Observation of the Superconducting Proximity Effect in the Surface State of SmB6 Thin Films

Abstract

The proximity effect at the interface between a topological insulator and a superconductor is predicted to give rise to chiral topological superconductivity and Majorana fermion excitations. In most topological insulators studied to date, however, the conducting bulk states have overwhelmed the transport properties and precluded the investigation of the interplay of the topological surface state and Cooper pairs. Here, we demonstrate the superconducting proximity effect in the surface state of SmB6 thin films which display bulk insulation at low temperatures. The Fermi velocity in the surface state deduced from the proximity effect is found to be as large as 105m/s , in good agreement with the value obtained from a separate transport measurement. We show that high transparency between the topological insulator and a superconductor is crucial for the proximity effect. The finding here opens the door to investigation of exotic quantum phenomena using all-thin-film multilayers with high-transparency interfaces.

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Plain Language Summary

Superconductivity in the topologically protected surface states of a three-dimensional topological insulator has been predicted to be a promising platform for exploring exotic quantum states such as Majorana fermion excitations. The superconducting proximity effect is a phenomenon occurring at a superconductor–normal-metal interface in which Cooper pairs diffuse into the normal metal, resulting in induced surface or local superconductivity in the normal metal. Although there have been previous experimental efforts focused on studying the superconducting proximity effect in bilayer structures between a superconductor and a chalcogenide topological insulator, suppressing the conducting bulk contribution and securing high interfacial transparency between a superconductor and a topological insulator have been major experimental bottlenecks to demonstrating superconductivity. Here, we demonstrate the superconducting proximity in the surface state of a topological insulator realized by a samarium hexaboride (SmB6 ) thin film with in situ deposition of a superconducting layer.

Known as a prototypical Kondo insulator, SmB6 has recently been reported to possess a topologically protected surface surrounding its insulting core. We ensure high interfacial quality between a superconductor and SmB6 by growing a superconducting layer immediately after fabrication of the SmB6 films, without breaking the vacuum in the deposition chamber. We extract physical parameters of the surface state of SmB6 , including the thickness of the surface state (approximately 6 nm), the normal coherence length, and the Fermi velocity; these data provide unambiguous evidence of the superconducting proximity effect induced in the surface of SmB6 .

Our findings provide unique insight into the surface state of SmB6 . The strong proximity effect we observe at the superconductor-SmB6 interface is an important stepping stone for pursuing novel quantum phenomena using thin-film topological insulator devices.