Rotational tuning of Hc2 anomalies in ErNi2B2C:: Angular-dependent superzone gap formation and its effect on the superconducting ground state

ErNi2B2C is a member of the RNi2B2C family of magnetic superconductors, with T-c approximate to 11 K and T-N approximate to 6 K. For magnetic fields applied along the magnetically hard c axis, the upper critical field H-c2 manifests a clear, sharp local maximum at T-N. For magnetic fields applied within the basal plane, features in H-c2 are less distinct and then are held induced changes in the local magnetic ordering. In order to address the relationship between H-c2 and local moment order, anisotropic magnetic and transport measurements on single crystals of ErNi2B2C will be presented. In particular, detailed H-T phase diagrams for different orientations of the applied field, as well as possible evidence of a superzone gap formation at the antiferromagnetic Chase transition and its effect on anisotropic upper critical field, will be discussed. The anomaly in H-c2 as well as sharp features in the normal-state resistivity can be tuned by changing the orientation of the applied field. This is consistent with tuning in and out of the 0.55a* magnetically ordered, superzone gapped phase.