Nuclear Spin-Lattice Relaxation of 139 La in the Superconducting Phase of La 1.8 Sr 0.2 CuO 4-δ

We report on zero-field nuclear spin-lattice relaxation measurements of 139La in La1.8Sr0.2CuO4-δ in the normal and the superconducting state. Below the superconducting transition temperature Tc the relaxation rate behaves like 1/T1 ~ exp [- Δ/kT] with activation energy Δ = 11.5 meV leading to a gap energy of 2Δ = 23 meV. With Tc = 38 K the ratio 2Δ/kTc = 7.1 is obtained, which is twice as large as the weak-coupling value of 3.5.