Ferromagnetic Quantum Critical Point in the Heavy-Fermion Metal YbNi 4 (P 1− x As x ) 2

A quantum critical point (QCP) occurs when quantum fluctuations, which do not go away even at absolute zero, cause a gradual (so-called second order) phase change. QCPs have been observed in ferromagnets, but for ferromagnetic metals, the evidence is less clear-cut and it is thought that, as the temperature is lowered, another order—such as superconductivity—will prevent the formation of a QCP. However, Steppke et al. (p. 933 ), using specific heat and magnetic susceptibility measurements, found strong evidence for a QCP in a quasi–one-dimensional heavy fermion material, YbNi 4 (P 1− x As x ) 2 , near an Arsenic substitution level of about 10%. The results present a challenge to theories about quantum criticality in ferromagnets. Precision low-temperature measurements reveal a divergence associated with quantum criticality in a ferromagnetic metal. 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 YbNi 4 (P 0.92 As 0.08 ) 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.