Field-enhanced diamagnetism in intense magnetic field in the pseudogap state of the cuprate $\rm Bi_2Sr_2CaCu_2O_{8+\delta}

In hole-doped cuprates, Nernst experiments imply that the superconducting state is destroyed by spontaneous creation of vortices which destroy phase coherence. Using torque magnetometry on $\rm Bi_2Sr_2CaCu_2O_{8+\delta}$, we uncover a field-enhanced diamagnetic signal $M$ above the transition temperature $T_c$ that increases with applied field to 32 Tesla and scales just like the Nernst signal. The magnetization results above $T_c$ distinguish $M$ from conventional amplitude fluctuations, and strongly support the vortex scenario for the loss of phase coherence at $T_c$.