We have studied the variation of transverse magnetoresistance of underdoped ${\mathrm{YBCO}}_{6.6}$ crystals, either pure or with reduced $T_c$ down to 3.5 K by electron irradiation, in fields up to 60 T. We find evidence that the superconducting fluctuation contribution to the conductivity is suppressed only above a threshold field $H_c^{\prime }(T)$, which is found to vanish at $T_c^{\prime }>T_c$. In the pure ${\mathrm{YBCO}}_{6.6}$ sample, $H_c^{\prime }$ is already 50 T at $T_c$. We find that increasing disorder weakly depresses $H_c^{\prime }(0)$, $T_c^{\prime }$, and $T_{\nu }$, the onset of the Nernst signal. Thus, these energy scales appear more characteristic of the 2D local pairing than the pseudogap temperature which is not modified by disorder.

• Received 26 October 2006

DOI:https://doi.org/10.1103/PhysRevLett.99.027003