On the Pseudogap State in Y- and La-Type High-Temperature Superconductors: Doping Dependence, Isotope Effects, and Electronic Properties Studied by Neutron Spectroscopy

Abstract

The doping dependence as well as the oxygen and copper isotope effects on the pseudogap temperature T* were investigated by neutron spectroscopic experiments of the relaxation rate of crystal-field excitations in rare-earth based Y- and La-type high-temperature superconductors. We found three essential results from our truly bulk-sensitive experiments: (1) For all doping levels we had T* > Tc, even in the optimally doped and overdoped regimes. (2) In bilayer high-Tc compounds we observed huge oxygen and copper isotope effects on T*, which give evidence that the pseudogap formation is governed by lattice modes involving both the oxygen and copper ions. In single layer high-Tc compounds, on the other hand, no shift of T* was found for the copper isotope substitution. (3) Our results obtained for all doping levels support the unusual temperature dependence of the gap function in the pseudogap region reported for underdoped Bi-type compounds, i.e., a breakup of the Fermi surface into disconnected arcs in the temperature range Tc < T < T*.