Nuclear relaxation behavior of the superconducting cuprates: ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_8$

Nuclear-magnetic-resonance data are presented and analyzed for the high-${\mathit{T}}_{\mathit{c}}$ compound ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{CaCu}}_2$ ${\mathrm{O}}_8$ for two oxygen doping levels. Both sample conditions lead to spin-gap behavior for the NMR shift, with a precursive downturn in the data at T>${\mathit{T}}_{\mathit{c}}$. In addition, the relaxation times ${\mathit{T}}_1$ obey the relation (${\mathit{T}}_1$T${)}^{\mathrm{-}1}$∝${\mathit{K}}^{\mathit{s}}$(T) at low temperatures (T≲100 K), where ${\mathit{K}}^{\mathit{s}}$(T) is the spin paramagnetic shift. This relation, which is also obeyed by other superconductors, is argued to be related to the spin-gap effects and thus incompatible with a Fermi-liquid approach to the understanding of these systems.