Metals near a magnetic instability

The ground state of a number of metals is in close proximity to a magnetic instability. In these metals, often pronounced deviations from Fermi-liquid behavior are observed, which are commonly referred to as non-Fermi-liquid (NFL) behavior. The scenario of incipient magnetic order is exemplified by CeCu6 with a nonmagnetic ground state, and long-range incommensurate antiferromagnetism in CeCu6-xAux for x > xc 0.1. The magnetic instability can be tuned by alloying or applying external pressure. At the quantum critical point (QCP), where the Neel temperature TN vanishes, the specific heat C varies as C/T ∝ ln(T/T0), independent of whether TN is tuned to zero by variation of x or p, and one observes a linear resistivity contribution Δρ ∝ T. These NFL features can be explained by quasi two-dimensional critical fluctuations, as observed for x = 0.1 with inelastic neutron scattering. In addition, unusual dynamic scaling laws are found in the vicinity of the QCP in CeCu6-xAux which are incompatible with simple spin-fluctuation scenarios. Here we compare in detail the QCP tuned by alloying with new data for x = 0.2, inducing a magnetic instability by a magnetic field. The observation that in case the specific heat and the resistivity at the critical field can be described by the three-dimensional spin fluctuation the latter scenario finds its correspondence in Lorentzian quasi-elastic linewidths observed with inelastic neutron scattering. The archetypal weak ferromagnet ZrZn2 has long been considered a candidate for p-wave superconductivity if samples can be made sufficiently pure. We recently found evidence for superconductivity in ZrZn2 single crystals of unpredicted purity. Surprisingly, superconductivity (with Tc ≅ 0.3 K) exists over the entire pressure range where ferromagnetism exists, and vanishes when ferromagnetism is suppressed at sufficiently high pressure