Transport properties of Bi2Se3 Topological Insulator Nanoribbon-Superconductor hybrid junctions

Sammanfattning: In recent years, topological superconductivity and Majorana zero-energy modes have attracted vast interest due to their potential for topologically protected quantum information processing. Hybrid devices involving a conventional s-wave superconductor (S) in proximity to a 3D Topological Insulator (TI) are expected to provide a platform for emulating and studying these phenomena. In superconductor-topological insulator-superconductor (S-TI-S) Josephson junctions, Majorana physics manifests as peculiar current-carrying bound states, i.e., Majorana bound states (MBS) localized on the topological surface of the 3D TI. In this thesis, we investigate the electrical transport properties of superconductor-topological insulator-superconductor (S-TI-S) Josephson junctions fabricated using Bi2Se3 nanoribbons and Al electrodes. We explore in-depth the size quantization effects and ballistic transport in S-TI-S junctions by studying the width dependence of critical current density in our junctions and Fabry-Pérot (FP) resonance arising from ballistic topological surface states (TSSs). We show that FP resonance survives in devices with width scales over a micrometre. Further characterization involves the measurement of the current phase relation (CPR) of our Al-Bi2Se3-Al junctions using the asymmetric SQUID measurements technique. The experimentally extracted CPR of our junctions is heavily skewed and supports transport by ballistic TSSs. The third part of the thesis developed around the microwave probing of Andreev bound state dynamics in Al-Bi2Se3-Al junctions. We use a circuit-QED-inspired layout where an RF-SQUID based on our S-TI-S junction is inductively coupled to a coplanar waveguide resonator. By studying the AC susceptibility of our junctions, we reveal bounds states with small energy gaps (or high transparency). In the final section of the thesis, we address the problem of the unavoidable bulk contributions to transport in our TINR-based devices and discuss some of our attempts to tackle the problem by employing electrostatic gates. We also explore the possibility of using ultrathin TI-nanoribbons, which are easy to control by a gate as compared to thick nanoribbons. The gate response of the conductivity indeed shows hints of size-induced subband quantization. Overall, the work presented in the thesis demonstrates the presence of highly transparent ballistic transport modes arising from TSSs in Al-Bi2Se3-Al junctions using a variety of DC and AC measurements. Our devices give hints that size control of the nanoribbons and geometry of the junctions can be instrumental in isolating the contributions of TSSs to the transport properties in the normal and superconducting state.