Pseudogap of the high-temperature superconductor La1.96-x Srx Ho0.04 Cu O4 as observed by neutron crystal-field spectroscopy

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 La1.96-x Srx Ho0.04 Cu O4 (x=0.11,0.15,0.20,0. 25). We found clear evidence for the opening of a pseudogap at T* > Tc 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.