Subharmonic Shapiro steps in the a.c. Josephson effect for a three-dimensional Weyl semimetal WTe2

Abstract

We experimentally study electron transport between two superconducting indium leads, coupled to a single WTe2 crystal, which is a three-dimensional Weyl semimetal. We demonstrate Josephson current in 5 μm long In-WTe2-In junctions, as confirmed by the observation of integer (1, 2, 3) and fractional (1/3, 1/2, 2/3) Shapiro steps under microwave irradiation. The demonstration of the fractional a.c. Josephson effect indicates the multivalued character of the current-phase relationship, which we connect with the Weyl topological surface states contribution to the Josephson current. In contrast to topological insulators and Dirac semimetals, we do not observe periodicity in the a.c. Josephson effect for WTe2 at different frequencies and power, which might reflect the chiral character of the Fermi arc surface states in a Weyl semimetal.