Superconducting magnetization above the irreversibility line inTl2Ba2CuO6+δ

Piezolever torque magnetometry has been used to measure the magnetization of superconducting ${\mathrm{Tl}}_2{\mathrm{Ba}}_2{\mathrm{CuO}}_{6+\mathrm{\delta }}.$ Three crystals with different levels of oxygen overdoping were investigated in magnetic fields up to 10 T. In all cases, the magnetization above the irreversibility line was found to depart from the behavior $M\sim \ln (\eta H_{c2\mathrm{}}/H)$ of a simple London-like vortex liquid. In particular, for a strongly overdoped ${(T}_c=15\mathrm{}\mathrm{K})$ crystal, the remnant superconducting order above the irreversibility line is characterized by a linear diamagnetic response $\left(M\sim H\right)$ that persists well above $T_c$ and also up to the highest field employed.