The oxygen isotope effect on the infrared photo-induced absorption spectra of La<Subscript>2</Subscript>CuO<Subscript>4</Subscript>

The infrared photo-induced spectra of the isotope-substituted stoichiometric La<Subscript>2</Subscript>CuO<Subscript>4</Subscript> samples were studied at T=20 K. Measured spectra were analyzed within the small polaron hopping theory. A small, but detectable shift of the effective phonon frequency on the substitution <Superscript>16</Superscript>O to <Superscript>18</Superscript>O is found for the high-energy surface carrier associated band (~4000 cm<Superscript>-1</Superscript>) and has a value of α<Subscript>O</Subscript>=0.3 ±0.2. The low-energy band (~1500 cm<Superscript>-1</Superscript>) originating from the bulk carriers doesn't show any difference within experimental error which is fully in agreement with theory predictions. The isotope shifts of the optically active oxygen-related phonon modes are described well by the harmonic approximation. Copyright EDP Sciences/Società Italiana di Fisica/Springer-Verlag 2006