Abstract
Measurements of conductance spectra in a superconductor-topological insulator-normal metal thin film junctions of NbN-BiSe-Au are reported. Junctions with ex situ and in situ prepared NbN-BiSe 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 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 BiSe-NbN interface, as they are absent in reference Au-NbN junctions without the topological layer sandwiched in between.
6 More- Received 23 July 2012
DOI:https://doi.org/10.1103/PhysRevB.86.144508
©2012 American Physical Society