Fractional ac Josephson effect as evidence of topological hinge states in a Dirac semimetal NiTe2

TL;DR

This study demonstrates fractional ac Josephson effect in NiTe2, providing evidence for topological hinge states through observed half-integer Shapiro steps and $ ext{ extpi}$ periodicity, distinct from surface state signatures.

Contribution

The paper provides experimental evidence for topological hinge states in NiTe2 via fractional Josephson effect, highlighting the role of hinge modes over surface states.

Findings

Observation of fractional Shapiro steps at half-integer values.

Absence of $4 extpi$ periodicity, indicating hinge state contribution.

Confirmation of $ extpi$ periodicity linked to topological hinge states.

Abstract

We experimentally investigate Josephson current between two 5~$\mu$m spaced superconducting indium leads, coupled to a NiTe$_2$ single crystal flake, which is a type-II Dirac semimetal. Under microwave irradiation, we demonstrate a.c. Josephson effect at millikelvin temperatures as a number of Shapiro steps. In addition to the integer ($n=1,2,3,4...$) steps, we observe fractional ones at half-integer values $n=1/2,3/2,5/2$ and 7/2, which corresponds to $\pi$ periodicity of current-phase relationship. In contrast to previous investigations, we do not observe $4\pi$ periodicity (disappearance of the odd $n=1,3,5...$ Shapiro steps), while the latter is usually considered as a fingerprint of helical surface states in Dirac semimetals and topological insulators. We argue, that our experiment confirms Josephson current through the topological hinge states in NiTe$_2$: since one can exclude bulk supercurrent in 5~$\mu$m long Josephson junctions, interference of the hinge modes is responsible for the $\pi$ periodicity, while stable odd Shapiro steps reflect chiral character of the topological hinge states.