Technical Report of ISSP Ser. A: Number 2801

Abstract

The temperature dependences of the Cu-63 nuclear spin-lattice relaxation rate T(sub 1)(exp - 1) and the Knight shift K(sub ab) at the planer Cu(2) site in Ca substituted YBa2Cu408 have been measured by the Nuclear Quadrupole Resonance (NQR) and the Nuclear Magnetic Resonance (NMR) techniques; where the notation 'ab' means the external field perpendicular to the c-axis. The temperature dependence of K(sub ab) in Ca substituted sample becomes weaker compared with the non-substituted sample, indicating that the electronic state in the CuO2 plane turned to be more metallic with Ca substitution. On the other hand, (T(sub 1)T)(exp - 1) in the non-substituted sample increased rapidly with increasing temperature and showed the maximum value at 160 K, so called the spin-gap temperature. Subsequently, (T(sub 1)T)(exp - 1) decreased with increasing temperature above the spin-gap temperature as the Curie-Weiss law. While the behaviors of (T(sub 1)T)(exp - 1) in Ca substituted samples were similar to that in the non-substituted YBa2Cu408, the spin-gap temperature decreased with increasing Ca (hole carrier) content. From these results, the existence of a negative linear relation between the spin-gap temperature and the superconducting transition temperature is concluded.

Ca置換YBa2Cu408の平面Cu(2)配置におけるCu-63核スピン-格子緩和速度T1｛-1｝とナイトシフトKabの温度依存性を核4重極共鳴(NQR)と核磁気共鳴(NMR)により測定した。ここで'ab'はc軸に直角な外部磁場を意味する。Ca置換試料のKabの温度依存性は不置換試料と較べ弱くなったが、これはCuO2面の電子状態がCa置換によってより金属的になったことを示している。一方、不置換試料の(T1T)｛-1｝は温度上昇とともに急速に増加し、いわゆるスピン-ギャップ温度160Kで最大値を示した。キュリー-ワイス則のとおり、スピン-ギャップ温度以上では(T1T)｛-1｝は温度上昇とともに減少した。スピン-ギャップ温度はCa(正孔キャリア)量の増加とともに減少した。以上のことから、スピン-ギャップ温度と超伝導転移温度の間の負の線形関係の存在を結論した。