Superconductive proximity in a Topological Insulator slab and excitations bound to an axial vortex

We consider the proximity effect in a Topological Insulator sandwiched between two conventional superconductors, by comparing s-wave spin singlet superconducting pairing correlations and odd-parity triplet pairing correlations with zero spin component orthogonal to the slab ("polar " phase). A superconducting gap opens in the Dirac dispersion of the surface states existing at the interfaces. An axial vortex is included, piercing the slab along the normal to the interfaces with the superconductors. It is known that, when proximity is s-wave, quasiparticles in the gap are Majorana Bound States, localized at opposite interfaces. We report the full expression for the quantum field associated to the midgap neutral fermions, as derived in the two-orbital band model for the TI. When proximity involves odd-parity pairing, midgap modes are charged Surface Andreev Bound States, and they originate from interfacial circular states of definite chirality, centered at the vortex singularity and decaying in the TI film with oscillations. When the chemical potential is moved away from midgap, extended states along the vortex axis are also allowed. Their orbital structure depends on the symmetry of the bulk band from where the quasiparticle level splits off.Comment: 13 pages no figures, accepted for publication in Phys. Rev.

Advantages of using YBCO-Nanowire-YBCO heterostructures in the search for Majorana Fermions

We propose an alternative platform to observe Majorana bound states in solid state systems. High critical temperature cuprate superconductors can induce superconductivity, by proximity effect, in quasi one dimensional nanowires with strong spin orbit coupling. They favor a wider and more robust range of conditions to stabilize Majorana fermions due to the large gap values, and offer novel functionalities in the design of the experiments determined by different dispersion for Andreev bound states as a function of the phase difference.Comment: 4 Pages, 3 figures, submission date 30-Apr-201

Embodied Cognition. Body, movement and sport for didactics

Embodied Cognition (EC) is an interdisciplinary and multiperspective scientific theory whose characteristic, from a cultural and professional perspective, opens up interesting scenarios in the field of psychopedagogy. In particular, a prolific and extremely interesting area of study is now provided by the contribution of EC to the world of didactics (Caruana & Borghi, 2013). This work is part of this research horizon, investigating how the key principles of Embodied Cognition offer new opportunities to enhance differences in learning processes (Gomez Paloma & Ianes, a cura di, 2014). Starting from the analysis of the body as a scientific mediator of the learning process on a neurobiological (Rizzolatti & Sinigaglia, 2006) and neurophenomenological (Gallese, 2006) level, the study focuses on the scientific evidence (Margiotta, 2014) that EC can provide to teachers in the field of didactics. It represents a concrete springboard for delineating and validating an “EC-Based” model (Gomez Paloma & Damiani, 2015) to enhance corporeality as a cognitive system and a learning/ contextualization setting for the building of professional skills in the field of education

Paramagnetic effect in YBaCuO grain boundary junctions

A detailed investigation of the magnetic response of YBaCuO grain boundary Josephson junctions has been carried out using both radio-frequency measurements and Scanning SQUID Microscopy. In a nominally zero-field-cooled regime we observed a paramagnetic response at low external fields for 45 degree asymmetric grain boundaries. We argue that the observed phenomenology results from the d-wave order parameter symmetry and depends on Andreev bound states.Comment: To be published in Phys. Rev.

High critical-current density and scaling of phase-slip processes in YBaCuO nanowires

YBaCuO nanowires were reproducibly fabricated down to widths of 50 nm. A Au/Ti cap layer on YBCO yielded high electrical performance up to temperatures above 80 K in single nanowires. Critical current density of tens of MA/cm2 at T = 4.2 K and of 10 MA/cm2 at 77 K were achieved that survive in high magnetic fields. Phase-slip processes were tuned by choosing the size of the nanochannels and the intensity of the applied external magnetic field. Data indicate that YBCO nanowires are rather attractive system for the fabrication of efficient sensors, supporting the notion of futuristic THz devices.Comment: 8 pages, 3 figures. Accepted for publication in Superconductor Science and Technolog

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