Ferromagnetic quantum criticality in the quasi-one-dimensional heavy fermion metal YbNi₄P₂

We present a new Kondo-lattice system, YbNi4P2, which is a clean heavy-fermion metal with a severely reduced ferromagnetic (FM) ordering temperature at TC = 0.17 K, evidenced by distinct anomalies in susceptibility, specific heat and resistivity measurements. The FM nature of the transition, with only a small ordered moment of ̃0.05μB, is established by a diverging susceptibility at TC with huge absolute values in the ferromagnetically ordered state, severely reduced by small magnetic fields. Furthermore, YbNi4P2 is a stoichiometric system with a quasi-one-dimensional crystal and electronic structure and strong correlation effects which dominate the low-temperature properties. This is reflected by a stronger-than- logarithmically diverging Sommerfeld coefficient and a linear-in-T resistivity above TC, which cannot be explained by current theoretical predictions. These exciting characteristics are unique among all correlated electron systems and make this an interesting material for further in-depth investigations.