64 research outputs found
Cryogenic Calibration Setup for Broadband Complex Impedance Measurements
Reflection measurements give access to the complex impedance of a material on
a wide frequency range. This is of interest to study the dynamical properties
of various materials, for instance disordered superconductors. However
reflection measurements made at cryogenic temperature suffer from the
difficulty to reliably subtract the circuit contribution. Here we report on the
design and first tests of a setup able to precisely calibrate in situ the
sample reflection, at 4.2 K and up to 2 GHz, by switching and measuring, during
the same cool down, the sample and three calibration standards.Comment: (6 pages, 6 figures
Destruction of superconductivity in disordered materials : a dimensional crossover
The disorder-induced Superconductor-to-Insulator Transition in amorphous
NbSi two-dimensional thin films is studied for different niobium
compositions through a variation of the sample thickness . We show that
the critical thickness , separating a superconducting regime from an
insulating one, increases strongly with diminishing , thus attaining values
of over 100 {\AA}. The corresponding phase diagram in the plane is
inferred and related to the three-dimensional situation. The two-dimensional
Superconductor-to-Insulator Transition well connects with the three-dimensional
Superconductor-to-Metal Transition
Observation of thermally activated glassiness and memory dip in a-NbSi insulating thin films
We present electrical conductance measurements on amorphous NbSi insulating
thin films. These films display out-of equilibrium electronic features that are
markedly different from what has been reported so far in disordered insulators.
Like in the most studied systems (indium oxide and granular Al films), a slow
relaxation of the conductance is observed after a quench to liquid helium
temperature which gives rise to the growth of a memory dip in MOSFET devices.
But unlike in these systems, this memory dip and the related conductance
relaxations are still visible up to room temperature, with clear signatures of
a temperature dependent dynamics
Effect of annealing on the superconducting properties of a-Nb(x)Si(1-x) thin films
a-Nb(x)Si(1-x) thin films with thicknesses down to 25 {\AA} have been
structurally characterized by TEM (Transmission Electron Microscopy)
measurements. As-deposited or annealed films are shown to be continuous and
homogeneous in composition and thickness, up to an annealing temperature of
500{\deg}C. We have carried out low temperature transport measurements on these
films close to the superconductor-to-insulator transition (SIT), and shown a
qualitative difference between the effect of annealing or composition, and a
reduction of the film thickness on the superconducting properties of a-NbSi.
These results question the pertinence of the sheet resistance R_square as the
relevant parameter to describe the SIT.Comment: 9 pages, 12 figure
A length scale for the superconducting Nernst signal above T in NbSi
We present a study of the Nernst effect in amorphous superconducting thin
films of NbSi. The field dependence of the Nernst coefficient
above T displays two distinct regimes separated by a field scale set by
the Ginzburg-Landau correlation length. A single function , with the
correlation length as its unique argument set either by the zero-field
correlation length (in the low magnetic field limit) or by the magnetic length
(in the opposite limit), describes the Nernst coefficient. We conclude that the
Nernst signal observed on a wide temperature () and field () range is exclusively generated by short-lived Cooper pairs.Comment: 4 pages, 4 figure
Magnetic field-induced quantum superconductor-insulator transition in
A study of magnetic-field tuned superconductor-insulator transitions in
amorphous thin films shows that quantum
superconductor-insulator transitions are characterized by an unambiguous
signature -- a kink in the temperature profile of the critical magnetic field.
Using this criterion, we show that the nature of the magnetic-field tuned
superconductor-insulator transition depends on the orientation of the field
with respect to the film. For perpendicular magnetic field, the transition is
controlled by quantum fluctuations with indications for the existence of a Bose
insulator; while for parallel magnetic field, the transition is classical,
driven by the breaking of Cooper pairs at the temperature dependent critical
field .Comment: 5 pages, 4 figure
Electron glass effects in amorphous NbSi films
We report on non equilibrium field effect in insulating amorphous NbSi thin
films having different Nb contents and thicknesses. The hallmark of an electron
glass, namely the logarithmic growth of a memory dip in conductance versus gate
voltage curves, is observed in all the films after a cooling from room
temperature to 4.2 K. A very rich phenomenology is demonstrated. While the
memory dip width is found to strongly vary with the film parameters, as was
also observed in amorphous indium oxide films, screening lengths and
temperature dependence of the dynamics are closer to what is observed in
granular Al films. Our results demonstrate that the differentiation between
continuous and discontinuous systems is not relevant to understand the
discrepancies reported between various systems in the electron glass features.
We suggest instead that they are not of fundamental nature and stem from
differences in the protocols used and in the electrical inhomogeneity length
scales within each material.Comment: Submission SciPos
Observation of the Nernst signal generated by fluctuating Cooper pairs
Long-range order is destroyed in a superconductor warmed above its critical
temperature (Tc). However, amplitude fluctuations of the superconducting order
parameter survive and lead to a number of well established phenomena such as
paraconductivity : an excess of charge conductivity due to the presence of
short-lived Cooper pairs in the normal state. According to an untested theory,
these pairs generate a transverse thermoelectric (Nernst) signal. In amorphous
superconducting films, the lifetime of Cooper pairs exceeds the elastic
lifetime of quasi-particles in a wide temperature range above Tc; consequently,
the Cooper pairs Nernst signal dominate the response of the normal electrons
well above Tc. In two dimensions, the magnitude of the expected signal depends
only on universal constants and the superconducting coherence length, so the
theory can be unambiguously tested. Here, we report on the observation of a
Nernst signal in such a superconductor traced deep into the normal state. Since
the amplitude of this signal is in excellent agreement with the theoretical
prediction, the result provides the first unambiguous case for a Nernst effect
produced by short-lived Cooper pairs
Temperature-dependent transport measurements with Arduino
The current performances of single-board microcontrollers render them attractive, not only for basic applications, but also for more elaborate projects, amongst which are physics teaching or research. In this article, we show how temperature-dependent transport measurements can be performed by using an Arduino board, from cryogenic temperatures up to room temperature or above. We focus on two of the main issues for this type of
experiments: the determination of the sample temperature and the measurement of its resistance. We also detail two student-led experiments: evidencing the magnetocaloric effect in Gadolinium and measuring the resistive transition of a high critical temperature superconductor
Axion searches with the EDELWEISS-II experiment
We present new constraints on the couplings of axions and more generic
axion-like particles using data from the EDELWEISS-II experiment. The EDELWEISS
experiment, located at the Underground Laboratory of Modane, primarily aims at
the direct detection of WIMPs using germanium bolometers. It is also sensitive
to the low-energy electron recoils that would be induced by solar or dark
matter axions. Using a total exposure of up to 448 kg.d, we searched for
axion-induced electron recoils down to 2.5 keV within four scenarios involving
different hypotheses on the origin and couplings of axions. We set a 95% CL
limit on the coupling to photons GeV in
a mass range not fully covered by axion helioscopes. We also constrain the
coupling to electrons, , similar to the more
indirect solar neutrino bound. Finally we place a limit on , where is the
effective axion-nucleon coupling for Fe. Combining these results we
fully exclude the mass range keV for DFSZ axions and
keV for KSVZ axions
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