Surface barrier in Hg-based polycrystalline superconductors.

Abstract

Pronounced effects of a surface barrier are frequently observed on polycrystalline superconductors of ${\mathrm{HgBa}}_{2}$${\mathrm{Ca}}_{\mathit{n}\mathrm{\ensuremath{-}}1}$${\mathrm{Cu}}_{\mathit{n}}$${\mathrm{O}}_{2\mathit{n}+2+\mathrm{\ensuremath{\Delta}}}$ [Hg-12(n-1)n] with n=1, 2, and 3. Compared with other known high-temperature superconductors, the effects at low temperatures are much stronger and persist over a wide range of magnetic fields. The present analysis shows that the enhancement of the irreversible magnetization of grains by the surface barrier is comparable to that by neutron irradiations. The magnitude of the surface supercurrent density is >4\ifmmode\times\else\texttimes\fi{}${10}^{7}$ A/${\mathrm{cm}}^{2}$ at T=40 K and H=45 kG, for Hg-1223. The upper and lower boundaries of the thickness of the vortex-free region and the surface-current density are analyzed as functions of temperature and field.