Heavy-Electron Behavior and Structural Change in Ca1.7Sr0.3RuO4

Sr$_2$RuO$_4$ is an unconventional superconductor with a tetragonal structure, whereas Ca$_2$RuO$_4$ is a Mott insulator with orthorhombic symmetry. The substituted Ca$_{2-x}$Sr$_x$RuO$_4$ has yielded a rich phase diagram that is just beginning to be explored in detail. Experimental investigation of the resistivity $\rho$, susceptibility $\chi$, specific heat C$_p$, Hall coefficient R$_H$, and X-ray diffraction of Ca$_{1.7}$Sr$_{0.3}$RuO$_4$ reveals a structural phase transition near T$_0$ = 190 K and heavy-Fermion (HF) behavior below a coherence temperature T$^*$ $\sim$ 10 K, resembling that of the $\it{f}$-electron HF compound UPt$_3$. The observation of T$^2$-dependence of $\rho$ below $\sim$ 0.5 K suggests a Fermi-liquid ground state. Based upon our data and theoretical calculations, we argue that the structural change at T$_0$ may be responsible for the formation of the HF state.