Tilting Mode Relaxation and Oxygen Isotope Effect in La2−xSrxCuO4 Studied by Electron Paramagnetic Resonance

The electron paramegnetic resonance (EPR) of Mn²⁺ doped into La_2?x Sr _x CuO₄ was used to probe the copper spin relaxation via the bottleneck effect for oxygen isotope (¹⁶O and ¹⁸O)–substituted samples. It was found that the EPR linewidth is larger for the ¹⁸O isotope samples than for the ¹⁶O samples. For x = 0.03, the linewidth for the ¹⁸O sample is larger than for ¹⁶O sample by a factor of 2. The isotope effect is pronounced at low temperatures and decreases with increasing Sr concentration. This effect is quantitatively explained by the Cu²⁺ S = 1/2 spin relaxation to the lattice via Dzyaloshinski terms coupled linearly to the local Q ₄/Q ₅ tilting modes of the CuO₆ octahedra as proposed by Kochelaev et al. [1] 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 (HTSC).