Differential resistance at the topological insulator-superconductor interface in a cross-bar geometry

We have studied the low temperature electronic transport properties of an interface between a topological insulator bismuth selenide and a superconducting niobium strip in a cross-bar geometry. In the regime where niobium is superconducting, we find that the zero-bias differential resistance increases with increasing magnetic field or temperature, which turns into a negative differential resistance just above the superconducting transition. We will discuss the possible origins of this phenomenon.