Andreev Oscillations in Normal-Superconducting-Normal Nanostructures

Abstract

We show that the voltage drop of specially prepared normal-superconducting-normal nanostructures show quantum Andreev oscillations as a function of magnetic field or input current. These oscillations are due to the interference of the electron wave function between the normal parts of the structure that act as reflective interfaces, i.e. our devices behave as a Fabry-Perot interferometer for conduction electrons. The observed oscillations and field periods are well explained by theory.The possibilities of exploiting quantum mechanical effects -with all the interferences and other phenomena occurring in real nano-devices -find new expectations that may lead to the fabrication of small devices with applications in new fields of technology as ballistic electronics and spintronics and flux devices combining normal and superconducting materials. Earlier work in semiconductors and STM experiments demonstrate the existence of quantum oscillations [1,2]. Recently, spin-polarized resonant tunneling in magnetic tunneling junctions showed large changes in the magnetoresistance due to the interference of the carrier wave function [3]. In this work we seek after quantum mechanical interference effects in the magnetoresistance using normal-superconducting structures without tunneling. Because one of the characteristics needed is ballistic transport our experiments have to be done at low temperatures and the systems should be designed to have Fermi wavelength of the order or larger than their relevant size.Assume a strip with a lateral structure M1/M2/M1, where M1 and M2 are two different materials with different Fermi energies E F and lengths L 1 and L 2 and where the electrical current passes through them. Because of the different E F 's between M1 and M2, the one particle potential can be described by a barrier U 2 of length L 2 in M2 that acts as a potential well where the wave function behaves coherently having multireflections, i.e. a kind of Fabry-Perot interferometer for electrons, a problem studied recently for the case of fluctuations in the magnetoresistance of graphene [4]. Inset in Fig. 1(a) shows the one-dimensional geometry of the system with the barrier U 2 (x) depending on the potential drop between the two ends of the trilayer. The transmittivity along such structure is [4,5]

Ferromagnetic- and Superconducting-Like Behavior of the Electrical Resistance of an Inhomogeneous Graphite Flake

“…Recently published high-resolution measurements of the magnetoresistance (MR) of micrometer small and several tens of nanometers thick graphite samples suggest the existence of inhomogeneous, granular superconductivity with critical temperature T c 25 K [14]. The claim of granular superconductivity in graphite is based on three main observations: (a) the existence of anomalous hysteresis loops of resistance versus magnetic field applied parallel to the c−axis that indicate the existence of Josephson-coupled superconducting grains, (b) the observation of oscillatory behavior in the magnetoresistance vs. field that can be related to 2 Andreev reflections of Copper pairs at the interfaces of superconducting and semiconducting regions, similar to a phenomenon observed recently in conventional superconducting-normal junctions [15], and (c) the phenomena observed in (a) and (b) vanish at T > T c , where T c denotes the temperature at which the resistance R(T ) reaches a maximum, i.e. R(T < T c ) shows metallic behavior with an anomalous large magnetoresistance.…”

Anomalous Anisotropy in Superconducting Nanodiamond Films Induced by Crystallite Geometry

“…Based on structural analysis, the observed anomalous anisotropy is well explained by modeling the superconducting nanodiamond films as networks of weak links (grain-boundary-grain junctions). A network composed of ordered uniform weak links should give rise to the emergence of the characteristic features of the Josephson effect, e.g., zero-bias supercurrent, modulations of the critical current as a function of magnetic field, and oscillations of the magnetic field dependence of resistivity [33], yet no such characteristic features of the Josephson effect have been reported for granular superconducting diamond. However, Josephson-effect-like traits such as current-induced Josephson noise have been observed in nanodiamond films [34], and strong intergrain and intragrain fluctuations of the superconducting order parameter have been reported for superconducting granular diamond by scanning tunneling microscopy/spectroscopy [10,15,18].…”