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York physicists uncover key property of elusive quantum state in two-dimensional magnets

Posted on 19 September 2018

Dr Ferreira's group discovered a mechanism by which the quantum nature of electrons manifests itself in atomically-thin artificial magnets, an important theoretical step that brings two-dimensional materials closer to applications in energy-efficient spin devices and quantum computing.

The findings have been published in Physical Review Letters

Materials that are only a few atoms thick fall in the class of two-dimensional materials, which encompasses a large variety of compounds, including graphene, insulators, and even superconductors. Atomically thin crystals can be stacked together to create artificial materials by design; a new frontier in interdisciplinary materials research propelled by the discovery of one-atom-thick graphene in 2004.

Scientists believe that artificial structures built from nano-magnets and graphene can host a quantum magnetic current, known as a “spin current”, which flows around the sample edges without any resistance.  Usually, one thinks of quantum mechanics as a theory of the ultra-small worldhowever certain materials behave quantum-mechanically as a whole, which could be used to build energy-efficient quantum computers. 

Quantum computers employ qubits - logic bits that can be zero (“OFF”), one (“ON”) or an arbitrary superposition of the two states (“ON & OFF”) so as to explore a huge number of possibilities simultaneously. Qubits would enable unprecedented speedup in certain computational tasks, but their sensitivity to environmental fluctuations renders their practical use extremely challenging. To overcome this issue, scientists have devised  “topological materials” capable of sheltering qubits from environmental noise.

Topological materials behave like rubber rings when stretched. Their fundamental properties resist high levels of disturbance. This topological protection manifests itself as dissipationless edge currents (see image). In other words, the electrons' motion along tiny strips bordering a topological sample cannot be easily stopped.

This exact quantisation of the Hall conductivity is used in high-accuracy resistance calibrations worldwide.

The research carried out at York's Condensed Matter Physics Institute has established a key signature in the operation of atomically-thin electronic devices. Dr Ferreira said: 

Metamaterials built from graphene and recently discovered atomically-thin ferromagnets could harbour electronic currents enjoying topological protection, a quantum effect so far only observed in topological insulators at very low temperaturesThe special phenomenon is called quantum anomalous Hall effec(QAHE)”. 

Since its theoretical proposal in 1988, the QAHhas exerted fascination as a unique macroscopic manifestation of the mind-bending quantum world. The hallmark of the QAHE is the quantisation of the Hall conductivity roughly speaking the amount of electrons moving towards the edgeof a current-carrying sample, which attains an integer value in terms of fundamental constants of nature, the electron charge and Planck constant (see Figure)This exact quantisation of the Hall conductivity is used in high-accuracy resistance calibrations worldwide.

Dr Ferreira added: “The observation of the QAHin a two-dimensional material requires samples with a well-established insulating energy gap in the bulkUltra-clean devices tuned into the topological regime are challenging to fabricate and thus only conventional charge transport has been reported so far. Surprisingly, our calculations predict that the echo of topologically nontrivial energy bands can be detected in dirty samples with a small energy gap. The key is a change of polarity in the Hall resistance of the device as it approaches the insulating regime. This effect foreruns the much sought-after topological insulating phase in a two-dimensional material, and thus can provide a guideline for future experiments.”

The calculations by Manuel Offidani, a PhD student in Dr Ferreira’s team, identified a key experimental signature. Manuel Offidani explains;

”We realised that in magnetised graphene-like systems, electrons have their tiny magnetic needle, called spin, pointing towards different directions depending on their energy. In particular, when lowering the energy close enough to the insulating gapthe electrons’ spin is reversed compared to its high-energy orientation.” 

According to their theory, the special magnetic texture of electrons in a two-dimensional ferromagnet could be detected as a reversal of the Hall conductivitywhen the density of charge carriers is lowered.

“The predicted phenomenon, allowing easy control over the electrons’ spin orientation, has implications for applications, beyond the fundamental aspect related to the anomalous Hall effect”.     

The York Condensed Matter Physics group’s discovery could serve as a guidance for making low-power consumption devices out of atomically thin ferromagnets.

The research was funded by the Royal Society and the Engineering and Physical Sciences Research Council (EPSRC).