Neutron spectroscopic study of the pseudogap formation in La _1.81Sr_0.15Ho_0.04CuO₄ at ambient and elevated pressure

In order to understand the mechanism of high-temperature superconductivity in cuprates it is important to elucidate the nature of the pseudogap formation. We have investigated the temperature dependence of the relaxation rate of crystal-field excitations in the optimally doped superconductor La _1.81 Sr_0.15Ho_0.04CuO₄ at hydrostatic pressures p = 0 and 0.8 GPa. The observed large pressure effect on the onset of the pseudogap formation temperature T*, together with the huge isotope effects on T* observed earlier for oxygen isotope substitution ( ¹⁶O versus ¹⁸O), qualitatively supports models based on phonon-mediated mechanisms of the pseudogap state.