Nuclear Spin-Lattice Relaxation of ¹³⁹La in the Superconducting Phase of La_1.8Sr_0.2CuO_4-δ

We report on zero-field nuclear spin-lattice relaxation measurements of 139La in La1.8Sr0.2CuO4-δin the normal and the superconducting state. Below the superconducting transition temperature Tcthe 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. © 1988 IOP Publishing Ltd.