Quantum electrical transport properties of topological insulators

Abstract

Three-dimensional (3D) topological insulators (TIs) are exotic quantum materials that have insulating bulk states and topological surface states (TSSs). The spins of the carriers, transported through the TSS, are locked to their momentum. The modulation of TSS can be carried out by applying an axial magnetic field to the TI nanoribbons (NRs) owing to the formation of one-dimensional subbands. Furthermore, TIs-based Josephson junctions are presumed to host Majorana bound states (MBS) in combination with superconducting contact. The topological Josephson current mediated by MBS is crucial for developing superconducting quantum information devices. In the first part of the thesis, we report the electrical detection of the spin-polarized current in TIs. The existence of the two-dimensional TSS is verified by measuring the angle of the magnetic field to the surface state dependent weak antilocalization (WAL) effect. Non-local current, which is regarded as a distinguishing property of TIs, is also observed with spin-momentum locking. In the second and third parts, we report the demonstration of one-dimensional subband formation by observing Aharonov–Bohm (AB) and Altshuler–Aronov–Spivak (AAS) oscillations, which are due to the quantum interference along the perimeter of the TI NRs. We propose that the relative amplitudes of the two quantum oscillations can be tuned by varying the channel length, exhibiting crossover from quasi-ballistic to diffusive transport regimes. Herein, we present an extensive investigation of the AB and AAS oscillations as a function of the gate voltage, revealing phase-alternating topological AB oscillations. Moreover, we observe the suppression of the quantum interference oscillation amplitudes near the Dirac point, which is attributed to the suppression of the phase coherence length within the low carrier density region. In the last part, we report the fabrication of the TI-based Josephson junction. Under the application of a microwave, the Josephson junction exhibits doubled Shapiro steps as a result of 4π the periodic current-phase relation due to Majorana zero modes. Therefore, we propose that TIs provide a useful platform for observing the topological Josephson current and developing superconducting quantum information devices.