NMR evidence for a pseudogap in Pb-doped Bi:2201 single crystal

A $^{63}$Cu nuclear magnetic resonance (NMR) study of a Pb-doped Bi:2201 system, Bi$_{1.6}$Pb$_{0.4}$Sr$_{2.05}$CuO$_y$, is presented. The temperature dependencies of the NMR peak shift and the nuclear spin-lattice relaxation (SLR) rate reveal the pseudogap that opens near $T^{\ast}=60\,$K, significantly above the superconducting critical temperature $T_c\simeq9\,$K at the NMR experiment field, 7 T oriented $H\parallel c$. The noticeable disparity between $T_c$ and $T^{\ast}$ and the behavior of Cu SLR at $T>T^{\ast}$ imply the underdoped state of the studied system. A relatively weak effect of the magnetic field on the superconductivity evidenced from small ($\approx$ 7 K) shift of the zero-field $T_{c0}=16\pm 1\,$K under the applied 7 T field suggests high upper critical field, $H_{c2}$, unusual for compounds with as low $T_{c0}$.