Magnetic properties of superconducting HgBa2CuO4+δ single crystals in the overdoped state before and after particle irradiation

We report on measurements of the mixed state properties of several superconducting HgBa2CuO4+δ single crystals in the overdoped regime with different transition temperatures and anisotropies. A pronounced fishtail is observed in all samples and all features qualitatively agree with predictions of the order-disorder theory of vortex matter. For instance, the peak field shifts to lower fields with increasing anisotropy and is accompanied by history effects of the magnetic moment. The irreversibility line becomes lower with increasing anisotropy, but different correlations are found for the critical current density. Particle irradiation was employed to modify the defect structure. Neutron irradiation reduces the anisotropy and considerably affects the irreversible properties, whereas electron irradiation leads only to small effects.