Probing theYb3+spin relaxation inY0.98Yb0.02Ba2Cu3Oxby electron paramagnetic resonance

Abstract

The relaxation of ${\text{Yb}}^{3+}$ in ${\text{YBa}}_{2}{\text{Cu}}_{3}{\text{O}}_{x}$ $(6<x<7)$ was studied using electron paramagnetic resonance. It was found that both electronic and phononic processes contribute to the ${\text{Yb}}^{3+}$ 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 $(\ensuremath{\sim}500\text{ }\text{K})$ optical phonons or an Orbach-type process via the excited vibronic levels of the ${\text{Cu}}^{2+}$ 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 ${\text{Yb}}^{3+}$ relaxation in ${\text{YBa}}_{2}{\text{Cu}}_{3}{\text{O}}_{6.98}$. It was shown that the electronic part of the ${\text{Yb}}^{3+}$ relaxation in the superconducting state follows the same temperature dependence as $^{63}\text{C}\text{u}$ and $^{17}\text{O}$ nuclear relaxations despite the huge difference between the corresponding electronic and nuclear relaxation rates.