U(1) Dirac quantum spin liquid candidate in triangular-lattice antiferromagnet CeMgAl11O19

Quantum spin liquid represents an intriguing state where electron spins are highly entangled yet spin fluctuation persists even at 0 K. Recently, the hexaaluminates RMgAl₁₁O₁₉ (R = rare earth) have been proposed to be a platform for realizing the quantum spin liquid state with dominant Ising anisotropic correlations. Here, we report detailed low-temperature magnetic susceptibility, muon spin relaxation, and thermodynamic studies on the CeMgAl₁₁O₁₉ single crystal. Ising anisotropy is revealed by magnetic susceptibility measurements. Muon spin relaxation and ac susceptibility measurements rule out any long-range magnetic ordering or spin freezing down to 50 mK despite the onset of spin correlations below ∼0.8 K. Instead, the spins keep fluctuating at a rate of 1.0(2) MHz at 50 mK. Specific heat results indicate a gapless excitation with a power-law dependence on temperature, C_m(T) ∝ T^α. The quasi-quadratic temperature dependence with α = 2.28(4) in zero field and linear temperature dependence in 0.25 T support the possible realization of the U(1) Dirac quantum spin liquid state.