Coupled magnon-phonon excitations in Sr2CuCl2O2 at high pressure

High-pressure optical experiments have been performed on midinfrared excita tions unique to the layered cuprates, yet whose physical origin has been un resolved. Measurements of high-pressure midinfrared spectra of Sr(2)CuCl(2) o(2) unambiguously identify the similar to 2800-cm(-1) absorption as an unu sual quasibound two-magnon + phonon excitation. We further show that the br oad peak centered near similar to 4000 cm(-1) is a four magnon + phonon exc itation, with spectral weight similar to 10(2) times larger than predicted by theory. These excitations reveal unusual latent interactions coupling ch arge, spin, and lattice degrees of freedom:in the CuO2 planes.