Quasiquartet crystal-electric-field ground state with possible quadrupolar ordering in the tetragonal compoundYbRu2Ge2

Abstract

We have investigated the magnetic properties of $\mathrm{Yb}{\mathrm{Ru}}_{2}{\mathrm{Ge}}_{2}$ by means of magnetic susceptibility $\ensuremath{\chi}(T)$, specific heat $C(T)$ and electrical resistivity $\ensuremath{\rho}(T)$ measurements performed on flux-grown single crystals. The Curie-Weiss behavior of $\ensuremath{\chi}(T)$ along the easy plane, the large magnetic entropy at low temperatures and the weak Kondo-like increase in $\ensuremath{\rho}(T)$ proves a stable trivalent Yb state. Anomalies in $C(T)$, $\ensuremath{\rho}(T)$, and $\ensuremath{\chi}(T)$ at ${T}_{0}=10.2\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, ${T}_{1}=6.5\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, and ${T}_{2}=5.7\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ evidence complex ordering phenomena. The magnetic entropy just above ${T}_{0}$ amounts to almost $R\phantom{\rule{0.2em}{0ex}}\mathrm{ln}\phantom{\rule{0.2em}{0ex}}4$, indicating that the crystal-electric-field (CEF) ground state is a quasiquartet instead of the expected doublet. The behavior at ${T}_{0}$ is rather unusual and suggests that this transition is related to quadrupolar ordering, being a consequence of the CEF quasiquartet ground state. The combination of a quasiquartet CEF ground state, a high ordering temperature, and the relevance of quadrupolar interactions makes $\mathrm{Yb}{\mathrm{Ru}}_{2}{\mathrm{Ge}}_{2}$ a rather unique system among Yb-based compounds.