High- Tc superconductors with antiferromagnetic order: limitations on spin-fluctuation pairing mechanism

Abstract

The 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∼( π, π) in the magnetic field H≪ H c2 at low temperatures 0< T≪ 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 ∥c -axis)≫R Q( H ⊥c -axis) . The quantum ( T=0) phase diagram at H=0 is constructed. The theory also predicts: (i) that the magnetic field can induce the AF order in the SC state; (ii) that the spin-fluctuation (SF) effective coupling constant g<0.1 eV is small, which gives small SC critical temperature T c (≪40 K)––thus questioning the SF mechanism of pairing in HTS oxides.