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 UPd 2 Al 3 , CeRu 2 and V 3 Si. 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)T c , slightly below H c2 (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).