Non-Fermi-liquid behaviour in magnetic d- and f-electron systems

A growing number of metals have been found to show systematic deviations from the predictions of Landau Fermi-liquid theory. The most straightforward examples are magnetic metals in which the Curie (T_C) or Néel (T_N) temperature has been suppressed to 0 K by the application of hydrostatic pressure. Such systems are discussed here within a spin fluctuation framework which provides quantitatively accurate descriptions of the d-electron systems MnSi and ZrZn₂ near their 'quantum critical points', but which appears to be less successful (in its simplest form) in antiferromagnetic f-electron systems, particularly CePd₂Si₂, which at its critical pressure has a resistivity of the form Δρ ∝ T^1.2±^0.1 over a temperature range extending from around 40 K to below 1 K. CeNi₂Ge₂, which is believed to be close to a quantum critical point at ambient pressure, shows similar behaviour. CeIn₃, CePd₂Si₂ and CeNi₂Ge₂ exhibit superconductivity, in the first two cases limited to a narrow region near the critical pressure, making these the first unambiguous examples of magnetically mediated superconductivity.