1. Department of Physics, University of Toronto, Toronto, ON M5S 1A7, Canada
2. Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA
3. Department of Condensed Matter Physics and Chemistry, Ris National Laboratory, 4000 Roskilde, Denmark
4. NEC Research, 4 Independence Way, Princeton, New Jersey 08540, USA
5. Department of Physics, University of Karlsruhe, D-76128 Karlsruhe, Germany
6. Institute for Solid State Physics, University of Tokyo, Roppongi 7-22-1, Minato-ku, Tokyo 106-8666, Japan
7. H. H. Wills Physics Laboratory, University of Bristol, Bristol BS8 1TL, UK

Correspondence to: B. Lake^1,2,3 Correspondence and requests for materials should be addressed to B.L. (e-mail: Email: bella@phonon.ssd.ornl.gov).

Abstract

A notable aspect of high-temperature superconductivity in the copper oxides is the unconventional nature of the underlying paired-electron state. A direct manifestation of the unconventional state is a pairing energy—that is, the energy required to remove one electron from the superconductor—that varies (between zero and a maximum value) as a function of momentum, or wavevector¹,²: the pairing energy for conventional superconductors is wavevector-independent³,⁴. 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 singlet⁵, so there will also be a pairing energy associated with transforming the spin singlet into the higher-energy spin triplet form without necessarily unbinding the charges. Here we use inelastic neutron scattering to determine thewavevector-dependence of spin pairing in La_2-xSr_xCuO₄, the simplest high-temperature superconductor. We find that the spin pairing energy (or 'spin gap') is wavevector independent, even though superconductivity significantly alters the wavevector dependence of the spin fluctuations at higher energies.