Abstract
The electron paramagnetic resonance (EPR) of ${\mathrm{Mn}}^{2+}$ doped into ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CuO}}_{4}$ was used to probe the copper spin relaxation via the bottleneck effect for oxygen isotope ${(}^{16}\mathrm{O}$ and ${}^{18}\mathrm{O})$ substituted samples. The concentration range x extended from 0.06 to 0.20 and the temperature dependence of the EPR linewidth was studied from 4 to 300 K. It was found that the EPR linewidth is larger for the ${}^{18}\mathrm{O}$ isotope samples than for the ${}^{16}\mathrm{O}$ samples. The isotope effect is pronounced at low temperatures and decreases with increasing Sr concentration. This effect is quantitatively explained by the ${\mathrm{Cu}}^{2+}S=1/2$ spin relaxation to the lattice via Dzyaloshinski terms coupled linearly to the local ${Q}_{4}{/Q}_{5}$ tilting modes of the ${\mathrm{CuO}}_{6}$ octahedra as proposed by B. I. Kochelaev [J. Supercond. 12, 53 (1999)]. The ${Q}_{4}{/Q}_{5}$ modes are coupled sterically to the ${Q}_{2}$ Jahn-Teller modes considered to be relevant for the (bi)polaron formation and thus for the high-temperature superconductivity.
Published Version
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