  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.
