YbPtGe2: A multivalent charge-ordered system with an unusual spin pseudogap

Abstract

We performed a study of the structural and physical properties of YbPtGe${}_{2}$. This compound is a multivalent charge-ordered system presenting an unusual spin pseudogap below 200 K. The crystal structure of YbPtGe${}_{2}$ is refined from single-crystal and powder high-resolution synchrotron x-ray diffraction data at different temperatures. Analysis of the structural features of YbPtGe${}_{2}$, together with a combined study of Yb ${L}_{\mathrm{III}}$ x-ray absorption spectroscopy, magnetic susceptibility $\ensuremath{\chi}(T)$, thermopower $S(T)$, and ${}^{171}$Yb and ${}^{195}$Pt NMR indicate half of the Yb atoms to be in an intermediate valence state with an electronic configuration close to 4${f}^{13}$ (Yb${}^{3+}$), while for the remaining Yb atoms the 4${f}^{14}$ (Yb${}^{2+}$) configuration with almost no valence fluctuations is most likely. A drastic drop of the magnetic susceptibility and a decrease of the isotropic shift ${}^{195}{K}_{\mathrm{iso}}(T)$ with decreasing temperature in the temperature range of 50--200 K evidence the opening of a spin pseudogap with an activation energy of $\ensuremath{\Delta}/{k}_{\mathrm{B}}$ $\ensuremath{\sim}$ 200 K. Surprisingly, transport properties do not show clear evidence for the opening of a charge gap, thus excluding a standard Kondo-insulator scenario. Possible origins for this unusual electronic (valence) behavior are discussed.