Quantum transport and induced superconductivity in selectively deposited topological insulator devices (2021)

Concepts for quantum computation technologies are based on the isolation of a quantum system that has two distinct quantum mechanical states. Famous examples like superconducting charge qubits, electronic spin qubits and nuclear spin qubits suffer from fast decoherence due to local perturbations like charge fluctuations or stray potentials. Topological quantum computation is based on the arrangement of two locally separated Majorana zero modes (MZMs) that can be manipulated by moving them around each other. The spatial separation makes the qubit state insusceptible to local perturbations. Localized MZMs can be realised in superconducting vortices within a metallic s-wave superconductor on top of a three-dimensional topological insulator (TI) surface. These zero dimensional quasiparticles can also be realized at the two ends of a quasi one-dimensional TI nanoribbon that is partially covered by a superconducting metal. For the experimental realization of MZMs this thesis focuses on the latter and is dedicated to study the electronic properties of the interface of a TI to a metallic superconductor and those of low-dimensional, confined TI nanoribbons. Signatures of Majorana bound states (MBSs) have been identified in Josephson junctions on molecular beam epitaxy (MBE) grown thin films of the (Bi,Sb)Te (BST) ternary compound TI. These arise due to the unique spin texture of the BST weak link that evokes perfectly transmitted modes. A novel in situ stencil technique has been developed to fabricate junctions with high interfacial quality that are protected by a dielectric capping layer. Junctions investigated have a high proximity coupling strength in between the superconducting metal and the TI as indicated by high IcRn products. Besides, a platform to selectively deposit TI nanoribbons using MBE has been developed. Magnetotransport measurements on nanoribbons of different cross-sectional areas of the binary compound Bi2Te3 have systematically been studied. Universal conductance fluctuations (UCFs) as well as Shubnikov-de Haas (SdH) oscillations are observed in a perpendicular magnetic field and a Berry phase of pi, a hallmark of self-interfering topological surface charges, has been identified. Once charges can traverse the nanoribbon perimeter phase-coherently quantized transverse momentum states develop. The anti-periodic boundary conditions of interfering charges due to the Berry phase of pi leads to a bandgap opening in the confined nanoribbon spectrum. Ahronov-Bohm (AB) and Al'tshuler-Aharonov-Spivak (AAS) oscillation patterns can be observed in an axial magnetic field that display a periodic band closing of transverse momentum states with increasing transverse mode index l. Electrostatic gating experiments are performed to identify the energetic spacing of these transverse momentum states. A combination of the selective area deposition of TIs and the in situ fabrication of devices has been developed to fabricate state of the art, confined Josephson junctions with a topological weak link. The physics in one-dimensional nanoribbon based Josephson junctions has been studied for junctions with different superconducting electrodes and novel MBE grown topological weak link materials. Results indicate a good proximity coupling and the observation of ballistic, topological surface modes. Based on these results, complex hybrid structures of topological matter and metallic superconductors can be envisioned to create and manipulate Majorana zero modes for topological quantum computation purposes.