Paper ET+NS+EM-ThM9
Point-Contact Spectroscopy Study of Topological Insulators and Superconductors

Thursday, November 1, 2012, 10:40 am, Room 16

Recently, much attention has been given to an intriguing class of materials, the so-called topological insulators. This type of material exhibits a band gap in the bulk, but gapless states on the edge or surface, which are protected by topological order and cannot be analogized to previous conventional semiconductors or insulators. When topological insulators are in contact with a superconductor (e.g., Nb, a conventional s-wave superconductor), novel proximity effect occurs. Theory predicts that the proximity induced superconducting state is spinless and p-wave like, and Majorana bound states may appear at the edges. On the other hand, in a related research avenue topological superconductors are predicted to possess unconventional pairing symmetries and gapless surface Andreev bound states. Theoretically massless Majorana fermions could be realized in such materials and used as a building block for topological quantum computation.

Here we present our point-contact spectroscopy studies of topological insulators and superconductors. Specifically, we use a superconducting Nb tip to approach the surface of topological insulators and measure the interface conductance as a function of bias voltage, temperature and magnetic field. Indeed, we find that a superconducting state can be induced at the interface when the Nb tip is in good contact with the topological insulator, as evidenced by observation of a zero-bias conductance peak in the point-contact spectra at a temperature below the superconducting transition temperature of Nb. Such an induced superconducting state is robust even in a magnetic field up to 1T. In the study of topological superconductors, we use a normal-metal Au tip to approach the surface, and a zero-bias conductance peak is also observed. Owing to accurate control of the point-contact barrier strength (tip/sample) in our experiments, the obtained spectra are free of artificial background, and therefore can be quantitatively compared with existing theories; good agreement is achieved.