Tunneling and bound states in near-edge NbN-$\rm Bi_2Se_3$ junctions fabricated by deposition through a wire shadow mask

Abstract

Transport measurements in thin film junctions of NbN-$\rm Bi_2Se_3$ exhibit tunneling as well as bound state resonances. The junctions are prepared by pulsed laser deposition of a NbN layer through a 25 $\mu$m wide gold wire shadow mask bisecting the wafer into two halves, on a $\rm Bi_2Se_3$ blanket film without further patterning. This results in two independent near-edge junctions connected in series via the 25 $\mu m$ long and 10 mm wide area of the uncapped $\rm Bi_2Se_3$ layer. Conductance spectra measured across the wire masked trench at different locations on the wafer show that some junctions have tunneling behavior with pronounced coherence peaks at $\pm 2\Delta$ where $\Delta \simeq$ 1 meV, while others have zero bias conductance peaks and series of bound states at higher bias. The later can be attributed to zero energy Majorana bound states or to the more conventional Andreev bound states. Based on the present results, we can not distinguish between these two scenarios.