Beyond a critical disorder, two-dimensional (2D) superconductors become insulating. In this Superconductor-Insulator Transition (SIT), the nature of the insulator is still controversial. Here, we ...present an extensive experimental study on insulating Nb
Si
close to the SIT, as well as corresponding numerical simulations of the electrical conductivity. At low temperatures, we show that electronic transport is activated and dominated by charging energies. The sample thickness variation results in a large spread of activation temperatures, fine-tuned via disorder. We show numerically and experimentally that this originates from the localization length varying exponentially with thickness. At the lowest temperatures, there is an increase in activation energy related to the temperature at which this overactivated regime is observed. This relation, observed in many 2D systems shows that conduction is dominated by single charges that have to overcome the gap when entering superconducting grains.
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 theory, these pairs generate a transverse thermoelectric (Nernst) signal. In two dimensions, the magnitude of the expected signal depends only on universal constants and the superconducting coherence length, so the theory can be rigorously tested. Here, we present measurements of amorphous superconducting films of Nb0.15Si0.85. In this dirty superconductor, the lifetime of Cooper pairs exceeds the elastic scattering lifetime of quasiparticles in a wide temperature range above Tc and, consequently, their Nernst response dominates that generated by the normal electrons. We resolved a Nernst signal, which persists deep inside the normal state. Its amplitude is in excellent agreement with the theoretical prediction. This result provides an unambiguous case for a Nernst effect produced by short-lived Cooper pairs.
A
bstract
Neutrinoless double-beta decay is a key process in particle physics. Its experimental investigation is the only viable method that can establish the Majorana nature of neutrinos, providing ...at the same time a sensitive inclusive test of lepton number violation. CROSS (Cryogenic Rare-event Observatory with Surface Sensitivity) aims at developing and testing a new bolometric technology to be applied to future large-scale experiments searching for neutrinoless double-beta decay of the promising nuclei
100
Mo and
130
Te. The limiting factor in large-scale bolometric searches for this rare process is the background induced by surface radioactive contamination, as shown by the results of the CUORE experiment. The basic concept of CROSS consists of rejecting this challenging background component by pulse-shape discrimination, assisted by a proper coating of the faces of the crystal containing the isotope of interest and serving as energy absorber of the bolometric detector. In this paper, we demonstrate that ultra-pure superconductive Al films deposited on the crystal surfaces act successfully as pulse-shape modifiers, both with fast and slow phonon sensors. Rejection factors higher than 99.9% of
α
surface radioactivity have been demonstrated in a series of prototypes based on crystals of Li
2
MoO
4
and TeO
2
. We have also shown that point-like energy depositions can be identified up to a distance of
∼
1 mm from the coated surface. The present program envisions an intermediate experiment to be installed underground in the Canfranc laboratory (Spain) in a CROSS-dedicated facility. This experiment, comprising
∼
3
×
10
25
nuclei of
100
Mo, will be a general test of the CROSS technology as well as a worldwide competitive search for neutrinoless double-beta decay, with sensitivity to the effective Majorana mass down to 70 meV in the most favorable conditions.
Neutrinoless double-beta (
0
ν
β
β
) decay is a hypothetical rare nuclear transition (
T
1
/
2
>
10
25
–
10
26
year). Its observation would provide an important insight into the nature of neutrinos ...(Dirac or Majorana particle) demonstrating that the lepton number is not conserved. This decay can be investigated with bolometers embedding the double-beta decay isotope (
76
Ge
,
82
Se
,
100
Mo
,
116
Cd
,
130
Te
...), which perform as low-temperature calorimeters (few tens of mK) detecting particle interactions via a small temperature rise read out by a dedicated thermometer. Cryogenic Rare-event Observatory with Surface Sensitivity (CROSS) aims at the development of bolometric detectors (based on
Li
2
MoO
4
and
TeO
2
crystals) capable of discriminating surface
α
and
β
interactions by exploiting superconducting properties of Al film deposited on the detector surface. We report in this paper the results of tests on prototypes performed at CSNSM (Orsay, France) that showed the capability of a-few-
μ
m
-thick superconducting Al film deposited on crystal surface to discriminate surface
α
from bulk events, thus providing the detector with the required pulse shape discrimination capability. The CROSS technology would further improve the background suppression and simplify the detector construction (no auxiliary light detector is needed to reject alpha surface events) with a view to future competitive double-beta decay searches.
Random coincidences of events could be one of the main sources of background in the search for neutrino-less double-beta decay of
100
Mo with macro-bolometers, due to their modest time resolution. ...Scintillating bolometers as those based on Li
2
MoO
4
crystals and employed in the CROSS and CUPID experiments can eventually exploit the coincident fast signal detected in a light detector to reduce this background. However, the scintillation provides a modest signal-to-noise ratio, making difficult a pile-up pulse-shape recognition and rejection at timescales shorter than a few ms. Neganov–Trofimov–Luke assisted light detectors (NTL-LDs) offer the possibility to effectively increase the signal-to-noise ratio, preserving a fast time-response, and enhance the capability of pile-up rejection via pulse shape analysis. In this article we present: (a) an experimental work performed with a Li
2
MoO
4
scintillating bolometer, studied in the framework of the CROSS experiment, and utilizing a NTL-LD; (b) a simulation method to reproduce, synthetically, randomly coincident two-neutrino double-beta decay events; (c) a new analysis method based on a pulse-shape discrimination algorithm capable of providing high pile-up rejection efficiencies. We finally show how the NTL-LDs offer a balanced solution between performance and complexity to reach background index
∼
10
-
4
counts/keV/kg/year with 280 g Li
2
MoO
4
(
100
Mo enriched) bolometers at 3034 keV, the Q
β
β
of the double-beta decay, and target the goal of a next generation experiment like CUPID.
Competing phenomena in low dimensional systems can generate exotic electronic phases, either through symmetry breaking or a non-trivial topology. In two-dimensional (2D) systems, the interplay ...between superfluidity, disorder and repulsive interactions is especially fruitful in this respect although both the exact nature of the phases and the microscopic processes at play are still open questions. In particular, in 2D, once superconductivity is destroyed by disorder, an insulating ground state is expected to emerge, as a result of a direct superconductor-to-insulator quantum phase transition. In such systems, no metallic state is theoretically expected to survive to the slightest disorder. Here we map out the phase diagram of amorphous NbSi thin films as functions of disorder and film thickness, with two metallic phases in between the superconducting and insulating ones. These two dissipative states, defined by a resistance which extrapolates to a finite value in the zero temperature limit, each bear a specific dependence on disorder. We argue that they originate from an inhomogeneous destruction of superconductivity, even if the system is morphologically homogeneous. Our results suggest that superconducting fluctuations can favor metallic states that would not otherwise exist.
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.
Received: 7 July 2018, Accepted: 27 September 2018; Edited by: A. Marti, M. Monteiro; Reviewed by: R. Marotti, Instituto de Física, Facultad de Ingeniería - Universidad de la República, Uruguay; DOI: http://dx.doi.org/10.4279/PIP.100007
Cite as: A Hilberer, G Laurent, A Lorin, A Partier, J Bobroff, F Bouquet, C Even, J M Fischbach, C A Marrache Kikuchi, M Monteverde, B Pilette, Q Quay, Papers in Physics 10, 100007(2018)
This paper, by A Hilberer, G Laurent, A Lorin, A Partier, J Bobroff, F Bouquet, C Even, J M Fischbach, C A Marrache Kikuchi, M Monteverde, B Pilette, Q Quay, is licensed under the Creative Commons Attribution License 4.0.
PDF
