Current-phase relations of InAs nanowire Josephson junctions: From interacting to multimode regimes

Abstract

Gate-tunable semiconductor-superconductor nanowires with superconducting leads form exotic Josephson junctions that are a highly desirable platform for two types of qubits: those with topological superconductivity (Majorana qubits) and those based on tunable anharmonicity (gatemon qubits). Controlling their behavior, however, requires understanding their electrostatic environment and electronic structure. Here we study gated InAs nanowires with epitaxial aluminum shells. By measuring current-phase relations (CPR) and comparing them with analytical and numerical calculations, we show that we can tune the number of modes, determine the transparency of each mode, and tune into regimes in which electron-electron interactions are apparent, indicating the presence of a quantum dot. To take into account electrostatic and geometrical effects, we perform microscopic self-consistent Schrodinger-Poisson numerical simulations, revealing the energy spectrum of Andreev states in the junction as well as their spatial distribution. Our work systematically demonstrates the effect of device geometry, gate voltage and phase bias on mode behavior, providing new insights into ongoing experimental efforts and predictive device design. 1 arXiv:1902.07804v1 [cond-mat.mes-hall]

Observation of Vanishing Charge Dispersion of a Nearly Open Superconducting Island

“…The rise in f 01 is accompanied by a strong decrease in δf 01 , suppressing by almost 2 orders of magnitude at the peak. This behavior is consistent with the presence of a quantum dot in the junction, which has been linked to peaks in the critical current that coincide with transparencies close to unity [25,26].…”

“…This systematic deviation indicates that the underlying junction potential might be shallower than captured by our model. We speculate that the discrepancies in Δ and the anharmonicities could be due to omitting the electron-electron interactions of the quantum dot, which has previously been found to suppress the critical current and alter the energyphase dependence of the ABS [26,[48][49][50].…”

“…However, recent advances in nanofabrication and nanowire growth [22] have enabled the development of superconductor-semiconductor-superconductor junctions with a small number of highly transmitting modes [23,24]. Experiments in such devices have detected a single mode with nearly perfect transmission, attributed to resonant tunneling through an accidental quantum dot in the junction [25,26]. Charge-sensitive devices connected to reservoirs via quantum-dot-based junctions are thus ideal for the investigation of near-ballistic Josephson junction behavior.…”

Suppressed Charge Dispersion via Resonant Tunneling in a Single-Channel Transmon

“…We attribute these features to resonant tunneling of Cooper pairs through an accidental quantum dot formed in the junction [ Fig. 1(d)], a common occurrence near full depletion [30,31]. We note that dc transport measurements (FET opened) of the switching current revealed corresponding resonances of similar width and spacing as a function of V Q , supporting our interpretation of resonant tunneling across the junction (see Supplemental Material [32]).…”

“…Minor deviations between experiment and model may be attributed to effects of electron-electron interactions in the quantum dot, which are not included in the model [31,41,42] as well as fluctuations in the ratio…”