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

TL;DR

This study investigates the wavevector dependence of spin pairing energy in a high-temperature superconductor, revealing a wavevector-independent spin gap despite changes in spin fluctuations caused by superconductivity.

Contribution

It provides direct experimental evidence that the spin gap in La_{2-x}Sr_xCuO_4 is wavevector-independent, challenging previous assumptions about spin pairing in high-T_c superconductors.

Findings

Spin gap is wavevector independent.

Superconductivity alters high-energy spin fluctuations.

Provides insight into spin pairing mechanisms.

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 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.