Gate-tunable band structure of the LaAlO3_3-SrTiO3_3 interface

The 2-dimensional electron system at the interface between LaAlO$_{3}$ and SrTiO$_{3}$ has several unique properties that can be tuned by an externally applied gate voltage. In this work, we show that this gate-tunability extends to the effective band structure of the system. We combine a magnetotransport study on top-gated Hall bars with self-consistent Schr\"odinger-Poisson calculations and observe a Lifshitz transition at a density of $2.9\times10^{13}$ cm$^{-2}$. Above the transition, the carrier density of one of the conducting bands decreases with increasing gate voltage. This surprising decrease is accurately reproduced in the calculations if electronic correlations are included. These results provide a clear, intuitive picture of the physics governing the electronic structure at complex oxide interfaces.Comment: 14 pages, 4 figure

In-situ spectroscopy of intrinsic Bi2Te3 topological insulator thin films and impact of extrinsic defects

Combined in-situ x-ray photoemission spectroscopy, scanning tunnelling spectroscopy and angle resolved photoemission spectroscopy of molecular beam epitaxy grown Bi2Te3 on lattice mismatched substrates reveal high quality stoichiometric thin films with topological surface states without a contribution from the bulk bands at the Fermi energy. The absence of bulk states at the Fermi energy is achieved without counter doping. We observe that the surface morphology and electronic band structure of Bi2Te3 are not affected by in-vacuo storage and exposure to oxygen, whereas major changes are observed when exposed to ambient conditions. These films help define a pathway towards intrinsic topological devices.Comment: 8 pages, 5 figure

Correlation between Superconductivity, Band Filling and Electron Confinement at the LaAlO3_{3}-SrTiO3_{3} Interface

By combined top- and backgating, we explore the correlation of superconductivity with band filling and electron confinement at the LaAlO$_3$-SrTiO$_3$ interface. We find that the top- and backgate voltages have distinctly different effects on the superconducting critical temperature, implying that the confining potential well has a profound effect on superconductivity. We investigate the origin of this behavior by comparing the gate-dependence of $T_c$ to the corresponding evolution of the band filling with gate voltage. For several backgate voltages, we observe maximum $T_c$ to consistently coincide with a kink in tuning the band filling for high topgate voltage. Self-consistent Schr\"odinger-Poisson calculations relate this kink to a Lifshitz transition of the second $d_{xy}$ subband. These results establish a major role for confinement-induced subbands in the phase diagram of SrTiO$_3$ surface states, and establish gating as a means to control the relative energy of these states.Comment: 17 pages, 5 figure

Scaling universality at the dynamic vortex Mott transition

The cleanest way to observe a dynamic Mott insulator-to-metal transition (DMT) without the interference from disorder and other effects inherent to electronic and atomic systems, is to employ the vortex Mott states formed by superconducting vortices in a regular array of pinning sites. Here, we report the critical behavior of the vortex system as it crosses the DMT line, driven by either current or temperature. We find universal scaling with respect to both, expressed by the same scaling function and characterized by a single critical exponent coinciding with the exponent for the thermodynamic Mott transition. We develop a theory for the DMT based on the parity reflection-time reversal (PT) symmetry breaking formalism and find that the nonequilibrium-induced Mott transition has the same critical behavior as the thermal Mott transition. Our findings demonstrate the existence of physical systems in which the effect of a nonequilibrium drive is to generate an effective temperature and hence the transition belonging in the thermal universality class

Gate-tuned Anomalous Hall Effect Driven by Rashba Splitting in Intermixed LaAlO3/GdTiO3/SrTiO3

The Anomalous Hall Effect (AHE) is an important quantity in determining the properties and understanding the behavior of the two-dimensional electron system forming at the interface of SrTiO3-based oxide heterostructures. The occurrence of AHE is often interpreted as a signature of ferromagnetism, but it is becoming more and more clear that also paramagnets may contribute to AHE. We studied the influence of magnetic ions by measuring intermixed LaAlO3/GdTiO3/SrTiO3 at temperatures below 10 K. We find that, as function of gate voltage, the system undergoes a Lifshitz transition, while at the same time an onset of AHE is observed. However, we do not observe clear signs of ferromagnetism. We argue the AHE to be due to the change in Rashba spin-orbit coupling at the Lifshitz transition and conclude that also paramagnetic moments which are easily polarizable at low temperatures and high magnetic filds lead to the presence of AHE, which needs to be taken into account when extracting carrier densities and mobilities.Comment: 11 pages, 8 figures; and supplemen

Microwave studies of the fractional Josephson effect in HgTe-based Josephson junctions

The rise of topological phases of matter is strongly connected to their potential to host Majorana bound states, a powerful ingredient in the search for a robust, topologically protected, quantum information processing. In order to produce such states, a method of choice is to induce superconductivity in topological insulators. The engineering of the interplay between superconductivity and the electronic properties of a topological insulator is a challenging task and it is consequently very important to understand the physics of simple superconducting devices such as Josephson junctions, in which new topological properties are expected to emerge. In this article, we review recent experiments investigating topological superconductivity in topological insulators, using microwave excitation and detection techniques. More precisely, we have fabricated and studied topological Josephson junctions made of HgTe weak links in contact with two Al or Nb contacts. In such devices, we have observed two signatures of the fractional Josephson effect, which is expected to emerge from topologically-protected gapless Andreev bound states. We first recall the theoretical background on topological Josephson junctions, then move to the experimental observations. Then, we assess the topological origin of the observed features and conclude with an outlook towards more advanced microwave spectroscopy experiments, currently under development.Comment: Lectures given at the San Sebastian Topological Matter School 2017, published in "Topological Matter. Springer Series in Solid-State Sciences, vol 190. Springer