Synthesis and study of the heavy-fermion compound Yb₅Pt₉

We report the synthesis of single crystals of a binary heavy-fermion system Yb₅Pt₉. An unusual double phase transition is observed in specific heat C_P measurements at 0.6 K and 0.65 K, signaling a magnetically ordered ground state. The complete magnetic field-temperature phase diagram of Yb₅Pt₉ is obtained from the magnetic field dependence of the electrical resistivity ρ(T) and specific heat C_P(T), and consists of two phase lines terminating at finite temperature critical endpoints. Electrical resistivity and specific heat measurements show that the magnetically ordered state is a Fermi liquid with strong electronic correlations, absent in the paramagnetic state. At higher temperatures, strong magnetic anisotropy is observed, which we ascribe to crystal electric field effects acting on a well localized Yb³⁺ moment, yielding a well separated doublet ground state, confirmed by inelastic neutron scattering measurements. Our measurements show that Yb₅Pt₉ is a heavy-fermion compound which is very near a quantum critical point.