Novel Superconducting Phases in Copper Oxides and Iron-oxypnictides: NMR Studies

TL;DR

This study uses site-selective NMR to explore the phase diagrams of antiferromagnetism and superconductivity in copper oxides and iron-oxypnictides, revealing the role of Mott physics and structural parameters in high-Tc superconductivity.

Contribution

It provides the first detailed layer-dependent NMR analysis of disorder-free CuO$_2$ planes and compares the mechanisms of high Tc in copper oxides and iron-oxypnictides.

Findings

Uniform AFM and SC coexistence in underdoped CuO$_2$ planes.

The AFM phase diagram aligns with strong correlation theories.

Structural parameters, not AFM fluctuations, drive Tc increase in iron-oxypnictides.

Abstract

We reexamine the novel phase diagrams of antiferromagnetism (AFM) and high-Tc$ superconductivity (HTSC) for a disorder-free CuO$_2$ 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 CuO$_2$ 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 CuO$_2$ 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 {\it 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 $\alpha$ of the FeAs$_4$ 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 $T_c$ higher significantly.