On the Magnetic Excitation Spectra of High-T_c Cu Oxides at Energies Up to the Region Far above the Resonance Energy

Abstract

The magnetic excitation spectra χ″(q,ω) of YBa₂Cu₃O_y and La214 systems have been studied. For a sample of the latter system, La_1.88Sr_0.12CuO₄, χ″(q,ω) has been measured up to ∼30 meV. Existing data of these two kinds of system have been analyzed at energies up to the values far above ω_r, around which χ″(q,ω) exhibits the commensurate peak. In the analyses, we have used the phenomenological expression of the generalized magnetic susceptibility χ(q,ω)=χ₀(q,ω)⁄{1+J(q)χ₀(q,ω)}, where χ₀(q,ω) is the susceptibility of the electrons without the exchange coupling J(q) among them. In the energy region up to ∼ω_r, it has been reported by the authors’ group that the expression can explain the characteristics of the q- and ω-dependences of the spectra of YBa₂Cu₃O_y (YBCO or YBCO_y). Here, it is also pointed out that the expression can reproduce the rotation of the four incommensurate peaks of χ″(q,ω) within the a^*–b^* plane about (π⁄a,π⁄a) [or so-called (π,π)] point by 45°, which occurs as ω goes to the region far above ω_r from an energy below ω_r. This indicates that for the rotation, “stripe” correlation is not required. For La214, the observed and calculated results of χ″(q,ω) are comparatively discussed up to the ω region far above ω_r. These results indicate that for YBCO, the characteristics of χ″(q,ω) can be explained qualitatively by the above expression throughout the ω region studied, while for the La214 system, effects of “stripes” on χ″(q,ω), which cannot be explained by the expression, are more significant in the low-energy region of ω (<ω_r) than at high energies.