Spin-liquid signatures in the quantum critical regime of pressurized CePdAl

CePdAl is a prototypical frustrated Kondo lattice with partial long-range order (LRO) at $T_{\mathrm{N}}=2.7$ K. Previous bulk experiments under hydrostatic pressure found signatures for a quantum critical regime that extends from $p_{\mathrm{c}}\approx 0.9$ GPa, where LRO disappears, up to $\sim 1.7$GPa\mathrm{.}Weemployedextensivemuonspinrelaxationandrotation($\mu$SR) experiments under pressure. The continuous and complete suppression of LRO at $p_{\mathrm{c}}$ is confirmed. Above $T_{\mathrm{N}}(p)$ and beyond $p_{\mathrm{c}}$, an additional crossover scale $T^{\ast }(p)$ characterizes the change from pure to stretched exponential relaxation in zero field. Remarkably $T^{\ast }(p)$ agrees with previously determined signatures of entropy accumulation above LRO. This coincidence microscopically evidences fluctuating frustrated spins at $T\leq T^\ast$ with spin-liquid behavior. Power-law divergences of the temperature and longitudinal field dependences of the relaxation rate, with time-field scaling, at pressures between $p_c$ and 1.7 GPa characterize this regime as quantum critical.