Studies on Magnetic Excitation Spectra of High-Tc Superconductors

Magnetic excitation spectra χ''( q ,ω) of YBa 2 Cu 3 O y , La 2- x Sr x CuO 4 , La 2- y - x Nd y Sr x CuO 4 and Nd 2- x Ce x CuO 4 are compared with those of model calculations, where q and ω are the wave vector and the energy of the excitations. In the model, the dynamical spin susceptibility is calculated by the expression χ( q ,ω)=χ 0 ( q ,ω)/{1+ J ( q )χ 0 ( q ,ω)}, where χ 0 ( q ,ω) is obtained by adopting the effective energy dispersion of the quasi particles which can reproduce the shapes of the Fermi surface, and J ( q ) is the exchange coupling between the Cu–Cu electrons. It is important to consider the experimentally observed quasi particle broadening (Γ) to reproduce the observed spectra of YBa 2 Cu 3 O y , while to reproduce the data of La 2- x Sr x CuO 4 and La 2- y - x Nd y Sr x CuO 4 , very small Γ values have to be used, even in the phases without the static “stripe” order. This may be caused by the existence of the “stripe” correlations or “stripe” fluctuations.