Topological superconductors can support localized Majorana states at their boundaries. These quasi-particle excitations have non-Abelian statistics that can be used to encode and manipulate quantum information in a topologically protected manner. While signatures of Majorana bound states have been observed in one-dimensional systems, there is an ongoing effort to find alternative platforms that do not require fine-tuning of parameters and can be easily scalable to large numbers of states. Here we present a novel experimental approach towards a two-dimensional architecture. Using a Josephson junction made of HgTe quantum well coupled to thin-film aluminum, we are able to tune between a trivial and a topological superconducting state by controlling the phase difference φ across the junction and applying an in-plane magnetic field. We determine the topological state of the induced superconductor *

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The observation of ultrarelativistic fermions in condensed-matter systems has uncovered a cornucopia of novel phenomenology as well as a potential for effective ultrafast light engineering of new states of matter. While the nonequilibrium properties of two- and three-dimensional (2D and 3D) hexagonal crystals have been studied extensively, our understanding of the photoinduced dynamics in 3D single-valley ultrarelativistic materials is, unexpectedly, lacking. Here, we use ultrafast scanning near-field optical spectroscopy to access and control nonequilibrium large-momentum plasmon-polaritons in thin films of a prototypical narrow-bandgap semiconductor Hg0.81Cd0.19Te. We demonstrate that these collective excitations exhibit distinctly nonclassical scaling with electron density characteristic of the ultrarelativistic Kane regime and experience ultrafast initial relaxation followed by a long-lived highly coherent state. Our observation and ultrafast control of Kane plasmon-polaritons in a semiconducting material using light sources in the standard telecommunications fiber-optics window open a new avenue toward high-bandwidth coherent information processing in next-generation plasmonic circuits.

Topological effects in edge states are clearly visible on short lengths only, thus largely impeding their studies. On larger distances, one may be able to dynamically enhance topological signatures by exploiting the high mobility of edge states with respect to bulk carriers. Our work on microwave spectroscopy highlights the responses of the edges which host very mobile carriers, while bulk carriers are drastically slowed down in the gap. Though the edges are denser than expected, we establish that charge relaxation occurs on short timescales, and suggests that edge states can be addressed selectively on timescales over which bulk carriers are frozen. arXiv:1903.12391v3 [cond-mat.mes-hall]

We present a novel low-temperature (30 °C) atomic layer deposition process for hafnium oxide and apply the layers as gate dielectric to fabricate devices out of the thermally sensitive topological insulator HgTe. The key to achieving self-limiting growth at these low temperatures is the incorporation of sufficiently long purge times ( ≥150 s) in the deposition cycles. We investigate the structural and compositional properties of these thin films using X-ray reflectometry and photoelectron spectroscopy, finding a growth rate of 1.6 Å per cycle and an atomic ratio of Hf/O of 1:1.85. In addition, we report on the transport properties of the microstructured devices, which are much enhanced compared to previous device generations. We determine a relative permittivity of ∼15 for our HfO2 layers. Our process considerably reduces the thermal load of the samples during microfabrication and can be adapted to a broad range of materials, enabling the fabrication of high-quality gate insulators on various temperature-sensitive materials.