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Unusual state of matter holds promise for transformative quantum technologies

(Nanowerk News) 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 in Nature Communications ("Gapless spin liquid in a square-kagome lattice antiferromagnet").
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 lack of ordered magnetic spin alignment in a solid structure 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 in experiments at the J-PARC facility in Japan. Importantly, the expected spin ordering or freezing was not detected in the inelastic neutron scattering spectra.
Instruments scientists Dr Richard Mole and Dr Dehong Yu used inelastic neutron scattering, a spectroscopic technique to detect the vibrations of atoms, on the Pelican instrument at ANSTO. Instrument scientist Dr Shinichiro Yano liaised with the group in Japan and also collaborated on the experiments.
“When we analysed the Pelican data at 25K 15 K and 48mK, we could see the same spin excitations and they persisted to the lowest temperature, which is only slightly above absolute zero,” said Mole.
In order to create these low-temperature environments for the sample, a technician at the Australian Centre for Neutron Scattering Gene Davidson used a dilution insert of liquid helium in a cryostat on the instrument.
“Although this sample environment has been used on other instruments, this was the first application on Pelican. It is very important in experiments relating to quantum phenomena,” said Mole.
“The spin excitations which we observed in the spectra provide one of the signatures for quantum spin liquid state, a novel phase of matter,” explained Yu.
“Quantum spin liquid state possesses extensive many-body entanglement, a kind of correlation or 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.”
“However, for a real piece of solid material at spin liquid state, all atoms in the material are completely frozen except the spin of the electrons.”
The investigators reported the spin liquid as ‘gapless’, an unusual characteristic that refers to a lack of discrete energy changes in the local environment.
The crystal structure of the new antiferromagnetic compound, KCu6AlBiO4(SO4)5Cl, that was synthesised by the group in Japan, has a geometry described as a square kagome lattice.
Unlike the regular kagome lattice, the square kagome lattice is comprised of two sub-lattices that are not equivalent. Each square kagome unit has three types of nearest neighbour magnetic interaction from the spins of copper atoms in the structure.
Other experimental studies, which included magnetic susceptibility, magnetisation, heat capacity, and muon spin relation measurements, also supported the presence of a quantum spin liquid in the material.
Source: Australian Nuclear Science and Technology Organisation (ANSTO)
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