Planar Tunneling Spectroscopy of Samarium Hexaboride(SmB6)

Abstract

As a well-known Kondo insulator originating from strong correlation, SmB6 has attracted much interest because of its surface states being potentially topological. In order to investigate this, we apply the technique of planar tunneling spectroscopy since it can detect the surface density of states (DOS). To acquire stable and clean tunneling spectra, we prepare SmB6 single crystal surfaces by polishing against alumina lapping film. Both (001) and (011) surfaces are studied using this method for comparison. With further ion-beam cleaning/etching under vacuum, tunnel junctions are formed by sputter deposition of Al (with approximate thickness between 2 5 nm) and subsequent plasma oxidation. Then two insulating strips are painted on the surface leaving a narrow conducting area at the center. Finally several silver counter-electrodes are deposited on the top using thermal evaporator. The junctions are defined by the intersections of the silver and the aluminum oxide barrier. Differential conductance is measured over wide ranges of temperature and magnetic field. Interesting features are detected at low temperature. For example, the overall asymmetric shape of conductance over the bias range from 300mV to 300mV, the multiple humps at negative bias voltage which do not evolve linearly with temperature, and the suppressed, linear conductance near zero bias. The asymmetric conductance is possibly a sign of Fano resonance in Kondo insulator, indicating that it might be conceivable to measure the bulk DOS using planar tunneling. The linear conductance at low bias, on the other hand, might be due to surface states. The gap feature of the spectra start to appear at around 50 K and the overall wavy background shape can sustain at around 100 K. *The work at UIUC is supported by the NSF DMR 12-06766.Ope