Spin Dynamics in Cuprates and Its Relation to Superconductivity

The relevance of magnetism for the mechanism responsible for high-temperature superconductivity remains an open and still interesting issue. The observation by inelastic neutron scattering of strong antiferromagnetic dynamical correlations in superconducting cuprates is discussed in relation to the unusual physical properties of the cuprates as well as in relation to the superconducting pairing. 1 Why neutron scattering in High-T c Cuprates ? Although conventional electron-phonon interaction alone within BCS 1 theory can be dismissed, fourteen years after its discovery 2 , the mechanism for high-temperature superconductivity in copper oxides is still debated. Among more exotic approach for superconductivity, spin fluctuations remain a serious candidate 3,4. Indeed, the systematic existence of strong antiferromagnetic (AF) dynamical correlations in the metallic and superconducting phases of all cuprates supports by principle that proposal. The observation of magnetic fluctuations around the AF wavevector Q AF ≡ (π, π) was first emphasized by copper Nuclear Magnetic Resonance (NMR) 5 and then widely reported by inelastic neutron scattering (INS). INS is actually playing an essential role on this matter as it is the only technique which directly measures the imaginary part of the spin susceptibility, Imχ(Q, ¯ hω), over a wide energy range (¯ hω ∼ 1 to 200 meV) and for any momentum transfer within the Brillouin zone. [In contrast, spin lattice relaxation rate in NMR experiments only probes a sum of Imχ(Q, ¯ hω) in momentum space weighting by atomic hyperfine tensor and at frequencies ω → 0.] In principle, the full determination of the spin susceptibility by neutron experiments would ultimately answer whether the mechanism for high-temperature superconductivity is due to AF fluctuations or not. The powerfulness of inelastic neutron scattering is unfortunately limited by the need of large single crystals (of cm 3 size) usually difficult to grow in complex systems such as high-T c cuprates. For that reason, only two cuprates families have been extensively studied by INS so far: La 2−x Sr x CuO 4 (LSCO) 6,7 Further, although they have common features (energy scale, absolute units) 10 , the spin susceptibility observed in the two systems also exhibits clear differences: namely the low energy spin fluctuations are peaked in LSCO at wavevectors Q δ = (π(1 ± δ), π) ≡ (π, π(1 ± δ)), incommensurate from the AF momentum. In YBCO, the spin 1 fluctuations are broader in momentum space but basically commensurate at Q AF (see section 4). More …