Spin and Charge Differentiation in Doped CuO₂ Planes Observed by Cu NMR/NQR Spectra

Abstract

One of the strange features in the Cu-NQR experiments in the CuO₂ plane of high-T_c Cu oxides is the existence of several distinctly separated NQR lines in the most of the doped cases with mixed valences. Since the NQR frequency in these Cu sites manifests itself the occupancy of the holes in the Cu d-orbital, hence the charged state, the experimental observation clearly demonstrates a “charge differentiation” in the CuO₂ plane. On the contrary, the spin state observed from the Cu Knight shift experiment has a uniform characteristic in the plane because only single NMR line has been observed for all the cases. This fact makes a great contrast to the case of vanadium oxides where the “spin differentiation” has been observed in the ⁵¹V NMR shift measurements. We are forced to argue that the above facts obtained by the Cu NMR and NQR for the CuO₂ plane are associated with a kind of “spin-charge separation”, namely the energy scale for the spin and charge fluctuations are well differentiated with respect to the NMR time scale (∼10⁻⁸ sec). This is one of the most important characteristics of the electronic state in high-T_c’s.