Kavli Affiliate: Gang Su
| First 5 Authors: Junsen Xiang, Chuandi Zhang, Yuan Gao, Wolfang Schmidt, Karin Schmalzl
| Summary:
Supersolid, an exotic quantum state of matter that consists of particles
forming an incompressible solid structure while simultaneously showing
superfluidity of zero viscosity [1], is one of the long-standing pursuits in
fundamental research [2, 3]. Although the initial report of $^4$He supersolid
turned out to be an artifact [4], this intriguing quantum matter has inspired
enthusiastic investigations into ultracold quantum gases [5-8]. Nevertheless,
the realization of supersolidity in condensed matter remains elusive. Here we
find evidence for a quantum magnetic analogue of supersolid — the spin
supersolid — in the recently synthesized triangular-lattice antiferromagnet
Na$_2$BaCo(PO$_4$)$_2$ [9]. Notably, a giant magnetocaloric effect related to
the spin supersolidity is observed in the demagnetization cooling process,
manifesting itself as two prominent valley-like regimes, with the lowest
temperature attaining below 100 mK. Not only is there an experimentally
determined series of critical fields but the demagnetization cooling profile
also shows excellent agreement with the theoretical simulations with an
easy-axis Heisenberg model. Neutron diffractions also successfully locate the
proposed spin supersolid phases by revealing the coexistence of
three-sublattice spin solid order and interlayer incommensurability indicative
of the spin superfluidity. Thus, our results indicate a strong entropic effect
of the spin supersolid phase in a frustrated quantum magnet and open up a
viable and promising avenue for applications in sub-Kelvin refrigeration,
especially in the context of persistent concerns about helium shortages [10,
11].
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