Topological IV-VI semiconductor heterostructures: A platform to study topological phase transitions and strain sensitive topology – TOPO-PLATFORM

A multilevel approach is used that encompasses all aspects from sample growth, strain, structural characterization and analysis, theoretical modeling as well as device fabrication. Samples are fabricated by molecular beam epitaxy, and characterized by X-ray diffraction and electron microscopy. The electronic structure and topology of the bands are evaluated by infrared magneto-spectroscopy as well as by angle-resolved photoemission spectroscopy. The design of the samples and the analysis of the spectroscopic data are based on the k.p theory which is particularly effective in these systems. The rapid exchange of ideas, samples, measurement and modeling results creates an effective feedback-loop that ensures rapid progress of the project.

After 18 months (the first third of the project) our project has already achieved the following significant results:

- Know-how for the growth by molecular beam epitaxy of multiple quantum wells and Pb(Sn)Se/PbEuSe superlattices with high mobility under reproducible conditions, on different substrates and different orientations,

- The demonstration of the formation of topological minibands in artificial superlattice topological crystalline insulator/conventional insulator. We have revealed that these minibands emerge from the hybridization of topological interface states that couple both across topological quantum wells and conventional insulating barriers,

- The first experimental construction of the composition/thickness phase diagram of topological quantum wells. As a result, our topological superlattices provide a new quasi-three-dimensional topological state that offers new perspectives for obtaining and controlling dissipationless spin currents.

The perspectives are the observation of exotic topological phenomena which bring into play electron or spin currents without dissipation and show how to tune them by external means in order to pave the way for the realization of devices based on the topology in which the topological character can be turned ON and OFF in a controllable manner.

First fabrication and demonstration of a fully tunable, quasi-three-dimensional topological superlattice, which provides a model for the realization of dissipationless spin currents:
Phys. Rev. B 103, 235302 (2021)

Many other publications and communications are planned in the project.

Topological materials, exhibiting Dirac surface states locked in spin and momentum, are revolutionizing our current understanding of phases of matter. Topological crystalline insulators (TCI) are a particular class of such materials, where non-trivial topology is induced by band inversion and crystalline symmetries. As a result, their topological properties are highly sensitive against internal or external perturbations (composition, temperature, strain, magnetic field, quantum confinement…). Therefore, TCIs provide a unique platform for investigation of topological phase transitions and tuning of topology and hold great promise for unravelling novel physics and opening up new avenues for quantum electronic devices.

The central objective of this joint ANR-FWF project is to answer the question of how the topological surface and interface states can be tuned and controlled by external means. To achieve these goals, we will establish a powerful platform for realization of novel topologically engineered TCI heterostructures based on the narrow gap IV-VI semiconductors and show how to combine strain and band structure engineering, hybridization of topological interfaces states, as well as electric and magnetic fields and piezo-voltages to manipulate the electronic wave functions and the topology of the systems. The new methodologies developed will allow us to demonstrate control of the topology using different knobs.

A multilevel approach will be employed that brings together the complementary expertise of two project partners from Ecole Normale Supérieure in Paris (ENS) and the Johannes Kepler University of Linz (JKU). This expertise covers all the relevant aspects for the project: growth, doping, band gap and strain engineering, scanning tunneling microscopy (STM), photoemission spectroscopy (ARPES), theory, quantum trans¬port, mag¬neto-optics and processing of prototypical devices.

Molecular beam epitaxy of PbSnTe and PbSnSe based TCI heterostructures and their characterization will be performed in the group of G. Springholz at JKU. Compensative doping, growth on lattice-matched substrates and dielectric gating will be used to optimize the carrier density and mobility. Particular focus will be on magneto-optical and transport investigations to fill the gap between pure surface studies (ARPES, STM) and possible TCI device applications. Magnetooptical spectroscopy and transport will be performed in the group of L.A. de Vaulchier, R.Ferreira and Y. Guldner at ENS to probe and modelize the topological surface and bulk states, and to quantify the evolution of the band structure across the topological phase transition. This will be complemented by ARPES and STM measurements performed by the JKU team. Different methods of symmetry breaking will be investigated. In particular, we will study the impact of strain and lattice deformations on the TCI band topology and its tuning by piezo elements as a proof-of-concept for functional devices. Symmetry breaking by magnetic doping and ferroelectricity induced by incorporation of Ge in PbSn(Se,Te) will be studied as well. One of the goals of this work will be to demonstrate the quantum spin Hall effect and the quantum anomalous Hall effect in TCI materials.

Overall, the project will establish TCI materials as a platform to study and manipulate band topology for possible integration in future quantum electronic devices.