May 12, 2022 
Quantum oneway street in topological insulator nanowires
(Nanowerk News) An international group of scientists have demonstrated that wires more than 100 times thinner than a human hair can act like a quantum oneway street for electrons when made of a peculiar material known as a topological insulator.

The discovery opens the pathway for new technological applications of devices made from topological insulators and demonstrates a significant step on the road to achieving socalled topological qubits, which it has been predicted can robustly encode information for a quantum computer.

To achieve this result, experimental physicists at the University of Cologne in the group of Professor Dr Yoichi Ando closely collaborated with the groups of Professor Dr Jelena Klinovaja and Professor Dr Daniel Loss at the University of Basel. Ando is also speaker of the Cluster of Excellence Matter and Light for Quantum Computing (ML4Q).

The study has now been published in Nature Nanotechnology ("Giant magnetochiral anisotropy from quantum confined surface states of topological insulator nanowires").

Topological insulators are materials in which a combination of quantum mechanics and the mathematical concept of topology – a combination that won the Nobel Prize in 2016 – produce conductive surfaces and insulating interiors. Topological insulators are highly promising candidates for future technologies and as potential platforms for quantum computing.

The researchers showed that, under the right circumstances, electric currents can flow more easily in one direction compared to the other, a process known as rectification. Rectification is the basis of most wireless technologies and so has a wide range of applications.

However, unlike the rectifiers found in, say, smartphones, the current rectification effect discovered in topological insulator nanowires arises as a result of quantum mechanics and is extremely controllable. Normally, quantum mechanical rectification effects are tiny. However, the unique degree of control made possible by modern device fabrication techniques resulted in a gigantic quantum rectification effect in topological insulator nanowires, far bigger than any previously discovered.

Usually, quantum rectification effects arise as a result of something known as spinorbit coupling, which is actually a mix of quantum mechanics and Einstein’s theory of relativity.

‘That strange mix normally results in tiny rectification effects. What is great about the topological insulator nanowires is that we can artificially produce essentially the same physics but with a much larger magnitude. This leads to a rectification effect that is really huge compared to other materials. It is also one of the aspects that makes topological insulators so exciting for applications in quantum computing,’ said Dr Henry Legg, Georg H. Endress postdoctoral fellow at the University of Basel and first author of the paper.

Ohm’s law states that the current flowing through a device is governed by the voltage drop across it and a quantity known as resistance. However, when quantum mechanics is at play, Ohm’s law sometimes needs to be corrected.

In particular, if a material or a device does not look the same when all its spatial properties are mirrored – socalled broken spatial inversion symmetry – applying a magnetic field means the quantum version of Ohm’s law allows current to flow more easily in one direction compared to the other. The size of the current rectification is determined by the difference between the resistances in each direction.

The high degree of control possible in topological insulator devices allowed the team of researchers to achieve a truly gigantic rectification effect compared to what had previously been observed.

‘It was very exciting when we saw what the theory had predicted. The rectification effect is highly controllable with small changes of gate voltage, and that could have quite promising technological implications,’ said Matthias Rößler, final year doctoral student in the group of Yoichi Ando and cofirst author of the paper. ‘It took a lot of late evenings in the lab, but it was worth it.’

Topological insulators have long been considered good candidates to serve as the basis for topological quantum computers, which promise unprecedented computing power. However, good control over topological insulator devices is essential to be able to produce topological qubits.

‘Our study demonstrates yet another novel quantum phenomenon that is possible in topological insulator devices and highlights their potential technological applications. It is also another key piece of evidence that topological insulators are ideal platforms for hosting Majoranas. The discovery of Majoranas would be a real game changer for quantum computing,’ Yoichi Ando, Professor of Experimental Physics at the University of Cologne and speaker of the ML4Q Cluster of Excellence, remarked.
