Kavli Affiliate: Gang Su
| First 5 Authors: Yuan Gao, Chuandi Zhang, Junsen Xiang, Dehong Yu, Xingye Lu
| Summary:
Supersolid is an exotic quantum state of matter that spontaneously hosts the
features of both solid and superfluid, which breaks the translation and U(1)
gauge symmetries. Here we study the spin dynamics in the triangular-lattice
compound Na$_2$BaCo(PO$_4$)$_2$, which is revealed in [Xiang et al., Nature
625, 270-275 (2024)] as a quantum magnetic analog of supersolid. We simulate
the easy-axis Heisenberg model with tensor network approach and uncover unique
dynamic traits. These features are manifested in two branches of excitations
that can be associated with the spin solidity and superfluidity, respectively.
One branch contains the U(1) Goldstone and roton modes, while the other
comprises pseudo-Goldstone and roton modes. The gapless Goldstone modes of the
in-plane superfluid order are confirmed by our inelastic neutron scattering
measurements. Together with the evident out-of-plane solid order indicated by
the magnetic Bragg peaks, our findings provide spectroscopic evidence for spin
supersolidity in this easy-axis antiferromagnet. Akin to the role of
phonon-roton modes — Landau elementary excitations — in shaping the helium
superfluid thermodynamics, the intriguing double magnon-roton dispersion here
determines the low-temperature thermodynamics of spin supersolid down to
sub-Kelvin regime, explaining the recently observed giant magnetocaloric effect
in Na$_2$BaCo(PO$_4$)$_2$.
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