Measurement of diamagnetic signal on UD YBCO

The ortho-II phase under-doped (UD) YBa$_2$Cu$_3$O$_{6+\delta}$ has shown a number of interesting phenomena such as quantum oscillations and field-driven charge ordering. An open question is the fate of the superconducting fluctuation in the magnetic field beyond the vortex melting field. To answer the question, we carried out the capacitance based cantilever torque magnetometry measurements on the $T_c = 60$ K phase YBa$_2$Cu$_3$O$_{6+\delta}$ up magnetic fields as high as 56T. At $T$ as low as 1.5 K, the magnetization hysteresis ends at 30 T, marking the melting of the vortex solid phase. Nonlinear diamagnetic signal exists beyond the melting field and persists at field 56T. Our observation suggests that the superconducting fluctuation persists into extensive field, as the Cooper pairing survives well above the vortex solid melting field.