Classifying Induced Superconductivity in Atomically Thin Dirac-Cone Materials

Classifying superconductivity in Moiré graphene superlattices

Several research groups have reported on the observation of superconductivity in bilayer graphene structures where single atomic layers of graphene are stacked and then twisted at angles θ forming Moiré superlattices. The characterization of the superconducting state in these 2D materials is an ongoing task. Here we investigate the pairing symmetry of bilayer graphene Moiré superlattices twisted at θ = 1.05°, 1.10° and 1.16° for carrier doping states varied in the range of n = (0.5 - 1.5) · 1012 cm-2 (where superconductivity can be realized) by analyzing the temperature dependence of the upper critical field Bc2(T) and the self-field critical current Jc(sf,T) within currently available models - all of which start from phonon-mediated BCS theory - for single- and two-band s-, d-, p- and d + id-wave gap symmetries. Extracted superconducting parameters show that only s-wave and a specific kind of p-wave symmetries are likely to be dominant in bilayer graphene Moiré superlattices. More experimental data is required to distinguish between the s- and remaining p-wave symmetries as well as the suspected two-band superconductivity in these 2D superlattices.

DC Self-Field Critical Current in Superconductor Dirac-Cone Material/Superconductor Junctions

Recently, several research groups have reported on anomalous enhancement of the self-field critical currents, Ic(sf,T), at low temperatures in superconductor/Dirac-cone material/superconductor (S/DCM/S) junctions. Some papers attributed the enhancement to the low-energy Andreev bound states arising from winding of the electronic wave function around DCM. In this paper, Ic(sf,T) in S/DCM/S junctions have been analyzed by two approaches: modified Ambegaokar-Baratoff and ballistic Titov-Beenakker models. It is shown that the ballistic model, which is traditionally considered to be a basic model to describe Ic(sf,T) in S/DCM/S junctions, is an inadequate tool to analyze experimental data from these type of junctions, while Ambegaokar-Baratoff model, which is generally considered to be a model for Ic(sf,T) in superconductor/insulator/superconductor junctions, provides good experimental data description. Thus, there is a need to develop a new model for self-field critical currents in S/DCM/S systems.