Evidence for an anomalous current–phase relation in topological insulator Josephson junctions

Abstract

Josephson junctions with topological insulator weak links can host low-energy Andreevbound states giving rise to a current-phase relation that deviates from sinusoidal behaviour. Of particular interest are zero-energy Majorana-bound states that form at a phase difference of p. Here we report on interferometry studies of Josephson junctions and superconducting quantum interference devices (SQUIDs) incorporating topological insulator weak links. We find that the nodes in single-junction diffraction patterns and SQUID oscillations are lifted and independent of chemical potential. At high temperatures, the SQUID oscillations revert to conventional behaviour, ruling out asymmetry. The node-lifting of the SQUID oscillations is consistent with low-energy Andreev-bound states exhibiting a nonsinusoidal current-phase relation, co-existing with states possessing a conventional sinusoidal current-phase relation. However, the finite nodal currents in the single-junction diffraction pattern suggest an anomalous contribution to the supercurrent possibly carried by Majorana-bound states, although we also consider the possibility of inhomogeneity.

Electrically Tunable Multiterminal SQUID-on-Tip

“…3b directly depicts the current-phase relation ( ) of junction 2. This correspondence holds for any form of current-phase relations 49 and therefore can provide a general tool, adding to existing techniques [52][53][54][55] , for direct probing of current-phase relations in various unconventional superconductor systems. In order to achieve a highly sensitive continuous operation with low flux noise 2 / = 2 3 / /|5 /5Φ |, one requires a high transfer function |5 /5Φ | and a low current noise 2 3 / at any value of the applied field.…”

Signature of a topological phase transition in the Josephson supercurrent through a topological insulator

“…Here, we would like to point to recently published work which indicates the presence of low-energy ABS in TIJJs. In this work Kurter, et al, 27,28 report anomalies in the diffraction patterns of Josephson junctions and superconducting quantum-interference devices (SQUIDs) made from exfoliated Bi 2 Se 3 and explain these by a non-sinusoidal contribution to the currentphase relationship (CPR) of TIJJs. The non-sinusoidal component is attributed to the presence of low-energy ABS in the junctions.…”

“…Unusual features in the diffraction patterns of SC-TI-SC were reported in Refs. 15,27,28 . Although we observed similar characteristics in some of our devices, we did not carry out a detailed study.…”

Approximating the Sachdev-Ye-Kitaev model with Majorana wires

“…Majorana fermions provide building blocks for many novel phenomena. As one notable example, Majorana-fermion zero modes [1,2] capture the essence of non-Abelian statistics and topological quantum computation [3,4], and correspondingly now form the centerpiece of a vibrant experimental effort [5][6][7][8][9][10][11][12][13][14][15][16]. More recently, randomly interacting Majorana fermions governed by the "Sachdev-YeKitaev (SYK) model" [17][18][19] were shown to exhibit sharp connections to chaos, quantum-information scrambling, and black holes-naturally igniting broad interdisciplinary activity (see, e.g., [20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39]).…”

“…We instead envision a realization [ Fig. 1(a)] that exploits Majorana zero modes germinated in proximitized semiconductor nanowires [41,42]-a leading experimental architecture for topological quantum information applications [5][6][7][8][9]11,[13][14][15].…”