Can a solid - a material with a rigid, spatially ordered structure - also be a superfluid that flows with zero viscosity? This was the question posed by theoretical physicist Anthony Leggett in 1970. A first observation of such "supersolid" behavior in helium-4, made in 2004, later turned out to be an experimental artifact. Today, with the exception of ultracold atomic gas simulations, such a state in solids remains elusive.   

Triangular lattice antiferromagnets are highly frustrated quantum spin systems. This property makes them promising hosts for exotic quantum spin states, including the quantum magnetic analog of the long-sought supersolid state, the spin supersolid.

In a new study published on January 10 in Nature, a team led by Prof. SU Gang from the University of Chinese Academy of Sciences (UCAS), in collaboration with Prof. SUN Peijie from the Institute of Physics of CAS, Prof. LI Wei from the Institute of Theoretical Physics of CAS, and Prof. JIN Wentao from Beihang University, has reported the signatures of the spin supersolid state in the triangular lattice magnet Na₂BaCo(PO₄)₂ (NBCP).

In addition, they also discover a giant magnetocaloric effect (MCE) in the spin supersolid phase. This is a property that may pave the way for helium-free cooling to temperatures below 1 Kelvin with solid materials. 

The researchers measured the temperature variation resulting from a magnetic field change in an adiabatic demagnetization process for high-quality single crystals of NBCP. By mapping out the entropy landscape of the material, they revealed two low-temperature regions with pronounced spin fluctuations.

To provide microscopic evidence for the coexistence of solid and superfluid spin orderings in these regions, they also performed neutron diffraction measurements, and compared these results with theoretical calculations using the intensive density matrix renormalization group technique. This led them to conclude that, for the first time, they had found a spin supersolid state in a real quantum magnet.

In the adiabatic demagnetization cooling measurements, they observed a lowest temperature of 94 milli-kelvin near the spin-supersolid transition. Such a sharp temperature drop is caused by the strong spin fluctuations inherent to spin supersolid, and NBCP remains cooled throughout the spin-supersolid phase, in sharp contrast to conventional spin-ordered states. The unique properties of NBCP make it a very promising quantum material coolant for sub-Kelvin refrigeration.

Unlike conventional dilution refrigeration methods, spin-supersolid cooling does not require helium - an element that is in short supply worldwide - so it could have important applications in space detection and quantum technologies, among others.

This study was supported by the National Natural Science Foundation of China, the Strategic Priority Research Program of CAS, the Scientific Instrument Developing Program, and the Project for Young Scientists in Basic Research.

Illustrations of spin supersolid state and its cooling effect near the supersolid–liquid transition in a triangular-lattice magnet (Image by Institute of Physics)