Scanning Tunnelling Microscopy and Transport Spectroscopy on Candidate Topological Systems

Abstract

Topologically non-trivial systems have emerged as a new area of research in condensed matter physics. The study of topologically nontrivial electronic systems is important because they provide a platform to experimentally investigate new low-energy excitations. In this thesis, I will discuss three such systems:Conventional Superconductivity in PdTe2: PdTe2 is a unique topological system where type II Dirac semimetallic phase coexists with a superconducting phase. This combined nature offers an interesting material candidate for investigation of possible topological superconductivity. We have performed high-resolution scanning tunnelling spectroscopic (STS) measurements to study the nature of superconductivity in this system. Theoretically, it has been speculated that superconductivity in such systems will be unconventional in nature. However, our experiments provide direct evidence of conventional superconductivity in PdTe2. High spin-polarization in Sb2Se3: Most of the A2B3 type chalcogenides (e.g. Bi2Se3, Bi2Te3 and Sb2Te3 etc.) are well known topological insulators. Though Sb2Se3 is a member of the same group, it is a band insulator under ambient conditions. A prominent quasiparticle interference (QPI) pattern observed in STM conductance mapping indicates the presence of backscattering, which is forbidden for a topological insulator. Interestingly, like most of the topological insulators, Sb2Se3 shows a high spin polarization ( 69 %) in transport current. To understand the origin of such high spin polarization, we performed band structure calculations which revealed presence of two trivial surface states with one undergoing large splitting due to Rashba type SOC and leading to high spin polarization. Tip-induced superconductivity in Pb0.6Sn0.4Te: The emergence of tip-induced superconductivity (TISC) has offered a possible way to detect topological superconductivity induced by a point contacts. We performed similar experiment on a topological crystalline insulator (TCI) Pb0.6Sn0.4Te with non-superconducting tips. Our experiments revealed that Ag/Pb0.6Sn0.4Te and Pd/Pb0.6Sn0.4Te point-contacts behave as a superconductor. As expected for a superconductor, we observed a sharp resistive transition at 6 K which evolved systematically with an applied magnetic field. This is also confirmed through temperature and magnetic field dependent studies of the point contact spectra. Moreover, on the basis of Hc Vs Tc phase diagram we predict that the induced superconductivity is conventional in nature. Furthermore, I will discuss low temperature and high vacuum point contact probe designing and fabrication.