RT Journal Article SR Electronic T1 Ferromagnetic Quantum Critical Point in the Heavy-Fermion Metal YbNi4(P1−xAsx)2 JF Science JO Science FD American Association for the Advancement of Science SP 933 OP 936 DO 10.1126/science.1230583 VO 339 IS 6122 A1 Steppke, Alexander A1 Küchler, Robert A1 Lausberg, Stefan A1 Lengyel, Edit A1 Steinke, Lucia A1 Borth, Robert A1 Lühmann, Thomas A1 Krellner, Cornelius A1 Nicklas, Michael A1 Geibel, Christoph A1 Steglich, Frank A1 Brando, Manuel YR 2013 UL http://science.sciencemag.org/content/339/6122/933.abstract AB Unconventional superconductivity and other previously unknown phases of matter exist in the vicinity of a quantum critical point (QCP): a continuous phase change of matter at absolute zero. Intensive theoretical and experimental investigations on itinerant systems have shown that metallic ferromagnets tend to develop via either a first-order phase transition or through the formation of intermediate superconducting or inhomogeneous magnetic phases. Here, through precision low-temperature measurements, we show that the Grüneisen ratio of the heavy fermion metallic ferromagnet YbNi4(P0.92As0.08)2 diverges upon cooling to T = 0, indicating a ferromagnetic QCP. Our observation that this kind of instability, which is forbidden in d-electron metals, occurs in a heavy fermion system will have a large impact on the studies of quantum critical materials.