Missions

Magneto-transport study of topological insulators up to 70 T: toward Quantum Anomalous Hall effect in topological hetero-structures

The main property of topological materials is the existence of metallic interfaces between two topologically inequivalent systems or simply between a topological material and vacuum, i.e. metallic surfaces for 3D topological insulators and metallic edges for 2D topological phases. As their existence is guaranteed by the material's band structure, these states are said to be “topologically protected” in the sense that their existence is not directly affected by disorder or geometry as long as it does not fundamentally change the band structure. Moreover, due to their chirality these edge states are protected against backscattering, making them dissipation-less, for instance in the Quantum Anomalous Hall effect (QAH).

With the recent discovery of magnetic 3D topological insulators (3DTIs), new routes have opened for fundamental research on topological materials. In particular, heterostructures of magnetic and non-magnetic 3DTIs offer promising routes for high-temperature QAH systems, with possible applications in metrology. Indeed, in the Quantum Hall effect the transverse quantization in units of e2/h can reach an extraordinary precision better than 10-10, which now defines the primary standard of current in metrology. Still, any complex heterostructure design must rely on the precise control and understanding of material, electronic and topological properties of 3DTIs. In this project, we propose to characterize the electronic properties of gated devices of locally grown epitaxial 3D topological insulator thin films (BiSb, BiSbTe and BiSbTeSe) in view of further 3DTI heterostructuring.

Given the relatively-low mobilities of 3DTI thin films (generally below ~1000 cm2/V.s), Shubnikov-de-Haas oscillations (Landau level spectroscopy) should be visible in gated nanostructures of epitaxial 3DTI using pulsed magnetic fields up to 70 T, allowing for the characterization of the different electronic populations in the heterostructure (effective mass, chemical potential, band bending) including the topological surface states, which is extremely difficult or impossible at low magnetic field. The LNCMI pulsed field facility is one of only four laboratories worldwide where this can be measured. The ultimate goal being to combine well-controlled low-doped topological insulators into topological heterostructures that would display the QAH effect.

Activités

• fabricate gated nanostructures of thin films of different 3D topological insulators (BiSb, BiSbTe, BiSbSeTe) in collaboration with the LAAS
Khaled, M. A. et al., ACS Appl Electron Mater 6, 3771–3779 (2024); Abdelrahman, N. et al., J Mater Chem C Mater 12, 18416-18426 (2024)

• study the electronic properties of the trivial and topological carrier populations (carrier density, mobility, effective mass, band bending) through Landau level spectroscopy at very-high magnetic field up to 70 T at the LNCMI
Veyrat, L. et al., Nano Lett 15, 7503–7507 (2015)

• collaborate with theoreticians at CEMES to compare experiments with the band structure first-principles calculations based on the density functional theory (DFT).

Compétences

- PhD in condensed matter physics (mandatory)
- Experience in nanofabrication (preferred)
- Experience in electrical transport (preferred)
- Scientifc English written and spoken (B2 level preferred, publications/conferences/collaborators)

Use of the Toulouse facility will require electric certification H0B0 (training is taken care of by the laboratory)

Contexte de travail

The National Intense Magnetic Field Laboratory (LNCMI), located across two sites (Toulouse and Grenoble), is a research facility under the French National Centre for Scientific Research (CNRS) and a Very Large Research Infrastructure (TGIR). It is affiliated with INSA Toulouse, Université Grenoble Alpes, and Université Paul Sabatier (Toulouse).

The LNCMI enables researchers to conduct experiments in some of the world's most intense magnetic fields. The Grenoble site provides continuous magnetic fields up to 42 tesla, while the Toulouse site offers pulsed fields up to 88 tesla.

The postdoctoral position will be based within the "Quantum Nanostructures and Topological Matter" ("Nano") group at LNCMI Toulouse, which consists of five permanent scientists.

This project strengthens and develops a local collaboration between three labs in Toulouse: the LNCMI (transport measurements at very high fields), the CEMES (theory, DFT), and the LAAS (epitaxial growth and nanofabrication).

Contraintes et risques

The position is expected to start no later than July 2026, with an initial duration of 18 months and the possibility of a 6-month extension at the end. Some national and international travel (a few weeks per year) will be required to present results at conferences and attend thematic schools for training.