High-T_{c} Superconductors with AF Order: Limitations on Spin-Fluctuation Pairing Mechanism

TL;DR

This paper investigates the interplay of antiferromagnetism and superconductivity in high-Tc superconductors, showing limitations of the spin-fluctuation pairing mechanism due to small coupling constants and predicting magnetic field effects.

Contribution

The study provides a microscopic theory explaining magnetic field effects on AF order in high-Tc superconductors and challenges the viability of the spin-fluctuation pairing mechanism.

Findings

Large increase in magnetic Bragg peak intensity under magnetic field

Predicted anisotropy in magnetic response with field orientation

Small spin-fluctuation coupling constant implying low Tc

Abstract

The very intriguing antagonistic interplay of antiferromagnetism (AF) and superconductivity (SC), recently discovered in high-temperature superconductors, is studied in the framework of a microscopic theory. We explain the surprisingly large increase of the magnetic Bragg peak intensity $I_{Q}$ at $Q\sim (\pi ,\pi)$ in the magnetic field $H\ll H_{c2}$ at low temperatures $0<T\ll T_{c},T_{AF}$ in $La_{2-x}Sr_{x}CuO_{4}$. Good agreement with experimental results is found. The theory predicts large anisotropy of the relative intensity $R_{Q}(H)=(I_{Q}(H)-I_{Q}(0))/I_{Q}(0)$%, i.e. $R_{Q}(H\parallel c-axis)\gg R_{Q}(H\perp c-axis)$. The quantum (T=0) phase diagram at H=0 is constructed. The theory also predicts: (i) the magnetic field induced AF order in the SC state; (ii) small value for the spin-fluctuation coupling constant $g<(0.025-0.05)$ $eV$. The latter gives very small SC critical temperature $T_{c}(\ll 40$ $K)$, thus questioning the spin-fluctuation mechanism of pairing in HTS oxides.