Direct observation of ballistic Andreev reflection

Abstract

An overview is presented of experiments on ballistic electrical transport in inhomogeneous superconducting systems which are controlled by the process of Andreev reflection. The initial experiments based on the coexistence of a normal phase and a superconducting phase in the intermediate state led to the concept itself. It was followed by a focus on geometrically inhomogeneous systems like point contacts, which provided a very clear manifestation of the energy- and directional dependence of the Andreev reflection process. The point contacts have in recent years evolved towards the atomic scale by using mechanical break-junctions, revealing in a very detailed way the dependence of Andreev reflection on the macroscopic phase of the superconducting state. In present day research the superconducting inhomogeneity is constructed by clean room technology and combines superconducting materials with, for example, low-dimensional materials and topological insulators. Alternatively the superconductor is combined with nano-objects, such as graphene, carbon nanotubes, or semiconducting nanowires. Each of these 'inhomogeneous systems' provides a very interesting range of properties, all rooted in some manifestation of Andreev reflection.Comment: 22 pages, 14 figures. Contribution to the special issue of JETP to honor A.F. Andreev's 75th birthda

Evolution of superconducting gaps in Th-substituted Sm1−xThxOFeAs studied by multiple Andreev reflection spectroscopy

“…The contact resistance increases with temperature, from R(4.2 K) ≈ 16 Ω to R(50 K) ≈ 41 Ω which agrees with the theory predictions for ballistic SnS-contacts 43 . The dynamic conductance spectra demonstrate pronounced dips at |V nL=1 | ≈ 24 mV, |V nL=2 | ≈ 12.3 mV being the SGS minima of n = 1, 2 order.…”

Estimation of Intraband and Interband Relative Coupling Constants from Temperature Dependences of the Order Parameter for Two-Gap Superconductors

“…These values are much larger than the estimated contact diameter a, thus providing local study of crystallites. Since a ≪ l, and the resistance of this contact decreases with temperature increasing, we conclude that our measurements are in the clean ballistic SnS-Andreev mode [40]. The same could be concluded for Mg 1−x Al x B 2 samples we used.…”

Subgap states in two-dimensional spectroscopy of graphene-based superconducting hybrid junctions

“…Tunneling conductance measurements at normal metal-superconductor junctions are a very useful tool to detect signatures of all these types of unconventional superconductivity [2]. In a ballistic junction, transport at voltages below the superconducting gap is mediated by Andreev reflections, where incident electrons are converted into holes in the normal metal creating Cooper pairs in the superconductor [28,29]. The presence of surface states in unconventional superconductors is connected to resonance peaks in the Andreev reflection probability, resulting in conductance peaks below the superconducting gap [30][31][32][33][34][35].…”

“…Unfortunately, tunneling spectroscopy of subgap resonances presents several experimental challenges, specially for nanoscale devices [29]. When considering hybrid junctions where the reservoirs and the intermediate scattering region are built from different materials, as sketched in Fig.…”

“…Additionally, quantum-coherent transport across the junction results in the emergence of Fabry-Pérot resonances [37,38]. In most of materials these effects can mask the experimental detection of novel phenomena associated to unconventional superconductivity [29]. However, recent experimental advances involving graphene-based nanoscale devices provide new ways to circumvent these challenges.…”