Spin gap and magnetic coherence in a clean high-temperature superconductor

A notable aspect of high-temperature superconductivity in the copper oxides is the unconventional nature of the underlying paired-electron state. A di rect manifestation of the unconventional state is a pairing energy-that is, the energy required to remove one electron from the superconductor-that va ries (between zero and a maximum value) as a function of momentum, or wavev ector(1,2): the pairing energy for conventional superconductors is wavevect or-independent(3,4). The wavefunction describing the superconducting state will include the pairing not only of charges, but also of the spins of the paired charges. Each pair is usually in the form of a spin singlet(5), so t here will also be a pairing energy associated with transforming the spin si nglet into the higher-energy spin triplet form without necessarily unbindin g the charges. Here we use inelastic neutron scattering to determine the wa vevector-dependence of spin pairing in La2-xSrxCuO4, the simplest high-temp erature superconductor. We find that the spin pairing energy (or 'spin gap' ) is wavevector independent, even though superconductivity significantly al ters the wavevector dependence of the spin fluctuations at higher energies.