Transport properties of Bi2Se3 Topological Insulator Nanoribbon-Superconductor hybrid junctions

Doctoral thesis, 2023

One of the celebrated equations in physics came from Paul A. M. Dirac in 1928. With the so-called Dirac equation, which describes spin half particles like electrons, he predicted the existence of an anti-electron or positron that is distinct from an electron. This led to the discovery of positron by Carl D. Anderson in 1933. Later, other antimatter particle counterparts of regular matter that comprise most of the universe were identified. When a particle and an antiparticle come together, they annihilate, producing energy. In 1937, Italian physicist Ettore Majorana devised an elegant modification to the Dirac equation. His equations predicted a particle, namely Majorana Fermion, that could be its own antiparticle. Many years after his prediction, even today, none of the elementary particles are shown to be Majorana Fermions.

Our modern electronics rely heavily on condensed matter physics, a field of physics which studies the properties of matter. In devices made of various materials, we can have collective excitations or quasi-particles, and unlike elementary particles, that are a fundamental property of the material. So, by cleverly engineering a hybrid material system, we can emulate particles different from elementary particles. It was predicted that Majorana fermions could be emulated using hybrid devices involving a conventional superconductor (S) and an unconventional metal. In our case, we use a 3D Topological Insulator (TI) Bi2Se3 nanoribbon, which is a special class of materials that are insulating in the bulk and have conducting metallic states on the surface which obey the Dirac equation.

In this thesis, we study electronic transport in superconductor-topological insulator-superconductor (S-TI-S) Josephson junctions in which Majorana physics might manifest as peculiar current-carrying bound states, i.e., Majorana bound states. Although Majorana bound states are not supposed to be present in our nanoscale devices under the explored experimental conditions, the precursors of such states can manifest in ballistic transport in the normal and superconducting state of our junctions. We observed that the metallic topological surface states carry most of the supercurrent in our Josephson junctions and they follow ballistic transport where electrons move smoothly with scattering over length scales over a micrometer. In the last part of the thesis investigate the dynamics of Andreev bound states originating from such ballistic transport modes. Our devices give hints that size control of the nanoribbon and geometry of our junctions can be instrumental to isolate the contribution of topological surface states to the transport properties in the normal and superconducting state.