We present a new Kondo-lattice system, YbNi 4 P 2 , which is a clean heavy-fermion metal with a severely reduced ferromagnetic ordering temperature at T C = 0.17 K, evidenced by distinct anomalies in susceptibility, specific-heat, and resistivity measurements. The ferromagnetic nature of the transition, with only a small ordered moment of ∼ 0.05 µ B , is established by a diverging susceptibility at T C with huge absolute values in the ferromagnetically ordered state, severely reduced by small magnetic fields. Furthermore, YbNi 4 P 2 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-thanlogarithmically diverging Sommerfeld coefficient and a linear-in-T resistivity above T C which cannot be explained by any current theoretical predictions. These exciting characteristics are unique among all correlated electron systems and makes this an interesting material for further in-depth investigations. arXiv:1108.4274v1 [cond-mat.str-el]

International audienceThe hydride CeRuSiH1.0 with space group P4/nmm was synthesized by exposure at 523 K of the heavy-fermion ternary silicide CeRuSi under 4 MPa of hydrogen gas. The investigation of the hydride by x-ray powder diffraction reveals that the hydrogenation induces a pronounced anisotropic expansion of the unit cell. Moreover, CeRuSiH1.0 presents two antiferromagnetic transitions at TN1=7.5(2) K and TN2=3.1(2) K evidenced by magnetization and specific heat measurements. Hydrogenation changes the moderate heavy-fermion compound CeRuSi, which has a gamma=220 mJ/mol K2, to an antiferromagnet, which has a smaller electronic coefficient gamma=26 mJ/mol K2. In other words, the hydrogen insertion diminishes the influence of the Kondo effect. The transition heavy-fermion behavior-->antiferromagnet can be well understood in terms of the classical Doniach diagram where the hydrogenation plays a role opposite to the pressure. The expansion of the lattice induced by hydrogen insertion is here much more important than the role of Ce-H bonding observed in other hydrogenated compounds CeCoSiH1.0 or CeCoGeH1.0, where an opposite transition (antiferromagnetic-->spin fluctuation) was evidenced