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# Opportunities in topological insulator devices

## Abstract

Topological insulators (TIs) hold promise as a platform for unique quantum phenomena. However, realizing these phenomena experimentally requires sophisticated devices. In this Technical Review, we discuss four topics of particular interest for TI devices: topological superconductivity, quantum anomalous Hall insulators as a platform for exotic phenomena, spintronic functionalities and topological mesoscopic physics. We also discuss the status and technical challenges in fabricating TI devices to address new physics.

## Key points

• Interesting quantum phenomena deriving from the peculiar properties of topological insulators (TIs) can be observed in TI devices. Fabrication of such devices should take into account the special challenges these materials pose for fabrication.

• In proximity to a conventional superconductor, TIs can realize a topological superconducting state hosting Majorana zero modes, representing the main ingredient for topological quantum computing, in which TIs can potentially have an advantage over semiconductor platforms.

• By magnetically doping a TI, the quantum anomalous Hall effect can be observed if the Fermi level is tuned into the magnetic exchange gap and chiral edge states arise that are expected to turn into chiral Majorana edge states if superconductivity is induced by the proximity effect.

• The spin-momentum-locked surface states of a TI are potentially useful for spintronic applications due to their current-induced spin polarization that interacts with ferromagnetic electrodes.

• Quantum confinement in mesoscopic-sized TI nanowires leads to the formation of a peculiar Dirac subband structure, which can be modified by magnetic and electric fields to open extended topological phases within which Majorana zero modes are expected if proximitized by a superconductor.

• Fabricating devices based on TIs and interfacing them with ferromagnets or superconductors requires well-tuned processes in order to preserve and control the surface state properties.

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## Relevant articles

• ### Giant magnetochiral anisotropy from quantum-confined surface states of topological insulator nanowires

Nature Nanotechnology Open Access 12 May 2022

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## Acknowledgements

The authors thank J. Feng, M. Rössler, D. Fan, L. Dang, F. Münning, G. Lippertz and A. Taskin for providing device pictures, and Y. Tokura, M. Kawasaki, H. F. Legg, D. Grützmacher, P. Schüffelgen and F. Yang for useful discussions. This work has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement no. 741121) and was also funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) under CRC 1238-277146847 (subprojects A04 and B01) and AN 1004/4-1-398945897, as well as under Germany’s Excellence Strategy — Cluster of Excellence Matter and Light for Quantum Computing (ML4Q) EXC 2004/1 — 390534769.

## Author information

Authors

### Contributions

Y.A. conceived the article, with input from O.B. O.B. and Y.A. wrote the manuscript.

### Corresponding author

Correspondence to Yoichi Ando.

## Ethics declarations

### Competing interests

The authors declare no competing interests.

### Peer review information

Nature Reviews Physics thanks Jie Shen and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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## Glossary

Chiral Majorana fermions

Fermionic particles that are their own antiparticles, arising as a 1D gapless edge mode, as expected in a proximitized quantum anomalous Hall insulator.

Topological qubits

A quantum system incorporating Majorana zero modes that allows quantum computation based on non-abelian exchange statistics to enable ‘digital’-like gate operation, as opposed to ‘analogue’-like conventional qubits.

Majorana zero modes

(MZMs). Localized bound states having a self-conjugate property, which can be viewed as an emergent Majorana particle obeying non-abelian exchange statistics.

Superconducting proximity effect

The extension of Cooper pairing from a superconductor into a normal system that is in close proximity.

Shapiro step

A step in the current–voltage characteristics of a Josephson junction upon microwave irradiation, caused by the AC Josephson effect.

Rashba-type band splitting

Splitting of a spin-degenerate band into a pair of spin-non-degenerate bands having opposite helical spin polarization.

Zeeman energy gap

An energy gap that is induced by the action of a magnetic field on the spin magnetic moment.

London penetration depth

The characteristic length over which a magnetic field penetrates into a superconductor.

Fraunhofer pattern

The magnetic field dependence of the critical current of a Josephson junction obeying the same pattern as the Fraunhofer diffraction.

Hexagonal warping

Deviation from the ideal circular Fermi surface of the Dirac cone caused by third-order spin–orbit coupling.

Size quantization

Discretization of the quantum-mechanical eigenenergies due to spatial confinement.

Kramers degeneracy

The ever-present double degeneracy of eigenstates of a fermionic system that is time-reversal invariant.

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Breunig, O., Ando, Y. Opportunities in topological insulator devices. Nat Rev Phys 4, 184–193 (2022). https://doi.org/10.1038/s42254-021-00402-6

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• DOI: https://doi.org/10.1038/s42254-021-00402-6

• ### Giant magnetochiral anisotropy from quantum-confined surface states of topological insulator nanowires

• Henry F. Legg
• Matthias Rößler
• Yoichi Ando

Nature Nanotechnology (2022)