Quantum critical point and intermediate valence fluctuations in CeRu2−xCoxGe2

Abstract

A detailed study of low-temperature properties across the series ${\mathrm{CeRu}}_{2\ensuremath{-}x}{\mathrm{Co}}_{x}{\mathrm{Ge}}_{2}$ ($0\ensuremath{\le}x\ensuremath{\le}2$) using magnetic susceptibility \ensuremath{\chi}($T$), isothermal magnetization $M$($H$), heat capacity $C$($T$), and electrical resistivity \ensuremath{\rho}($T$) is presented. Using doping as a tuning parameter, a crossover from a Ruderman-Kittel-Kasuya-Yosida (RKKY) dominated region ($0\ensuremath{\le}x\ensuremath{\le}1$) to a Kondo dominated region ($x\ensuremath{\ge}1.5$) is evident. \ensuremath{\chi}($T$) and \ensuremath{\rho}($T$) curves, analyzed in terms of theoretical models proposed by Sales and Freimuth for the Co ($x\ensuremath{\ge}1.5$) rich compounds, suggest an intermediate valence state of the Ce ion. The intricate balance between the competing RKKY and Kondo effect is attributed to the volume change upon Co substitution, owing to its smaller ionic size, which enhances the hybridization between $4f$ and conduction electrons. A quantum critical point (QCP) in the (T, x) phase diagram is reached for the critical concentration (${x}_{c}\ensuremath{\sim}1.5$) where the competing RKKY and Kondo energy scales tend to zero. Deviation in \ensuremath{\chi}($T$), $C$($T$), and \ensuremath{\rho}($T$) from Fermi-liquid behavior is observed in the vicinity of the QCP and the former is seen to recover with the application of magnetic field. Further support for the quantum criticality comes from the universal scaling behavior of $M$($H$), \ensuremath{\chi}($T$), and \ensuremath{\rho}($T$) data for ${x}_{c}\ensuremath{\sim}1.5$. Also, the valence fluctuations in the vicinity of the compound showing non-Fermi-liquid behavior may suggest the possible role of valence fluctuations in the QCP, which makes the present series an interesting case.