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.
Topological insulator materials like HgTe exhibit unique electronic properties at their interfaces and so peculiar attention has to be paid concerning the growth optimization. Molecular beam epitaxy ...of tensile-strained HgTe/CdTe is investigated as a function of the growth temperature. Crystal quality is checked by using high resolution X-rays diffraction. By combining several material characterization techniques such as scanning transmission electronic microscopy, time-of-flight secondary ion mass spectroscopy and X-rays reflectivity, we report sharp interface morphology with nanometer-scale Hg/Cd diffusion lengths.
•High crystal quality HgTe/CdTe heterostructures.•Sharp interfaces.•Nanometer-scale interdiffusion.•Morphological and compositional information provided by EDX-STEM coupled images.•Well-suited structures to investigate topological insulator physics.
We demonstrate evidences of electronic transport via topological Dirac surface states in a thin film of strained HgTe. At high perpendicular magnetic fields, we show that the electron transport ...reaches the quantum Hall regime with vanishing resistance. Furthermore, quantum Hall transport spectroscopy reveals energy splittings of relativistic Landau levels specific to coupled Dirac surface states. This study provides insights in the quantum Hall effect of topological insulator (TI) slabs, in the crossover regime between two- and three-dimensional TIs, and in the relevance of thin TI films to explore circuit functionalities in spintronics and quantum nanoelectronics.
A multi-tiered data structure, analysis toolkit and data processing management system has been constructed using ROOT and CouchDB. This system is well suited for experiments that acquire many ...computer files of raw data over the course of months or years, that are distributed to different computing centers and further reduced in size by several steps of data processing. Data handling for experiments searching for rare events extracted from digitized pulse traces typically fit this description. An implementation of this system has been constructed for the EDELWEISS-III experiment and is described here in some detail. This solution may also serve as a prototype system for the proposed EURECA experiment.
We report on the study of the Disordered-induced Superconductor-Insulator Transition (D-SIT) in NbxSi1−x thin films. These films, synthesized by electron-beam co-deposition, are continuous, ...amorphous, homogeneously disordered and structurally stable for a wide range of compositions, thicknesses and annealing temperature and thus particularly well suited for the study of D-SIT. We present an analysis of the D-SIT induced by three different parameters: the thickness, the Nb composition that changes the electronic density of states and the annealing temperature that changes the microscopic disorder. The annealing changes quantum interference patterns that decreases the local conductance. Our results show that the effect of the thickness on the destruction of superconductivity is very distinct from those of the composition or the annealing. We point out this material is particularly interesting to disentangle the effect of the parameters driving this quantum phase transition.
We report on a study of the transport properties of Nb thin films. By varying the thickness of the films from 263 Å to 25 Å, we observed a depression of the superconductivity. Magnetic field was also ...applied up to 6 T, inducing the disappearance of the superconductivity and the onset of an insulating behavior. The results were compared to those we have already obtained on a highly disordered system, a-NbxSi1−x, to understand whether the same mechanisms for the disappearance of the superconductivity could be at play in pure metallic thin films and in highly disordered systems.
Superconducting a-NbxSi1-x thin films experience a lowering of the Tc until the superconductivity disappears through a Superconductor-Insulator Transition (SIT). We here present transport ...measurements on 2D a-NbxSi1-x films, close to the SIT, for different compositions and thicknesses. We investigate the lowering of the Tc in light of existing theories, especially the one developed by Finkel'stein.
Near-surface-events are a major limitation to the performance of cryogenic massive germanium heat and ionisation detectors for dark matter search, due to their incomplete charge collection. We ...present here a powerful method of surface event identification based on the transient heat signal of a Ge bolometer, equipped with two NbSi high impedance thin film sensors. Calibration runs using electrons and low energy gamma particles from a
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Cd source show highly effective surface event rejection down to the heat threshold energy. Neutron and gamma source calibrations were realised to get information on the fiducial volume of the Ge absorber. First results from low background data taking are discussed.
The disorder-induced superconductor-to-insulator transition in amorphous Nb sub(x)Si sub(1-x) two-dimensional thin films is studied for different niobium compositions x through a variation of the ...sample thickness d. We show that the critical thickness d sub(c), separating a superconducting regime from an insulating one, increases strongly with diminishing x, thus attaining values of over 100 Angstrom. The corresponding phase diagram in the (d, x) 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.
A new design of a cryogenic germanium detector for dark matter search is presented, taking advantage of the coplanar grid technique of event localisation for improved background discrimination. ...Experiments performed with prototype devices in the EDELWEISS II setup at the Modane underground facility demonstrate the remarkably high efficiency of these devices for the rejection of low-energy β, approaching 105. This opens the road to investigate the range beyond 10−8 pb in the WIMP–nucleon collision cross-sections, as proposed in the EURECA project of a one-ton cryogenic detector mass.
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