Center for Functional Nanomaterials Seminar

Topological insulators(TI) are an interesting class of quantum materials which have an insulating bulk state and time reversal symmetry protected non-trivial conducting states on the surfaces(3D) or edges(2D) .These symmetry protected states make them stable to local perturbations. An s-wave superconductor that is proximity coupled to the topological surface states on a strong 3D topological insulator can be considered equivalent to a 2D spinless px+ipy superconductor- a fully gapped topological super conductor(TSC). This can be regarded as engineering an artificial TSC system from well-studied systems, instead of trying to directly realize p-wave superconductivity. This system can host Majorana Bound States(MBS), quasiparticle excitations exhibiting non-standard exchange statistics. These MBS would emerge in pairs, localized as magnetic vortex cores within a S-TI-S linear Josephson Junction(JJ). These pairs can then be utilized to create a topological qubit, that can be manipulated by local application of external magnetic fields to perform anyon braiding operations for a fault-tolerant topological quantum computer. A single JJ is hence a natural system to study the proximity-induced TSC that is the basis for using Majorana Fermion based topological qubits. Bi2Te3, Sb2Te3 and BixSb2-xTe3 (BST) are all V-VI narrow gap semiconductors as well as 3D TIs. BST is a 3D TI whose composition can be tuned between the naturally n-type Bi2Te3 and p-type Sb2Te3 to suppress bulk carriers. Bulk carriers need to be minimized to realize purely topological superconductivity. Based on ARPES data of various compositions of BST, Bi0.8Sb1.2Te3 was found to be bulk insulating and therefore the carriers are due to topological surface states, with the Fermi level only crossing the surface states lying in between the bulk bands. Molecular beam epitaxy is used to grow thin films of Bi0.8Sb1.2Te3 on sapphire substrates, which are then used as the weak link in fabricating coplanar TSC-TI-TSC JJs with niobium electrodes used for proximity-induced superconductivity in the TI layer i.e. the TSC. For BST this is localized only in the 2D electron gas in contact with the niobium layer. Single Josephson Junctions of varying dimensions were fabricated and studied in a cryostat. DC I-V measurements are used to measure the contact transparency between Nb and BST and to calculate the proximity-induced superconducting energy gap in the TI. I introduce a new method to more accurately extract critical currents in overdamped Josephson Junctions at various temperatures by accounting for the thermal noise current. This is important since all the devices have small critical currents due to not having any bulk carriers and allows for more accurate characterization of the BST material. Differential conductance measurements give more information into the nature of the proximity-induced TSC, with some interesting geometric resonances systematically observed in all devices.