Unconventional magnetism in multivalent charge-ordered YbPtGe${}_2$ probed by ${}^{195}$Pt- and ${}^{171}$Yb-NMR

Detailed ${}^{195}$Pt- and ${}^{171}$Yb nuclear magnetic resonance (NMR) studies on the heterogeneous mixed valence system YbPtGe${}_2$ are reported. The temperature dependence of the ${}^{195}$Pt-NMR shift ${}^{195}K(T)$ indicates the opening of an unusual magnetic gap below 200,K. ${}^{195}K(T)$ was analyzed by a thermal activation model which yields an isotropic gap $\Delta/k_B \approx 200$K\mathrm{.}Incontrast,thespin-latticerelaxationrate$^{195}$($1/T_1$) does not provide evidence for the gap. Therefore, an intermediate-valence picture is proposed while a Kondo-insulator scenario can be excluded. Moreover, ${}^{195}$($1\mathrm{/}{T}_1$) follows a simple metallic behavior, similar to the reference compound YPtGe${}_2$. A well resolved NMR line with small shift is assigned to divalent ${}^{171}$Yb. This finding supports the proposed model with two sub-sets of Yb species (di- and trivalent) located on the Yb2 and Yb1 site of the YbPtGe${}_2$ lattice.