Oxygen and copper isotope effects on the pseudogap in the high-temperature superconductor<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">La</mml:mi></mml:mrow><mml:mrow><mml:mn>1.81</mml:mn></mml:mrow></mml:msub></mml:mrow><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">Ho</mml:mi></mml:mrow><mml:mrow><mml:mn>0.04</mml:mn></mml:mrow></mml:msub></mml:mrow><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">Sr</mml:mi></mml:mrow><mml:mrow><mml:mn>0.15</mml:mn></mml:mrow></mml:msub></mml:mrow><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">CuO</mml:mi></mml:mrow><mml:mrow><mml:mn>4</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>studied by neutron crystal-field spectroscopy

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 ${T}^{*}\ensuremath{\approx}60\mathrm{K},$ far above ${T}_{c}\ensuremath{\approx}32\mathrm{K}.$ 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., ${T}^{*}{(}^{63}\mathrm{Cu})\ensuremath{\approx}{T}^{*}{(}^{65}\mathrm{Cu})\ensuremath{\approx}60\mathrm{K}.$ 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.

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