Influence of Topological Edge States on the Properties of Al/Bi2Se3/Al Hybrid Josephson Devices

In superconductor-topological insulator-superconductor hybrid junctions, the barrier edge states are expected to be protected against backscattering, to generate unconventional proximity effects, and, possibly, to signal the presence of Majorana fermions. The standards of proximity modes for these types of structures have to be settled for a neat identification of possible new entities. Through a systematic and complete set of measurements of the Josephson properties we find evidence of ballistic transport in coplanar Al-Bi2Se3-Al junctions that we attribute to a coherent transport through the topological edge state. The shunting effect of the bulk only influences the normal transport. This behavior, which can be considered to some extent universal, is fairly independent of the specific features of superconducting electrodes. A comparative study of Shubnikov - de Haas oscillations and Scanning Tunneling Spectroscopy gave an experimental signature compatible with a two dimensional electron transport channel with a Dirac dispersion relation. A reduction of the size of the Bi2Se3 flakes to the nanoscale is an unavoidable step to drive Josephson junctions in the proper regime to detect possible distinctive features of Majorana fermions.Comment: 11 pages, 14 figure

Suspended InAs nanowire Josephson junctions assembled via dielectrophoresis

We present a novel technique for the realization of suspended Josephson junctions based on InAs semiconductor nanowires. The devices are assembled using a technique of drop-casting guided by dielectrophoresis, which allows one to finely align the nanostructures on top of the electrodes. The proposed architecture removes the interaction between the nanowire and the substrate which is known to influence disorder and the orientation of the Rashba vector. The relevance of this approach in view of the implementation of hybrid Josephson junctions based on semiconducting nanowires coupled with high-temperature superconductors is discussed

Macroscopic quantum tunnelling in spin filter ferromagnetic Josephson junctions.

The interfacial coupling of two materials with different ordered phases, such as a superconductor (S) and a ferromagnet (F), is driving new fundamental physics and innovative applications. For example, the creation of spin-filter Josephson junctions and the demonstration of triplet supercurrents have suggested the potential of a dissipationless version of spintronics based on unconventional superconductivity. Here we demonstrate evidence for active quantum applications of S-F-S junctions, through the observation of macroscopic quantum tunnelling in Josephson junctions with GdN ferromagnetic insulator barriers. We show a clear transition from thermal to quantum regime at a crossover temperature of about 100 mK at zero magnetic field in junctions, which present clear signatures of unconventional superconductivity. Following previous demonstration of passive S-F-S phase shifters in a phase qubit, our result paves the way to the active use of spin filter Josephson systems in quantum hybrid circuits.We acknowledge financial support from COST Action MP1201 [NanoSC COST], by Progetto FIRB HybridNanoDev RBFR1236VV001 and by Regione Campania through POR Campania FSE 2007/2013, progetto MASTRI CUP B25B09000010007.This is the final version. It was first published by NPG at http://www.nature.com/ncomms/2015/150609/ncomms8376/full/ncomms8376.html#abstract

Electrodynamics of Josephson junctions containing strong ferromagnets

Triplet supercurrents in multilayer ferromagnetic Josephson junctions with misaligned magnetization can penetrate thicker ferromagnetic barriers compared to the singlet component. Although the static properties of these junctions have been extensively studied, the dynamic characteristics remain largely unexplored. Here we report a comprehensive electrodynamic characterization of multilayer ferromagnetic Josephson junctions composed of Co and Ho. By measuring the temperature-dependent current-voltage characteristics and the switching current distributions down to 0.3 K, we show that phase dynamics of junctions with triplet supercurrents exhibits long (in terms of proximity) junction behavior and moderately damped dynamics with renormalized capacitance and resistance. This unconventional behavior possibly provides a different way to dynamically detect triplets. Our results show new theoretical models are required to fully understand the phase dynamics of triplet Josephson junctions for applications in superconducting spintronics.DM, RC, FT would like to thank NANOCOHYBRI project (Cost Action CA 16218). NB acknowledges funding from the British Council through UKIERI programme and Loughborough University. MGB acknowledges funding from EPSRC Programme Grant EP/N017242/1

The not-so-massive black hole in the microquasar GRS1915+105

We present a new dynamical study of the black hole X-ray transient GRS1915+105 making use of near-infrared spectroscopy obtained with X-shooter at the VLT. We detect a large number of donor star absorption features across a wide range of wavelengths spanning the H and K bands. Our 24 epochs covering a baseline of over 1 year permit us to determine a new binary ephemeris including a refined orbital period of P=33.85 +/- 0.16 d. The donor star radial velocity curves deliver a significantly improved determination of the donor semi-amplitude which is both accurate (K_2=126 +/- 1 km/s) and robust against choice of donor star template and spectral features used. We furthermore constrain the donor star's rotational broadening to vsini=21 +/-4 km/s, delivering a binary mass ratio of q=0.042 +/- 0.024. If we combine these new constraints with distance and inclination estimates derived from modelling the radio emission, a black hole mass of M_BH=10.1 +/- 0.6 M_sun is inferred, paired with an evolved mass donor of M_2=0.47 +/- 0.27 M_sun. Our analysis suggests a more typical black hole mass for GRS1915+105 rather than the unusually high values derived in the pioneering dynamical study by Greiner et al. (2001). Our data demonstrate that high-resolution infrared spectroscopy of obscured accreting binaries can deliver dynamical mass determinations with a precision on par with optical studies

Rotation and Macroturbulence in Metal-poor Field Red Giant and Red Horizontal Branch Stars

We report the results for rotational velocities, Vrot sin i, and macroturbulence dispersion, zeta(RT), for 12 metal-poor field red giant branch stars and 7 metal-poor field red horizontal branch stars. The results are based on Fourier transform analyses of absorption line profiles from high-resolution (R ~ 120,000), high-S/N (~ 215 per pixel) spectra obtained with the Gecko spectrograph at CFHT. We find that the zeta(RT) values for the metal-poor RGB stars are very similar to those for metal-rich disk giants studied earlier by Gray and his collaborators. Six of the RGB stars have small rotational values, less than 2.0 km/sec, while five show significant rotation, over 3 km/sec. The fraction of rapidly rotating RHB stars is somewhat lower than found among BHB stars. We devise two empirical methods to translate the line-broadening results obtained by Carney et al. (2003, 2008) into Vrot sin i for all the RGB and RHB stars they studied. Binning the RGB stars by luminosity, we find that most metal-poor field RGB stars show no detectable sign, on average, of rotation. However, the most luminous stars, with M(V) <= -1.5, do show net rotation, with mean values of 2 to 4 km/sec, depending on the algorithm employed, and these stars also show signs of radial velocity jitter and mass loss.Comment: accepted for publication in the Astronomical Journa

A novel numerical modelling approach for keratoplasty eye procedure

Objective of the work is to investigate stress and deformation that conrneal tissue and donor graft undergo during endothelial keratoplasty. In order to attach the donor graft to the cornea, different air bubble pressure profiles acting on the graft are considered. This study is carried out by employing a three-dimensional nonlinear finite element methodology, combined with a contact algorithm. The ocular tissues are treated as isotropic, hyper-elastic and nearly-incompressible materials. The contact algorithm, based on the penalty-based node-to-surface approach, is used to model the donor graft-corneal interface region. First, the proposed computational methodology is tested against benchmark data for bending of the plates over a cylinder. Then, the influence of geometrical and material parameters of the graft on the corneal contact-structural response is investigated. The results are presented in terms of Von Mises stress intensity, displacement and mean contact force. Results clearly indicate that the air bubble pressure plays a key role in the corneal stress and strain, as well as graft stiffness and thickness

Dark Matter in the Universe: Evidence, Candidates and Searches

Contribution from the students of the 4th CERNCLAF School of High-Energy Physics who participated in the Discussion Session addressing the issues of dark matter in the Universe