Overcoming magnetic disorder in quantum insulators

Overcoming magnetic disorder is key to exploiting the unique properties of QAH insulators.

A Monash-led team has demonstrated that the breakdown in topological protection is caused by magnetic disorder, explaining previous observations that topological protection could be restored by application of stabilising magnetic fields.

“The study paves a clear research pathway towards use of MTIs in low-energy topological electronics,” said lead author FLEET PhD candidate Qile Li (Monash University).

The challenge

When combined, magnetism and topology can yield the quantum anomalous Hall effect (QAHE) allowing for electrical currents to flow without resistance along one-dimensional edges across macroscopic distances.

Yet the current flow along these topologically protected, one-dimensional edges has proven to be far from robust, with the QAHE breaking down in magnetically doped topological insulators at temperatures higher than 1 kelvin, well below the temperatures predicted by theory.

A new class of materials, known as intrinsic magnetic topological insulators (MTIs), for example MnBi₂Te₄, possess both non-trivial topology and intrinsic magnetism and are predicted to offer more robust QAHE at higher temperatures than magnetically doped topological insulators.

In MnBi₂Te₄ it has been shown that the QAHE can survive up to 1.4 K, and interestingly, this can rise to 6.5 K with the application of stabilising magnetic fields, providing hints at the mechanisms which are driving the breakdown of topological protection.

However, 6.5 K is still well below the 25 K that is predicted by theory. To advance these materials towards potential applications it is necessary to raise that temperature to the hardline limit set by the magnetic band gap energy and magnetic transition temperature. And this requires a better understanding of the precise mechanisms involved in the breakdown of topological protection at the material surface.

Image caption: Conductance map taken with a scanning tunnelling microscope showing the gapless edge state and its coupling to metallic bulk states. Image credit: Monash University.

Studying interplay between surface disorder, band gap fluctuation and edge state

To fully understand what was happening the Monash-led team used direct, atomically precise measurement of the interplay between surface disorder, local fluctuations in the band gap energy, and chiral edge state.

The team used low-temperature scanning tunnelling microscopy and spectroscopy (STM/STS) to study five-layer, ultra-thin film MnBi₂Te₄.

How the band gap fluctuates was studied at the location of crystal defects, as well as at the edge and interior of the five-layer film — to understand what might cause the breakdown of QAHE.

The team also applied low magnetic fields, observing the band gap and QAHE could be restored.

The applied magnetic fields are well below the spin-flop transition for MnBi₂Te₄.

Results in five-layer MnBi₂Te₄ reveal a magnetic villain

The research team found long-range fluctuations in band gap energy in the interior of the film, ranging between 0 (gapless) and 70 MeV, and not correlated to individual surface defects.

Directly observing the breakdown of topological protection shows that the gapless edge state, the hallmark signature of a QAH insulator, hybridises with extended gapless regions in the bulk.

These results demonstrate that:

The gapless edge state in MnBi₂Te₄ is directly coupled to extended percolating bulk metallic regions arising from band gap fluctuations caused by magnetic surface disorder.

Band gap fluctuations can be greatly reduced by applying a magnetic field, increasing the average exchange gap to 44 MeV, close to predicted values.

Image caption: Demonstrating how the exchange gap spatial fluctuation caused by surface magnetic disorder can be reduced significantly by applying a perpendicular magnetic field. Image credit: Monash University.

“These results provide insight on the mechanism of topological breakdown and how it can be restored in a magnetic field,” said corresponding author FLEET Associate Investigator Dr Mark Edmonds (also at Monash).

The research findings have been published in Advanced Materials.

Top image credit: iStock.com/Greyfebruary