| Research Abstract |
Main results obtained in this research period are the followings.
1. We have carried out measurements of the magnetic excitation spectra χ"(ω) in single crystals of Yba ィイD22ィエD2CuィイD23ィエD2OィイD26+yィエD2 (YBCOィイD26+yィエD2)(y〜0.4 and 0.45). At 295Κ, the χ"-ω curves of both samples are slightly concave in the low energy region, which suggest that the singlet correlation exists even at room temperature. We can conclude that the fluctation of the singlet formation persists far above the so-called spin gap temperature ΤィイD2SGィエD2 (〜150Κ), possibly to ΤィイD20ィエD2 at around which various transport anomalies set in (for example Hall coefficients), and that the singlet correlation and the antiferromaginetic correlation begin to develop almost simultaneously.
2. Recently, it has been reported that in the magnetic excitation spectra of a 0.5% Zn-doped YBCOィイD27ィエD2 sample, a sharp peak (resonance peak) can be observed at (π, π) even far above ΤィイD2cィエD2. The results is rather similar to that observed in underdoped YBCO samples, suggesting that the doping significantly enhance the singlet correlation (or the pseudo gap) even in the optimally doped system. In order to see this possibly, we have measured various quantities which are sensitive to the existence of the singlet pairs, the BィイD22uィエD2 phonon, NMR relaxation rate and the in-plane resistivity of YBΤィイD2a2ィエD2(CuィイD21-xィエD2ZnィイD2xィエD2)ィイD23ィエD2OィイD27ィエD2(x〜0.005). These quantities do not exhibit such the anomalous Τ-dependence as observed in underdoped YBCO samples, or no evidence for the enhancement on the pseudo gap by the Zn doping has been found. Then, for the complete understanding of the Zn-doping effects on the magnetic excitation spectra, it seems to be necessary to study its detailed momentum-energy space structure composed of the resonance-peak component centered at (π, π) and another component which has peak at the inbommensurate q-point.
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