In-Situ Fabricated Low-Dimensional Topological Insulator - Superconductor Hybrid Junctions: A Platform for Majorana Fermions

Abstract

Topological Insulators are relatively a new class of materials in condensed matter physics that exhibit insulating bulk band structure with linearly dispersed and spin polarized metallic surface states which arise from the band inversion caused by the strong spin-orbit interactions. Helical spin texture in these surface states prohibits direct backscattering that makes such materials exotic for spintronic applications. On the other hand, the hybrid structures of three-dimensional topological insulator and s-wave superconductor with p-wave pair correlation symmetry, in low-dimensional systems, are predicted to give rise to confined quasiparticle excitations at the interface called Majorana Zero Modes (MZMs) that act as the basic building blocks of a topological quantum bit for the fault tolerant quantum computation. A stepwise overview of the all challenges and achievements in obtaining low dimensional hybrid TI-SC devices including the novel device fabrication techniques to high quality topological insulator thin film growth, from prevention of degradation of TI surface states to in-situ lithography techniques, is presented.TI thin films are selectively grown on the pre-designed nano-scale patterns using molecular beam epitaxy (MBE) and are characterized at ultra-low temperatures. Selective area growth (SAG) technique facilitated the minimum requirement of fabrication steps after the TI growth. High quality films of binary topological compounds based on Bismuth, Antimony, Tellurium and Selenium are grown and investigated. In the Te based compounds the Fermi level is tailored to the Dirac Point (DP) using a p-n junction heterostructures, and improved with reduced point defects, using the ternary BixSb(2-x)Te3 system. Surface degradation and aging issue is encountered with capping the TI with thin film of aluminum-oxide on top. Superconducting contacts are prepared utilizing the pre-standardized techniques of stencil lithography and on-chip fabricated shadow mask and hence low-dimensional Joseph Junctions (JJ) of various geometries have been prepared. Transport studies performed focus on quantum oscillations in magneto-conductance measurements as well as on induced superconductivity facilitating superconducting leads with a contact separation of only a few tens of nanometers. In the TI-SC hybrid devices, the initial signatures of 4-π periodic supercurrent and missing first Shapiro step are observed. Systems with WTI and TCI phases are the subject of future investigations