Anomalous peak effect in heavy-fermion, intermediate-valence and A15 superconductors: Evidence for a Fulde-Ferrell-Larkin-Ovchinnikov state?



An experimental study based upon magnetic and dilatometric measurements is presented for the clean high-κ superconductors UPd2Al3, CeRu2 and V3Si. All three compounds show an enhanced spin susceptibility. Their superconducting state is strongly Pauli limited, and an anomalous peak effect is observed at T<(0.8-0.9)Tc, slightly below Hc2(T). This phenomenon appears to be qualitatively consistent with a first-order transition between weak and collective pinning, caused by the formation of a staggered order parameter in a generalized Fulde-Ferrell-Larkin-Ovchinnikov phase (M. Tachiki et al., Z. Phys. B, in press).