Pseudogap of the high-temperature superconductorLa1.96−xSrxHo0.04CuO4as observed by neutron crystal-field spectroscopy

Abstract

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 ${\mathrm{La}}_{1.96\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{Ho}}_{0.04}\mathrm{Cu}{\mathrm{O}}_{4}$ $(x=0.11,0.15,0.20,0.25)$. We found clear evidence for the opening of a pseudogap at ${T}^{*}g{T}_{c}$ 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.