Nonmagnetic Impurity Effects on Spectrum of Antiferromagnetic Spin Fluctuations and Nuclear Spin-Lattice Relaxation Rate in High-T_c Superconductivity : Condensed Matter: Electronic Properties, etc.

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We theoretically investigate antiferromagnetic (AF) spin fluctuations in impurity-doped high-T_c superconductivity. We assume a d_<x^2-y^2>-wave superconductor with strong AF spin fluctuations, and take into account nonmagnetic impurities within the t-matrix approximation. In the clean system, the imaginary part of the AF spin susceptibility (Im_X(Q,w)) is suppressed far below the excitation gap E_g = 2 × Max[Δ_p] (Δ_p: superconducting order parameter), while it has a peak slightly below E_g. On the other hand, in the presence of unitarity scatterers, it is shown that Im_X(Q,w) far below E_g increases with decreasing temperature even in the superconducting state. As a result, at low termperatures, Im_X(Q,w) has two peaks far and slightly below E_g as observed in Zn-doped YBa_2Cu_3O_<6.97>. In addition, because of the enhancement of the low-energy AF spin fluctuations, the nuclear spin-lattice relaxation rate on Cu ((1/T_1T)_<Cu>) also increases at low temperatures in contrast to the case of the clean superconductivity. This result can explain the recent experiment on (1/T_1T)_<Cu> in Zn-doped YBa_2Cu_4O_8.
社団法人日本物理学会の論文
2000-09-15