Zhidong Hao

New mechanisms for irreversibility in high‐Tc superconductors (invited)

We discuss the zero‐field cooled (ZFC), field‐cooled (FC), and remanent (rem) magnetization of YBaCuO crystals in the context of an extended Bean critical‐state model. We introduce a novel way to include the reversible magnetization in this theory and show how this can explain the concave‐downwards shape of the ZFC magnetization near Tc. We explain the persistence of remanence in a temperature range where FC and ZFC magnetizations merge in terms of a residual irreversibility arising from a step in the reversible B(H) curve. We also introduce the concept of a flux‐trapping depth which helps explain the field dependence of the Meissner effect.

Residual irreversibility in high-Tc superconductors

Magnetization measurements on many single crystals of YBaCuO as well as BiSrCaCuO show a non-vanishing remanent magnetization in a region where the field-cooled and zero-field-cooled magnetizations coincide. We call this kind of remanence “residual irreversibility”. We argue that this behavior is not compatible with over-simplified critical-state models or surface barriers. We show how it can arise from weak “residual” pinning in a regime where vortices are further apart than the penetration depth and the reversible B(H) curve shows a sharp initial rise just above Hcl. The “residual irreversibility” can be enhanced in crystals with an inhomogeneous or non-superconducting core, as seen in some of our thicker YBa/CuO crystals and also in BiSrCaCuO. We also show how the field-cooled Meissner fraction can measure the superconducting volume fraction in the low-field limit.

Vortices in anisotropic type-II superconductors

We present a generalized variational model of an isolated vortex, valid in the anisotropic case when the external magnetic field is along one of the symmetry directions. We discuss the effects of the field anisotropy on the core and magnetic pinning of individual vortices.