Nernst effect, fluctuation diamagnetism and vortices above Tc in cuprates

Nernst-effect and torque magnetometry experiments have provided evidence that, in the hole-doped cuprates, long-range phase coherence vanishes at the critical temperature $T_c$, while the pair condensate survives to a much higher ``onset" temperature $T_{onset}$. In the Nernst experiment, the vortex current produced by a gradient generates a Josephson $E$-field perpendicular to the applied field $\bf H$. In cuprates, this large Nernst signal $e_N$ persists to $T_{onset}\sim$ 130 K. Extensive Nernst experiments in the cuprates LSCO, Bi 2201, and 2212 yield a 3D phase diagram $(x,T,H)$ in fields up to 45 T. This picture has been confirmed by high-resolution torque magnetometry. In a tilted $\bf H$, local planar supercurrents associated with vortices above $T_c$ produce a torque that deflects a cantilever. At each $T$, the diamagnetic magnetization inferred matches the field profile of the Nernst $e_N$. The high-resolution measurement of the diamagnetic susceptibility $\chi$ over 5 field decades uncovers an unusual, fragile ``London rigidity'' that exists in the pseudogap state of Bi 2212 and 2201. The magnetization curves below $T_c$ also provide a reliable determination of the upper critical field $H_{c2}$ which is found to scale linearly with $T_{onset}$. I will also preview evidence for pairing without phase coherence at 0.35 K in LSCO for $x < x_c$ in fields to 30-45 T. \newline \newline In collaboration with Yayu Wang, Lu Li, Joseph G. Checkelsky, Michael Naughton, Seiki Komiya, Shimpei Ono, Yoichi Ando, Shin-ichi Uchida and Genda Gu.