Transport through Andreev bound states in a graphene quantum dot

When a low-energy electron is incident on an interface between a metal and superconductor, it causes the injection of a Cooper pair into the superconductor and the generation of a hole that reflects back into the metal—a process known as Andreev reflection. In confined geometries, this process can give rise to discrete Andreev bound states (ABS), which can enable transport of supercurrents through nonsuperconducting materials and have recently been proposed as a means of realizing solid-state qubits1–3. Here, we report transport measurements of sharp, gate-tunable ABS formed in a superconductor–quantum dot (QD)–normal system realized on an exfoliated graphene sheet. The QD is formed in graphene beneath a superconducting contact as a result of a workfunction mismatch4,5. Individual ABS form when the discrete QD levels are proximity-coupled to the superconducting contact. Owing to the low density of states of graphene and the sensitivity of the QD levels to an applied gate voltage, the ABS spectra are narrow and can be continuously tuned down to zero energy by the gate voltage. Although signatures of Andreev reflection and bound states in conductance have been widely reported6, it has been difficult to directly probe individual ABS. In superconductor (SC)–graphene structures, most previous work has focused on the nature of the supercurrent in well-coupled Josephson junctions7–10. SC–QD hybrids in graphene have not been studied, although recent work has predicted11–14 and demonstrated15 that ABS can be isolated by coupling them to discreteQDenergy levels.However, theABSpeaks in previous SC–QD experiments were strongly broadened, either by the large lead density of states15 or by the lack of a tunnel barrier16. In the work described in this Letter, sharp subgap conductance peaks are obtained by tunnelling into a proximity-coupled QD formed within graphene, a high-mobility zero-gap semiconductor17. We focus on the two lowest-energy conductance peaks that occur below the superconducting gap, and show that they are a signature of transport by means of ABS. The spectral pattern of these peaks as a function of gate and bias voltage is consistent with a simple theoretical model of ABS spectra presented below, and can be accurately fittedwith amore detailedmicroscopic calculation. The data shown in this Letter were taken from one single-layer graphene device (sample A) and one multilayer device (sample B, approximately ten layers thick). Similar behaviour was seen in three other devices (two single layer and one bilayer). As the features are robust on adding layers, it is evident that a precise Dirac-point band structure is not a requirement. The sample geometry and measurement circuit are shown in Fig. 1a; the location of the quantum dot that forms beneath the SC probe is depicted in