Proximity-induced superconductivity in a dual topological insulator and crossover from gapped-to-gapless Dirac surface states in a magnetic topological insulator

Abstract

The combination of non-trivial topology and superconductivity is predicted to give rise to exotic Majorana fermions at the vortex core, forming a topological superconductor (TSC). TSCs, crucial for fault-tolerant quantum computing, typically exhibit p-wave superconductivity. However, due to a lack of suitable candidate materials, alternative approaches involve artificially creating the TSC phase by interfacing topological materials with swave superconductor. Since the prediction of the artificial creation of TSC by interfacing strong topological insulator (STI)/s-wave superconductor (SC) in 2008, this domain has been heavily pursued both theoretically and experimentally. A few years later, topological superconductivity was experimentally discovered in the doped topological insulator, CuxBi2Se3 employing the point-contact spectroscopy. Subsequently, interfacing Dirac semimetal (e.g. Cd3As2) with SC has resulted in superconducting correlations in the Fermi arcs. An integral part of this thesis deals with investigating the electrical transport properties of the dual topological insulator/s-wave superconductor heterojunction. This thesis is organized as follows: In Chapter 1, we introduce various types of Hall effects, including the Ordinary Hall effect (OHE), Anomalous Hall effect (AHE), and Spin Hall effect (SHE). We delve into their quantum mechanical counterparts and explore the concept of topology originating from the integer quantum Hall effect (IQHE). Additionally, we discuss different types of topological insulating phases, such as weak topological insulators (WTIs), strong topological insulators (STIs), topological crystalline insulators (TCIs), and magnetic topological insulators (MTIs). We also provide a basic overview of superconductivity and topological superconductivity. Towards the end, the 2-D Blonder-Tinkham-klapwijk (BTK) model is discussed in detail along with the simulations for metal/superconductor heterojunctions in a range of temperature and interfacial barriers. In chapter 2, the experimental techniques used during the course of this thesis are described. These include single crystal growth, characterization techniques, fabricating mesoscopic devices, and performing electrical transport measurements in different cryostats. In chapter 3, we have presented the first work of the thesis, where we comprehensively explore the superconducting proximity effects at the dual topological insulator/s wave superconductor (BiSe/NbSe2) interface by means of electrical transport measurements. These include magnetoresistance, resistance vs. temperature, and differential conductance measurements. The results show unconventional superconductivity at the heterojunction, which potentially originates due to the topological superconductivity. In chapter 4, we investigated the superconducting proximity effects at the flat-land region of BiSe (i.e., unproximitized region of BiSe in the BiSe/NbSe2 heterostructure) by performing the electrical transport on BiSe probes far from the interface. Interestingly, all the transport measurements consistently demonstrate that the electrical transport properties of DTI BiSe get dominated by the superconducting proximity effects. This is of great importance as the probes are far away from the interface, the presence of such strong superconducting correlation into BiSe suggests longer proximity length and high-quality interface devices. In chapter 5, we have probed the electrical transport properties of quasi 2-D exfoliated flakes of magnetic topological insulator MnBi2Te4 in various temperature regimes. At very low temperatures, our observations on the temperature dependence of longitudinal conductivity at various constant out-of-plane magnetic fields revealed the gapped Dirac surface states. In addition, for the first time, we have observed the weak-antilocalization (WAL) effect in the magnetoresistance measurements near the antiferromagnetic to paramagnetic transition, and beyond. Thus our observations shed light on the gapped and gapless nature of surface states in MnBi2Te4 in different temperature regimes. In chapter 6, we have discussed the main conclusions from the thesis and the possible future work that can serve as an extension of the present work. In chapter 7, we have provided the crystal growth of Fe3GeTe2 and CrI3 using the chemical vapor transport method. Additionally, we have also presented the characterization results obtained through SEM-EDS, including the stoichiometries of these crystals.