  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 wavevector: the
pairing energy for conventional superconductors is wavevector-independent. The
wavefunction describing the superconducting state will include not only the
pairing of charges, but also of the spins of the paired charges. Each pair is
usually in the form of a spin singlet, 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 the wavevector-dependence of spin pairing in
La_{2-x}Sr_xCuO_4, 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.
