Emergence of low-energy spin waves in superconducting electron-doped cuprates

Abstract

In order to fully utilize the technological potential of unconventional superconductors, an enhanced understanding of the superconducting mechanism is necessary. In the best performing superconductors, the cuprates, superconductivity is intimately linked with magnetism, although the details of this coupling remain elusive. In search of clarity in the magnetism-superconductivity relationship, we focus on the electron-doped cuprate $\mathrm{Nd}_{1.85}\mathrm{Ce}_{0.15}\mathrm{CuO}_{4-\delta}$ (NCCO). NCCO has an antiferromagnetic ground state when synthesized, and only becomes superconducting after a reductive annealing process. This makes NCCO an ideal template to study how the magnetism differs in the superconducting and non-superconducting state, while keeping the material template as constant as possible. Using neutron spectroscopy, we reveal that the as-grown crystal exhibits a large spin pseudogap in the magnetic fluctuation spectrum. Upon annealing, defects that are introduced by the commonly employed synthesis method are removed and the spin pseudogap is significantly reduced. While the spin pseudogap in the annealed sample is likely an effect of superconductivity, we argue that the spin pseudogap in the as-grown sample is caused by the absence of long-wavelength spin waves. The defects in as-grown NCCO thus play the dual role of suppressing both superconductivity and low-energy spin waves, highlighting a potential connection between these two phenomena.