Strong surface-pinning effects in polycrystalline HgBa2CuO4+δ superconductors

Magnetic studies on polycrystalline HgCa2CuO4+δ superconductors reveal pronounced pinning by the grain surfaces at low temperatures over a wide range of magnetic fields. The response to small ac fields shows that up to 80% of the width of the dc magnetic hysteresis loop in high fields originates from shielding supercurrent flowing in the surface sheath, a vortex-free region with thickness of ∼10 nm. At 10 K and 15 kG, the observed persistent surface current has a density of ∼5×107 A/cm2 when averaged over the barrier thickness. The study suggests that a high current density may be achieved in very thin films by surface pinning.