Sep 16, 2020  
Reviewing the quantum anomalous Hall effect(Nanowerk News) A collaboration across three FLEET nodes has reviewed the fundamental theories underpinning the quantum anomalous Hall effect (Small, "Quantum Anomalous Hall Effect in Magnetic Doped Topological Insulators and Ferromagnetic SpinGapless Semiconductors – A Perspective Review"). 

The quantum anomalous Hall effect (QAHE) is one of the most fascinating and important recent discoveries in condensedmatter physics.  
It is key to the function of emerging ‘quantum’ materials, which offer potential for ultralow energy electronics.  
QAHE causes the flow of zeroresistance electrical current along the edges of a material.  
QAHE in topological materials: key to lowenergy electronics 

Topological insulators, recognised by the Nobel Prize in Physics in 2016, are based on a quantum effect known as the quantum anomalous Hall effect.  
“Topological insulators conduct electricity only along their edges, where oneway ‘edge paths’ conducts electrons without the scattering that causes dissipation and heat in conventional materials,” explains lead author Muhammad Nadeem.  
QAHE was first proposed by 2016 Nobelrecipient Prof Duncan Haldane (Manchester) in the 1980s, but it subsequently proved challenging to realize QAHE in real materials. Magneticdoped topological insulators and spingapless semiconductors are the two best candidates for QAHE.  
The quantum Hall effect (QHE) is a quantummechanical version of the Hall effect, in which a small voltage difference is created perpendicular to a current flow by an applied magnetic field.  
The quantum Hall effect is observed in 2D systems at low temperatures within very strong magnetic fields, in which the Hall resistance undergoes quantum transitions — ie, it varies in discrete steps rather than smoothly.  
QAHE describes an ‘unexpected’ (ie, ‘anomalous’) quantisation of the transverse ‘Hall’ resistance, accompanied by a considerable drop in longitudinal resistance.  
QAHE is referred to as ‘anomalous’ because it occurs in the absence of any applied magnetic field, with the driving force instead provided by either a) spinorbit coupling or b) intrinsic magnetization.  
Researchers seek to enhance these two driving factors in order to strengthen QAHE, allowing for topological electronics that would be viable for roomtemperature operation.  
It’s an area of great interest for technologists,” explains Xiaolin Wang. “They are interested in using this significant reduction in resistance to significantly reduce the power consumption in electronic devices.”  
“We hope this study will shed light on the fundamental theoretical perspectives of quantum anomalous Hall materials,” says coauthor Prof Michael Fuhrer (Monash University), who is Director of FLEET.  
The study 

The collaborative, theoretical study concentrates on these two mechanisms: 1)large spinorbit coupling (interaction between electrons’ movement and their spin); and 2) strong intrinsic magnetization (ferromagnetism).  
Four models were reviewed that could enhance these two effects, and thus enhance QAHE, allowing topological insulators and spin fullypolarized zerogap materials (spin gapless semiconductors) to function at higher temperatures.  
“Among the various candidate materials for QAHE, spingapless semiconductors could be of potential interest for future topological electronics/spintronics applications”, explains Muhammad Nadeem. 
Source: ARC Centre of Excellence in Future LowEnergy Electronics Technologies (FLEET)  
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