Andreev reflection in ballistic normal metal/graphene/superconductor junctions

Abstract

We report the study of ballistic transport in normal metal/graphene/superconductor junctions in edge-contact geometry. While in the normal state, we have observed Fabry-Pérot resonances suggesting that charge carriers travel ballistically, the superconducting state shows that the Andreev reflection at the graphene/superconductor interface is affected by these interferences. Our experimental results in the superconducting state have been analyzed and explained with a modified Octavio-Tinkham-Blonder-Klapwijk model taking into account the magnetic pair-breaking effects and the two different interface transparencies, i.e. between the normal metal and graphene, and between graphene and the superconductor. We show that the transparency of the normal metal/graphene interface strongly varies with doping at large scale, while it undergoes weaker changes at the graphene/superconductor interface. When a cavity is formed by the charge transfer occurring in the vicinity of the contacts, we see that the transmission probabilities follow the normal state conductance highlighting the interplay between the Andreev processes and the electronic interferometer.

Probing the current-phase relation of graphene Josephson junctions using microwave measurements

“…While this is unexpected, we note that without knowing exactly how many ABS channels are active in the JJ, it is not possible to extract a number for τ , as the measured anharmonicity coefficient only returns information on τ i = N τ . Additional experiments, such as extracting the transparency for each channel from multiple Andreev reflection via voltage-biased measurements [37][38][39][40], or direct measures of both E J and the anharmonicity coefficient in transmon qubits [41][42][43][44], would be required to draw further conclusions.…”

Ballistic Graphene Cooper Pair Splitter

“…Graphene-superconductor interfaces exhibit very peculiar Andreev processes at low energy [40][41][42][43][44][45], and nonlocal processes have been predicted to occur under various conditions [46][47][48], including in the quantum Hall regime [28][29][30][31]. Thanks to the progress in device fabrication [49], graphene appears to be a clean and highly tunable two-dimensional material that couples very well to superconductors where a ballistic effect can be directly observed [50][51][52][53][54][55][56][57][58]. However, to our knowledge, very limited studies on nonlocal processes in clean graphene have been reported until now [59,60].…”

A Ballistic Graphene Cooper Pair Splitter

“…[7] for the case of single walled carbon nanotube. We employ a modified Octavio-Tinkham-Blonder-Klapwijk (OTBK) model [47][48][49][50][51] along with a threeterminal beam splitter model to explain our experimental observation. The advantage of this implementation is that the Andreev reflection takes place locally at a single interface, and is not limited by contact spacing.…”

“…We have used the dry transfer technique for the fabrication of h-BN/graphene/h-BN van der Waals heterostructure which is similar to the one described in Wang et al [34]. Edge contacts to the encapsulated graphene layer were established in a self-aligned manner as described in Kraft et al [39], and adapted for two different contact materials in Pandey et al [51]. Details of the devices are shown in the Supplemental Material [52].…”

“…In order to interpret the experimental data, we employ a modified Octavio-Tinkham-Blonder-Klapwijk (OTBK) model [47][48][49][50][51] for the transport between each pair of termi-nals, as indicated by the solid lines in Fig. 3(a).…”