Triangular lattice antiferromagnets are highly frustrated quantum spin systems. This feature makes them promising hosts for exotic quantum spin states, including the quantum magnetic analogue of the long-sought supersoild state, namely, spin supersolid. In a new study published on January 10 in Nature, Prof. LI Wei from Institute of Theoretical Physics, Chinese Academy of Sciences (CAS), in collaboration with Prof. SU Gang from University of CAS, Prof. SUN Peijie from Institute of Physics, CAS, and Prof. JIN Wentao from Beihang University, has reported the discovery of spin supersolid state in triangular lattice magnet Na₂BaCo(PO₄)₂ (NBCP). Additionally, they also reveal a giant magnetocaloric effect (MCE) near the spin supersolid transition. This finding paves a vaible way for helium-free sub-Kelvin cooling with frustrated quanutm magnets.  

With high-quality single crystals of NBCP, the researchers measured the temperature variation upon changing magnetic field in an adiabatic demagnetization process. By mapping out the material’s entropy landscape, they revealed two low-temperature regimes with pronounced spin fluctuations. To provide microscopic evidence for the coexistence of solid and superfluid spin orderings in these regions, they also conducted neutron diffraction measurements, and compared these results with theoretical calculations obtained with the intensive density matrix renormalization group technique. This led them to conclude that a spin supersolid state has been identified in a real-world quantum magnet for the first time.

In the adiabatic demagnetization cooling measurements, they observe a lowest temperature of 94 milli-kelvin near the spin-supersolid transition. Such a sharp temperature decrease is caused by the strong spin fluctuations inherent to spin supersolid, and NBCP remains cooled within the whole spin-supersolid phase, in sharp contrast to conventional spin-ordered states. The unique features of NBCP make it a very promising quantum material coolant for sub-Kelvin refrigeration. Unlike conventional dilution refrigeration methods, here the spin supersolid cooling does not require helium — an element that is in short supply globally — so it may find important applications in space detections and quantum technologies, among others.

This study entitled “Giant magnetocaloric effect in spin supersolid candidate Na2BaCo(PO4)2”, published on Nature 625, 270–275 (2024), was supported by the National Natural Science Foundation of China, the Chinese Academy of Sciences through the Strategic Priority Research Program, Scientific Instrument Developing Program, and the Project for Young Scientists in Basic Research.

Fig.1 Illustrations of spin supersolid state (left) and its cooling effect near the spin supersolid transition (right) in a triangular-lattice magnet (insets).

Link:

https://www.nature.com/articles/s41586-023-06885-w

Contact:

Wei Li

Email: w.li@itp.ac.cn

Institute of Theoretical Physics, CAS