Magnetization curves in underdoped cuprates measured at low T in fields up to 45 Tesla

Torque magnetometry is capable of resolving the weak diamagnetic which extends to temperatures $T\gg T_c$ in hole-doped cuprate crystals. Recently, we reported$^1$ that the magnetization $M$ above $T_c$ scales accurately as the Nernst signal $e_N$ and remains robust to fields of 33 T. The results strongly support the scenario in which thermally created vortices destroy long-range phase coherence at $T_c$. We have extended these studies to explore the fluctuation magnetization to 45 T in underdoped LSCO and Bi 2201 in a series of samples doped near the critical value $x_c\sim$ 0.055. We investigate the loss of phase coherence as we decrease $x$ below $x_c$ keeping $T$ as low as 0.5 K. We use the $M$-$H$ curves to explore how singular phase fluctuations occur as superfluidity is destroyed when $x$ approaches the insulating Mott limit at low $T$.\\ $^*$Supported by NSF grant (DMR 0213706).\\ 1. Yayu Wang et al., Phys. Rev. Lett., {\bf 95}, xxx \emph{in press}, cond-mat/0503190; Lu Li \etal, Eurpophys. Lett. {\bf 72}, 451-457 (2005).