Our more recent work on probing the topological states of PbSnSe in a superlattice structure has been published in Physics Review B. It is a milestone demonstrating a full spectroscopic study of the Dirac states of a topological material in a heterostructure using a non-surface sensitive probe. Our study also shows how the versatility of the PbSnSe system allows us to tune the Dirac mass and penetration depth of the topological states by simply changing the temperature. This is of important fundamental relevance to engineer Dirac fermions with a variable mass using an external continuous knob, without having to alter the sample characteristics.

# Author: baa

# Topological insulator capacitors in the RF regime

Our recent work on rf-capacitor devices made out of layered hexagonal-boron nitride (hBN) and Bi2Se3 stacks was recently published in Physical Review Applied .

This work probes the rf-response of surface Dirac electrons in Bi2Se3 obtained by CVD-growth on hBN. It is the first investigation that addresses this issue, that is of capital importance to the future use of topological insulators in high-mobility transistors and GHz devices. One of key hurdles that scientists are confronted with in topological insulators is their native defects, that creates free mobiles electrons. Our device essentially operates as a gate-channel stack, where the gate is use to simultaneously deplete free charge carriers and probe the quantum capacitance of the system. The quantum capacitance is related to the electronic compressibility or the density of states of the system. We succeed in observing the Dirac point in Bi2Se3 via capacitance measurements performed at GHz frequencies. Although several obstacles remain to be overcome before topological materials can be implemented as transistors channels, our work achieves a two-fold result: (i) our devices employs a new combination of mechanical exfoliation and CVD growth to realize a hBN-Bi2Se3-hBN sandwich structure. (ii) we observe the Dirac response of surface states at GHz frequencies.

Bi2Se3/hBN stack embedded in RF waveguide.

# Negative Magnetoresistance in topological crystalline insulators

Our new article on the observation and the explanation of negative magnetoresistance in PbSnSe topological crystalline insulator was highlighted among the editor’s suggestions of Physical Review Letters.

General audience summary

Topological insulators are promising materials for spintronic data processing and quantum computing. They are materials that can be made insulating in the bulk, but always conduct at the surface like metals. This is due to the fact the in such materials surface electrons have their spin coupled to their momentum. Spin transport is therefore inherent to these materials even in the absence of ferromagnetism. A second reason is the fact that one can create robust Majorana fermions using these spin-momentum locked electrons. Majoranas are potentially useful for use as qubits in quantum computers.

Up to now most studies were concerned with studying manifestations of the topological properties of these materials via their ‘exotic’ surface electrons. In our recent work, we go beyond this paradigm, and reveal manifestations of topological properties in the insulating bulk states. When materials are subjected to strong magnetic fields, their electrons are quantized into orbits, referred to as Landau orbits. In a topological material, we demonstrate that a fundamental signature from bulk electrons is an anomalous behavior of these Landau orbits, that yields a resistance that drops as a function of increasing magnetic field, contrary to what usually happens in conventional semiconductors.

With this work (Physical Review Letters 119 106606 (2017)) and our previous work on optical spectroscopy of these Landau levels (Nature partner journals, Quantum Materials 2 26 (2017)), we demonstrate bulk signatures of the topological properties of charge carriers in these materials.