  In the search of quantum spin liquid (QSLs), spin-1/2 triangular-lattice
Heisenberg antiferromagnets (TLHAFs) have always been viewed as fertile soils.
Despite the true magnetically-ordered ground state, anisotropy has been
considered to play a significant role in stabilizing a QSL state. However, the
nature and ground state of the most anisotropic case, the triangular-lattice
Ising antiferromagnet (TLIAF), remains elusive and controversial. Here, we
report specific heat and thermal conductivity measurements on a
newly-discovered Ising-type QSL candidate PrMgAl11O19. At zero field, the
magnetic specific heat shows a quadratic temperature dependence. On the
contrary, no direct positive magnetic contribution to thermal conductivity was
detected, ruling out the presence of mobile gapless fermionic excitations.
Further analysis of phonon thermal conductivity reveals that the phonons are
strongly scattered by thermally-activated magnetic excitations out of a gap,
which exhibits a linear dependence with magnetic field. These results
demonstrate that the spin-1/2 TLIAF PrMgAl11O19 has a gapped Z2 QSL ground
state.
