Anharmonicity and Weak Mode Assignment in La$\text{}_{2-x}$Sr$\text{}_{x}$CuO$\text{}_{4}$, with Oxygen Isotopic Substitution

The oxygen isotope effect on the first- and second-order Raman active modes of high-quality La$\text{}_{2-x}$Sr$\text{}_{x}$Cu$\text{}^{16,18}$O$\text{}_{4}$ (80% substitution $\text{}^{16}$O by $\text{}^{18}$O) polycrystalline compounds with x=0.00, 0.015 was investigated at low temperatures. In the second-order Raman scattering several weak and strong peaks were observed and assigned to oxygen atom vibrations. With the isotopic substitution the apex oxygen mode follows quite well the mass harmonic law at all temperatures studied, while the soft mode is anharmonic in the temperature range of 77-180 K and for higher temperatures its anharmonicity is strongly increased. The temperature dependence of the energy of the soft mode for x=0.0 shows also a modification at≈180 K and an abnormal behaviour at≈280 K, which may be related with the transition to the antiferromagnetic phase.