Zero energy bound states in tunneling conductance spectra at the interface of an s-wave superconductor and a topological insulator in NbN-$Bi_2Se_3$-Au thin film junctions

TL;DR

This study investigates conductance spectra in NbN-$Bi_2Se_3$-Au junctions, revealing zero energy bound states linked to the interface, with differences based on interface preparation affecting the spectral features.

Contribution

It demonstrates the importance of tunneling barriers in observing zero energy bound states at superconductor-topological insulator interfaces.

Findings

Zero bias conductance peaks observed in ex-situ junctions.

Flat conductance spectra in in-situ junctions with lower resistance.

Zero energy bound states likely originate at the $ m Bi_2Se_3$-NbN interface.

Abstract

Measurements of conductance spectra in a superconductor - topological insulator - normal metal thin film junctions of NbN-$\rm Bi_2Se_3$-Au are reported. Junctions with ex-situ and in-situ prepared $\rm NbN-Bi_2Se_3$ interfaces were studied. At low temperatures, all the ex-situ junctions showed coherence peaks in their conductance spectra, but imbedded robust zero bias conductance peaks were observed only in junctions with a metallic or a metal to insulator transition below $\rm T_c$ of the NbN electrode. The in-situ junctions which had about two orders of magnitude lower resistance at low temperatures, generally showed flat conductance spectra at low bias, with no coherence or broad Andreev peaks, since the critical current of the NbN electrode was reached first, at voltage bias below the energy gap of the superconductor. A weak zero bias conductance peak however, was observed in one of these junctions. We conclude that significant tunneling barriers, as in the ex-situ prepared junctions, are essential for the observation of coherence peaks and the zero energy bound states. The later seem to originate in the $\rm Bi_2Se_3$-NbN interface, as they are absent in reference Au-NbN junctions without the topological layer sandwiched in between.