Intermediate valence and spin fluctuations near a quantum critical point in CeRu2−xCoxGe2

We show the presence of a quantum critical point in $\mathrm{Ce}{\mathrm{Ru}}_{2\text{\ensuremath{-}}x}{\mathrm{Co}}_{x}{\mathrm{Ge}}_{2}$ $(0\ensuremath{\le}x\ensuremath{\le}2)$ using detailed heat capacity and muon spin relaxation $(\ensuremath{\mu}\mathrm{SR})$ measurements. The millikelvin heat capacity and $\ensuremath{\mu}\mathrm{SR}$ results for the critical composition ${x}_{\mathrm{C}}\ensuremath{\sim}1.5$ are consistent with the spin fluctuation model of the quantum critical point. The time-field-temperature scaling of the heat capacity and $\ensuremath{\mu}\mathrm{SR}$ data further suggests the role of critical and cooperative spin fluctuations near the antiferromagnetic quantum critical point. In addition, the temperature-dependent x-ray absorption near edge structure reveals intermediate valence fluctuation down to the lowest temperature for the compositions $x\ensuremath{\ge}1.5$. The noninteger valence for the critical composition ${x}_{\mathrm{C}}\ensuremath{\sim}1.5$ is further demonstrated by room temperature x-ray photoelectron spectroscopy. Interestingly, the quantum criticality in $\mathrm{Ce}{\mathrm{Ru}}_{2\text{\ensuremath{-}}x}{\mathrm{Co}}_{x}{\mathrm{Ge}}_{2}$ is governed by the survival of valence and spin fluctuations down to the lowest temperature.