We studied the isotope, pressure, and doping effects on the pseudogap temperature $T^{*}$ by neutron spectroscopic experiments of the relaxation rate of crystal-field excitations in ${\mathrm{La}}_{1.96-x}{\mathrm{Sr}}_x{\mathrm{Ho}}_{0.04}\mathrm{Cu}{\mathrm{O}}_4$ $(x=0.11,0.15,0.20,0.25)$. We found clear evidence for the opening of a pseudogap at $T^{*}>T_c$ in the whole doping range. Furthermore, the effect of oxygen isotope substitution was found to produce an upward shift of $T^{*}$, whereas the application of pressure on the optimally doped compound $(x=0.15)$ results in a downward shift of $T^{*}$. The opposite effect of oxygen isotope substitution and pressure application indicates that electron-phonon induced effects contribute to the pseudogap formation. The temperature dependence of the relaxation data is consistent with a gap function of predominantly $d$-wave symmetry.

• Received 9 March 2006

DOI:https://doi.org/10.1103/PhysRevB.74.184520