SPIN-PHONON COUPLING IN THE SINGLE-LAYER EXTENDED T-J MODEL

We consider the implications of spin-phonon coupling within the slave- boson, mean-field treatment of the extended t-J model of a high-temper ature superconductor. In materials such as YBaCuO, where the CuO2 plan e is buckled, this interaction is linear in O displacement along the c axis, and the coupling constant is found to be large. We calculate th e spin effects on phonon dynamics, finding at and below the spin singl et formation temperature changes in frequency shift and linewidth broa dening for certain, key phonon modes which correspond well with experi ment near the optimal doping level. Furthermore, the theory predicts t hat phonon anomalies in underdoped compounds should show evidence of s pin-gap phenomena with the same characteristic temperature as that fou nd in NMR studies; this is exactly as observed in YBaCuO systems, sugg esting a possible unified understanding of the anomalies in magnetic a nd lattice properties. Finally, such a coupling affords the possibilit y of a small isotope effect, and our estimate is in good agreement wit h recent site-selective O-substitution experiments.