Nuclear spin lattice relaxation of ¹³⁹La in La_1.8Sr_0.20uO_4-δ and La ₂CuO₄

Nuclear spin-lattice relaxation of 139La in (I) La1.8Sr0.2CuO4-δ and La2CuO4 was investigated. Below the superconducting transition temperature Tc = 38 K the relaxation rate in (I) follows an Arrhenius law with an activation energy Δ = 11.5 meV, leading to a gap energy of 2Δ/kTc = 7. The relaxation time of 139La in (II), however, shows a completely different temperature dependence, which can be explained by antiferromagnetic fluctuations. © 1988 Elsevier Science Publishers B.V.