The Transport Properties of High-Temperature Superconductor/Graphene Junctions

Abstract

Charge transport in graphene (Gr) has been studied using conventional superconductors (SCs) in different types of devices. Graphene Josephson junctions (Gr-JJ), using s-wave superconductors, have shown supercurrents that are gate tunable. Andreev reflection (AR), which occurs at both Gr-SC interfaces in a Gr-JJ is responsible for the supercurrent. A prelude to realizing Gr-JJ using unconventional SCs, such as Bi2Sr2CaCu2O8+x (BSCCO), is to understand transport in a single BSCCO/Gr junction which can reveal unique aspects of AR. We have found that charge transfer from BSCCO into graphene creates a hole-doped region at the boundary of the BSCCO/Gr junction. This doped region renders two different interfaces, one with a superconducting graphene region and the other with a bare graphene region. Together they contribute to two scattering mechanisms – AR and Klein tunneling (KT), respectively, that create resonance states. We use a global gate to simultaneously gate the doped and bare graphene regions and observe an asymmetry in conductance as doping configuration changes from p+-p to p-n, a hallmark of KT. A resonant interference condition occurs for every two round trips of carriers within the doped region, which produces oscillations in conductance while sweeping the gate voltage. We observe the suppression of the oscillation amplitude above a junction bias ranging between 10 to 15 mV in over half-a-dozen BSCCO/Gr devices. This energy is consistent with the expected size of the induced gap of Δi ~15 meV measured by others. Furthermore, we have studied the transport properties in BSCCO-Gr-BSCCO junctions that were fabricated by mechanically cracking a single BSCCO flake, to control the separation length from 20 nm to 3 μm between the SCs, while setting it on graphene. We observe that the oscillation period increases as junction length decreases. In long junctions, the period tends to that of the single junction. The oscillations suppress for bias between 20 to 30 meV in the BSCCO-Gr-BSCCO junction which corresponds to 2Δi, as expected for transport between two SCs. These results show the rich transport phenomena in junctions of BSCCO and graphene that may lead to the realization of gate-tunable high-temperature graphene Josephson junctions for quantum computing applications. Superconducting d-wave proximity in graphene could create an effective p-wave pairing, which is one of the ingredients to create Majorana excitations.