Novel superconducting phases in copper oxides and iron-oxypnictides: NMR studies Original Research Article

We reexamine the novel phase diagrams of antiferromagnetism (AFM) and high-T c superconductivity (HTSC) for a disorder-free CuO2 plane based on an evaluation of local hole density (p ) by site-selective Cu-NMR studies on multilayered copper oxides. Multilayered systems provide us with the opportunity to research the characteristics of the disorder-free CuO2 plane. The site-selective NMR is the best and the only tool used to extract layer-dependent characteristics. Consequently, we have concluded that the uniform mixing of AFM and SC is a general property inherent to a single CuO2 plane in an underdoped regime of HTSC. The T =0 phase diagram of AFM constructed here is in quantitative agreement with the theories in a strong correlation regime which is unchanged even with mobile holes. This Mott physics plays a vital role for mediating the Cooper pairs to make T c of HTSC very high. By contrast, we address from extensive NMR studies on electron-doped iron-oxypnictides La1111 compounds that the increase in T c is not due to the development of AFM spin fluctuations, but because the structural parameters, such as the bond angle αα of the FeAs4 tetrahedron and the a-axis length, approach each optimum value. Based on these results, we propose that a stronger correlation in HTSC than in FeAs-based superconductors may make Tc higher significantly.