Transport properties of Bi2Se3 Topological Insulator Nanoribbon-Superconductor hybrid junctions
Doctoral thesis, 2023

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-PĂ©rot (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.

Bismuth selenide

surface states

Topological insulator

ac susceptibility

Josephson junctions

superconductivity

SQUID

andreev bound states

Kollektorn, MC2, Kemivägen 9, Chalmers.
Opponent: Professor Alexander Brinkman, Faculty of Science and Technology, University of Twente, The Netherlands.

Author

Ananthu Pullukattuthara Surendran

Chalmers, Microtechnology and Nanoscience (MC2), Quantum Device Physics

Ballistic transport on micrometer scale revealed by Fabry-Pérot-like resonances in Bi2Se3 nanoribbon devices.

Quantum Confinement and Coherent Transport in Ultrathin Bi2Se3 Nanoribbons

AC Susceptibility of a Bi2Se3 nanoribbon Josephson junction

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.

Areas of Advance

Nanoscience and Nanotechnology

Roots

Basic sciences

Infrastructure

Nanofabrication Laboratory

Subject Categories

Condensed Matter Physics

ISBN

978-91-7905-944-6

Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 5410

Publisher

Chalmers

Kollektorn, MC2, Kemivägen 9, Chalmers.

Opponent: Professor Alexander Brinkman, Faculty of Science and Technology, University of Twente, The Netherlands.

More information

Latest update

2/29/2024