The oxygen and copper isotope effects on the relaxation rate of crystal-field excitations in the optimally doped high-temperature superconductor ${\mathrm{La}}_{1.81}{\mathrm{Ho}}_{0.04}{\mathrm{Sr}}_{0.15}{\mathrm{CuO}}_4$ have been investigated by means of inelastic neutron scattering. For the ${}^{16}\mathrm{O}$ compound there is clear evidence for the opening of an electronic pseudogap in the normal state at $T^{*}\approx 60\mathrm{K},$ far above $T_c\approx 32\mathrm{K}.$ Upon substituting ${}^{16}\mathrm{O}$ by ${}^{18}\mathrm{O},$ $T^{*}$ is shifted to about 70 K. On the other hand, no shift is found for the copper isotope substitution, i.e., $T^{*}{(}^{63}\mathrm{Cu}\left)\approx T^{*}{(}^{65}\mathrm{Cu}\right)\approx 60\mathrm{K}.$ These results, together with the large oxygen and copper isotope effects on $T^{*}$ for ${\mathrm{HoBa}}_2{\mathrm{Cu}}_4{\mathrm{O}}_8,$ give evidence that the pseudogap formation in the single-layer LSCO and bilayer YBCO type compounds is governed by an additional local mode in the latter.

• Received 20 August 2002

DOI:https://doi.org/10.1103/PhysRevB.66.184506