Conduction spectroscopy of a proximity induced superconducting topological insulator

The combination of superconductivity and the helical spin-momentum locking at the surface state of a topological insulator (TI) has been predicted to give rise to p-wave superconductivity and Majorana bound states. The superconductivity can be induced by the proximity effect of a an s-wave superconductor (S) into the TI. To probe the superconducting correlations inside the TI, dI/dV spectroscopy has been performed across such S-TI interfaces. Both the alloyed Bi$_{1.5}$Sb$_{0.5}$Te$_{1.7}$Se$_{1.3}$ and the stoichiometric BiSbTeSe$_2$ have been used as three dimensional TI. In the case of Bi$_{1.5}$Sb$_{0.5}$Te$_{1.7}$Se$_{1.3}$, the presence of disorder induced electron-electron interactions can give rise to an additional zero-bias resistance peak. For the stoichiometric BiSbTeSe$_2$ with less disorder, tunnel barriers were employed in order to enhance the signal from the interface. The general observations in the spectra of a large variety of samples are conductance dips at the induced gap voltage, combined with an increased sub-gap conductance, consistent with p-wave predictions. The induced gap voltage is typically smaller than the gap of the Nb superconducting electrode, especially in the presence of an intentional tunnel barrier. Additional uncovered spectroscopic features are oscillations that are linearly spaced in energy, as well as a possible second order parameter component.Comment: Semiconductor Science and Technology; Special Issue on Hybrid Quantum Materials and Device

Non-triviality matters: examining the interplay between s-wave superconductivity and topological surface states

The main focus of this thesis is to understand the correlations present at the s-wave/three-dimensional topological insulator interface both theoretically and experimentally. In the future, devices containing these kind of interfaces can be used to create and manipulate a Majorana zero-energy mode which serves as a building block for a topological quantum bit.\ud \ud In Chapter 1 the basics around quantum computation is considered and we show that the observation of a 4pi periodicity in a superconductor/topological insulator Josephson junction is the hallmark for the detection of a Majorana fermion mode. Chapter 2 studies in more detail the properties and conditions of the existence of this 4pi periodicity. We show that only one channel contributes to the 4pi periodicity but by exploiting the effect of magnetization, caused by a ferromagnet, the number of channels contributing to a 4pi period increases.\ud \ud Chapter 3 presents measurements on the topological insulator Bi1.5Sb0.5Te1.7Se1.3 (BSTS) flakes. We show that at low temperatures the bulk conductance is neglible and that we succeeded to realize a Josephson supercurrent through the surface states. This is confirmed by the observation of a supercurrent and Fraunhofer pattern. \ud \ud Chapter 4 describes the calculations on the superconducting correlation present a the s-wave proximized topological surface states. We show that in the time-reversal symmetry case there is an equal admixture of s and p-wave correlations. As soon as time-reversal symmetry is broken, p-wave symmetry becomes dominant. The dominant p-wave correlations cause the appearance of both a conductance dip a the gap energy and a conductance peak at zero-bias. \ud \ud Chapter 5 combines the results of the previous chapters to study the conductance spectra on Au/BSTS/Nb devices. A conductance dip appears at the induced gap which could so far not be explained by trivial s-wave theory. \ud \ud In Chapter 6 we present a tight binding model of a three-dimensional topological insulator to model the properties of a few quintuple layers. We were able to explain the kink observed in scanning tunneling spectroscopy data on a Bi2Te3 thin film by fitting the tight binding model to the density of states obtained by existing ab initio calculations