observed. Our results open interesting prospects for the study
of screening currents in any superconductor.Comment: 4 pages, 5 figure
Scanning Tunneling Spectroscopy on the novel superconductor CaC6
We present scanning tunneling microscopy and spectroscopy of the newly
discovered superconductor CaC. The tunneling conductance spectra, measured
between 3 K and 15 K, show a clear superconducting gap in the quasiparticle
density of states. The gap function extracted from the spectra is in good
agreement with the conventional BCS theory with = 1.6 0.2
meV. The possibility of gap anisotropy and two-gap superconductivity is also
discussed. In a magnetic field, direct imaging of the vortices allows to deduce
a coherence length in the ab plane 33 nm
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