Unconventional Josephson junctions with topological Kondo insulator weak links

Abstract

Proximity-induced superconductivity in three dimensional (3D) topological insulators forms a new quantum phase of matter and accommodates exotic quasiparticles such as Majorana bound states. One of the biggest drawbacks of the commonly studied 3D topological insulators is the presence of conducting bulk that obscures both surface states and low energy bound states. Introducing superconductivity in topological Kondo insulators such as SmB6, however, is promising due to their true insulating bulk at low temperatures. In this work, we develop an unconventional Josephson junction by coupling superconducting Nb leads to the surface states of a SmB6 crystal. We observe a robust critical current at low temperatures that responds to the application of an out-of-plane magnetic field with significant deviations from usual Fraunhofer patterns. The appearance of Shaphiro steps under microwave radiation gives further evidence of a Josephson effect. Moreover, we explore the effects of Kondo breakdown in our devices, such as ferromagnetism at the surface and anomalous temperature dependence of supercurrent. Particularly, the magnetic diffraction patterns show an anomalous hysteresis with the field sweep direction suggesting the coexistence of magnetism with superconductivity at the SmB6 surface. The experimental work will advance the current understanding of topologically nontrivial superconductors and emergent states associated with such unconventional superconducting phases.

Signatures of Majorana bound states and parity effects in two-dimensional chiral p-wave Josephson junctions

“…In particular, lateral Josephson junctions offer a promising alternative to their nanowire counterparts for localizing MBS even at low magnetic fields 14,[23][24][25][26] . To summarize the key ideas, starting with the proposed setting by Fu and Kane 23 , the principle is to juxtapose appropriate thin film materials in patterned geometries so as to create networks of lateral topological Josephson junctions.…”

“…Here, we turn to the essential task of laying out and analyzing crucial steps leading to such a functioning device. While several of these components have been investigated in different contexts, our purpose here is to have a focused treatment in the specific context of lateral junctions, as directly relevant to current experimental capabilities, including on-going work 25,26,[34][35][36][37][38] . We center our analysis on a simple, minimal model of a lateral topological Josephson junction.…”

Kondo effect and superconductivity in niobium with iron impurities

“…The competition between superconductivity and Kondo effect has been of great interests 39 – 42 . In recent years, the Kondo effect has also been studied in many new superconductors materials 31 – 35 , in 2D topological superconductors and 3D topological insulators 43 , 44 . Similar resisivity minimum was also observed in high- superconductors 45 .…”

Majorana bound state parity exchanges in planar Josephson junctions

“…The original nanowire geometries, while facing setbacks, laid down the foundations for integrating the fields of condensed matter, material physics and quantum information science in designing viable platforms, and computational schemes [2][3][4][5][6][7][8]. In recent times, extended junctions have become popular platforms to host MBS even at low magnetic field strengths [9][10][11][12][13]. Early proposals involve proximityinduced superconductivity on the surface of topological insulators in 2D and 3D with an externally applied magnetic field resulting in spatially separated MBS nucleating in Josephson vortices [2,9,10,[13][14][15][16][17][18][19][20][21].…”