Probing the Yb${}^{3+}$ spin relaxation in Y${}_{0.98}$Yb${}_{0.02}$Ba${}_2$Cu${}_3$O${}_x$ by Electron Paramagnetic Resonance

Abstract

The relaxation of Yb${}^{3+}$ in YBa${}_2$Cu${}_3$O${}_x$ ($6<x<7$) was studied using Electron Paramagnetic Resonance (EPR). It was found that both electronic and phononic processes contribute to the Yb${}^{3+}$ relaxation. The phononic part of the relaxation has an exponential temperature dependence, which can be explained by a Raman process via the coupling to high-energy ($\sim$500 K) optical phonons or an Orbach-like process via the excited vibronic levels of the Cu${}^{2+}$ ions (localized Slonczewski-modes). In a sample with a maximum oxygen doping $x$=6.98, the electronic part of the relaxation follows a Korringa law in the normal state and strongly decreases below $T_c$. Comparison of the samples with and without Zn doping proved that the superconducting gap opening is responsible for the sharp decrease of Yb${}^{3+}$ relaxation in YBa${}_2$Cu${}_3$O${}_{6.98}$. It was shown that the electronic part of the Yb${}^{3+}$ relaxation in the superconducting state follows the same temperature dependence as ${}^{63}$Cu and ${}^{17}$O nuclear relaxations despite the huge difference between the corresponding electronic and nuclear relaxation rates.

8 pages, 6 figures