Quantum Griffiths singularity in the stoichiometric heavy-fermion system CeRh4Al15

We present a detailed investigation of the stoichiometric ${\mathrm{CeRh}}_{4}{\mathrm{Al}}_{15}$ single-crystal compound using the temperature dependence of the heat capacity ${C}_{\text{P}}(T$), electrical resistivity $\ensuremath{\rho}(T$), magnetic susceptibility $\ensuremath{\chi}(T$), and magnetization $M(H$) measurements for a magnetic field $H$ applied in the basal plane and along the $c$ axis. The low-temperature power-law behavior of $C/T\ensuremath{\propto}\ensuremath{\chi}\ensuremath{\propto}{T}^{\ensuremath{-}1+\ensuremath{\alpha}}$, the scaling behaviors of field-dependent magnetization at different temperatures up to 50 K and temperature-dependent heat capacity at various fields up to 9 T, and the $T$-linear resistivity are found to be consistent with the formation of quantum Griffiths singularities in the non-Fermi-liquid (NFL) regime. We further investigate the spin dynamics of a polycrystalline sample of ${\mathrm{CeRh}}_{4}{\mathrm{Al}}_{15}$, using zero-field (ZF) and longitudinal-field (LF) muon spin relaxation ($\ensuremath{\mu}\mathrm{SR}$) measurements. ZF-$\ensuremath{\mu}\mathrm{SR}$ measurements do not reveal any sign of long-range magnetic ordering down to 70 mK. The electronic relaxation rate $\ensuremath{\lambda}$ below 0.5 K increases rapidly and shows a thermal-activation-like characteristic $[T\mathrm{log}(\ensuremath{\lambda})\ensuremath{\sim}\phantom{\rule{4pt}{0ex}}T]$ over the entire measured temperature range between 70 mK and 4 K, indicating the presence of low-energy spin fluctuations in ${\mathrm{CeRh}}_{4}{\mathrm{Al}}_{15}$. LF-$\ensuremath{\mu}\mathrm{SR}$ measurements show a time-field ($t/{H}^{\ensuremath{\eta}}$) scaling of the $\ensuremath{\mu}\mathrm{SR}$ asymmetry, indicating the quantum critical behavior of this compound. Furthermore, an inelastic neutron scattering study of the polycrystalline sample reveals two crystal field excitations near 19 and 33 meV. These features collectively provide strong evidence of NFL behavior in ${\mathrm{CeRh}}_{4}{\mathrm{Al}}_{15}$ due to the formation of the Griffiths phase close to a $T\phantom{\rule{4pt}{0ex}}\ensuremath{\rightarrow}$ 0 K quantum critical point.