Exponential temperature dependence of the Yb 3+ relaxation in Y 0.98 Yb 0.02 Ba 2 Cu 3 O x

摘要

The relaxation of Yb 3+ in YBa 2 Cu 3 O x (x = 6.1 and 6.4) was studied using Electron Para- magnetic Resonance (EPR). In these samples the Yb 3+ relaxation is dominated by a phonon mechanism. It was shown that the conventional Raman two-phonon process involving acoustic phonons can not describe the temperature dependence of the Yb 3+ relaxation. Instead, the Raman process involving optical phonons or an Orbach-like process via the excited vibronic levels of Cu 2+ ions with energy ? = 500(50)K is responsible for the phononic part of the Yb 3+ relaxation in YBa 2 Cu 3 O x . The present results provide clear experimental evidence that optical phonons or local vibrations are the dominant source of spin-lattice relaxation at sufficiently high temperatures, which cannot be described by the traditional approach using the Debye approximation.