Possible gapless quantum spin liquid behavior in the triangular-lattice Ising antiferromagnet PrMgAl$_{11}$O$_{19}$

TL;DR

This study presents evidence for a gapless quantum spin liquid state in the Ising antiferromagnet PrMgAl$_{11}$O$_{19}$ on a triangular lattice, characterized by persistent spin fluctuations and a continuum of magnetic excitations.

Contribution

It provides the first experimental evidence of a gapless QSL in an Ising-type magnet on a triangular lattice, expanding the understanding of QSL states beyond Heisenberg models.

Findings

No magnetic order or spin freezing down to 1.8 K.

Observation of a gapless broad continuum in magnetic excitations.

Magnetic specific heat shows a quasi-quadratic temperature dependence.

Abstract

Quantum spin liquids (QSLs) represent a novel state where spins are highly entangled but do not order even at zero temperature due to strong quantum fluctuations. Such a state is mostly studied in Heisenberg models defined on geometrically frustrated lattices. Here, we turn to a new triangular-lattice antiferromagnet PrMgAl$_{11}$O$_{19}$, in which the interactions are believed to be of Ising type. Magnetic susceptibility measured with an external field along the $c$ axis is two orders of magnitude larger than that with a field in the $ab$ plane, displaying an ideal easy-axis behavior. Meanwhile, there is no magnetic phase transition or spin freezing observed down to 1.8 K. Ultralow-temperature specific heat measured down to 50 mK does not capture any phase transition either, but a hump at 4.5 K, below which the magnetic specific heat exhibits a quasi-quadratic temperature dependence that is consistent with a Dirac QSL state. Inelastic neutron scattering technique is also employed to elucidate the nature of its ground state. In the magnetic excitation spectra, there is a gapless broad continuum at the base temperature 55~mK, in favor of the realization of a gapless QSL. Our results provide a scarce example for the QSL behaviors observed in an Ising-type magnet, which can serve as a promising platform for future research on QSL physics based on an Ising model.