Quantum Critical Behavior in Heavy-Fermion Iron Oxypnictide Ce(Ru$_{\mathrm{1-x}}$Fe$_{\mathrm{x}})$PO

Quantum phase transition in itinerant ferromagnets is one of the major topics in a strongly correlated electron system, since it has been suggested to be always first order when the ferromagnetic (FM) order is suppressed by pressure or chemical doping [1]. In order to obtain universal features of the FM quantum criticality, we have studied the two-dimensional heavy-fermion (HF) system Ce(Ru$_{\mathrm{1-x}}$Fe$_{\mathrm{x}})$PO from microscopic $^{31}$P-NMR measurements [2-4]. A HF ferromagnet CeRuPO turns into a HF paramagnet by an isovalent Fe substitution for Ru. We found that Ce(Ru$_{0.15}$Fe$_{0.85})$PO shows critical fluctuations down to $\sim$ 0.3 K, as well as the continuous suppression of Curie temperature and the ordered moments by the Fe substitution. These experimental results suggest the presence of a FM quantum critical point (QCP) at around x $=$ 0.86, which is a rare example among itinerant ferromagnets. In addition, we point out that the critical behaviors in Ce(Ru$_{0.15}$Fe$_{0.85})$PO share a similarity with those in YbRh$_{2}$Si$_{2}$ [5], where the local criticality of f electrons has been discussed [6]. We reveal that Ce(Ru$_{\mathrm{1-x}}$Fe$_{\mathrm{x}})$PO is a new system to study FM quantum criticality in HF compound. \\[4pt] [1] D. Belitz, T. R. Kirkpatrick, and J. Rollb\"uhler, Phys. Rev. Lett. \textbf{94}, 247205 (2005).\\[4pt] [2] S. Kitagawa\textit{ et al.}, Phys. Rev. Lett. \textbf{107,} 277002 (2011).\\[0pt] [3] S. Kitagawa \textit{et al.}, K. Ishida, T. Nakamura, M. Matoba, and Y. Kamihara, Phys. Rev. Lett. \textbf{109}, 227004 (2012).\\[0pt] [4] S. Kitagawa \textit{et al.}, J. Phys. Soc. Jpn. \textbf{82}, 033704 (2013). \\[0pt] [5] K. Ishida \textit{et al.}, Phys. Rev. Lett. \textbf{89}, 107202 (2002).\\[0pt] [6] Q. Si, S. Rabello, K. Ingersent, and J. L. Smith, Nature (London) \textbf{413}, 804 (2001).