NMR study of the spin dynamics in ${\mathrm{Tl}}_2$${\mathrm{Ba}}_2$${\mathrm{CuO}}_{\mathit{y}}$ (${\mathit{T}}_{\mathit{c}}$=85 K)

In order to understand the spin dynamics in the over-doped region of the high-${\mathit{T}}_{\mathit{c}}$ oxides, the temperature (T) dependence of the Knight shift and the spin-lattice relaxation time (${\mathit{T}}_1$) at the Cu and the O sites of the ${\mathrm{CuO}}_2$ plane were measured in the normal state of ${\mathrm{Tl}}_2$ ${\mathrm{Ba}}_2$ ${\mathrm{CuO}}_{\mathit{y}}$ with maximum ${\mathit{T}}_{\mathit{c}}$ (${\mathit{T}}_{\mathit{c}}$=85 K). The T dependence of the Knight shift at the O site is almost isotropic. In addition, the transferred hyperfine coupling constant is estimated to be fairly large at the Cu site. These facts indicate that the hybridization between the Cu 3d and the O 2p is strong in ${\mathrm{Tl}}_2$ ${\mathrm{Ba}}_2$ ${\mathrm{CuO}}_{\mathit{y}}$ and the doped holes do not have strong O 2p character. The T dependence of the Knight shift at the O site is the same as that of the Cu site, suggesting that the single-spin-component model is likely in this over-doped system. Based on a comparison between the ${\mathit{T}}_1$ of the Cu and the O sites, the antiferromagnetic correlation is considered to be very weak in this system, and the antiferromagnetic correlation length (ξ/a) is estimated to be 1.2 at 120 K. Although the reason is unclear, the so-called psuedo-spin-gap behavior is observed below 120 K at the Cu site, and the relation ${\mathit{T}}_1$TK=const was found to be more likely at the O site than the Korringa-type relation. In addition, the ratio of the ${\mathit{T}}_1$ at the Cu and the O sites becomes T independent in the vicinity of the ${\mathit{T}}_{\mathit{c}}$. These phenomena are also observed in the lightly doped system; thus they may be considered as universal features of the high-${\mathit{T}}_{\mathit{c}}$ oxides.