INDUSTRY NEWS
BREAKING NEWS Unusual State of Matter in New Material Holds Promise for Transformative Quantum Technologies
Spin-Gapless Semiconductors Review: More Candidates for Next-Generation Low Energy and High Efficient Spintronics
ANSTO has provided supporting experimental evidence of a highly unusual quantum state, a quantum spin liquid (QSL), in a two-dimensional material as reported by an international collaboration led by Tokyo University of Science. Materials with quantum spin liquid states could be used in the development of spintronic devices, quantum computers and other transformative quantum technologies. In a quantum spin liquid, an elusive state of matter that is the subject of much investigation worldwide, the electron spins, in a magnetic material, never align, but continue to fluctuate even at the lowest temperatures. This phenomena has been described as a fluctuating liquid-like state. Low energy spin excitations, evidence of a QSL, were detected at a range of very low temperatures during experiments in Japan. Importantly, the expected spin ordering or freezing was not detected in the inelastic neutron scattering spectra. Scientists Dr Richard Mole and Dr Dehong Yu used inelastic neutron scattering, a spectroscopic technique to detect the vibrations of atoms. “When we analysed the Pelican data at 25K, 15K and 48mK, we could see the same spin excitations and they persisted to the lowest temperature, which is only slightly above absolute zero,” Dr Mole said. In order to create these low-temperature environments, a special type of cryostat, called a dilution insert, was optimised on the Pelican instrument. “A quantum spin liquid state possesses extensive many-body entanglements, a kind of correlation, or a link between all the spins. As an analogy, think of a bucket of water with several fishing floats on the surface. If you disturb one float, all the floats will also be disturbed,” Dr Yu explained.
The University of Wollongong recently published an extensive review of spin-gapless semiconductors (SGSs). SGSs are a new class of zero gap materials that have fully spin polarised electrons and holes. The study enhances the search for materials that would allow for ultra-fast, ultra-low energy spintronic electronics, with no wasted dissipation of energy from electrical conduction. The defining property of SGS materials relates to their ‘bandgap’ – the gap between the material’s valence and conduction bands – which defines their electronic properties. In general, one spin channel is semiconducting with a finite band gap, while the other spin channel has a closed (zero) band gap. The band structures of the SGSs can have two types of energymomentum dispersions: Dirac (linear) dispersion or parabolic dispersion. The new review investigates both Dirac and the three sub-types of parabolic SGSs in different material systems. For Dirac type SGS, the electron mobility is two to four orders of magnitude higher than in classical semiconductors. Very little energy is needed to excite electrons in an SGS, charge concentrations are very easily ‘tuneable’. The Dirac type spin gapless semiconductors exhibit fully spin polarised Dirac cones, and offer a platform for spintronics and low-energy consumption electronics through dissipation-less edge states, driven by the quantum anomalous Hall effect. In a spin-gapless semiconductor, conduction and valence band edges touch in one spin channel, and no threshold energy is required to move electrons from occupied (valence) states to empty (conduction) states. This property gives these materials unique properties, as their band structures are extremely sensitive to external influences such as pressure or magnetic field. Most SGS materials are all ferromagnetic materials with high Curie temperatures.
Inelastic neutron scattering data of KCu6AlBiO4(SO4)5Cl. Pelican measurements plotted at h. Image courtesy of ANSTO.
46 | SEPTEMBER 2020
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LEFT: The band structures of parabolic and Dirac type SGS materials with spin-orbital coupling, which leads to the quantum anomalous Hall effect. ABOVE: FLEET Chief Investigator Professor Xiaolin Wang
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