Paramagnons and high-temperature superconductivity in a model family of cuprates

Cuprate superconductors have the highest critical temperatures (Tc) at ambient pressure, yet
a consensus on the superconducting mechanism remains to be established. Finding an
empirical parameter that limits the highest reachable Tc can provide crucial insight into this
outstanding problem. Here, in the first two Ruddlesden-Popper members of the model Hg-
family of cuprates, which are chemically nearly identical and have the highest Tc among all
cuprate families, we use inelastic photon scattering to reveal that the energy of magnetic
fluctuations may play such a role. In particular, we observe the single-paramagnon spectra to
be nearly identical between the two compounds, apart from an energy scale difference of
~30% which matches their difference in Tc. The empirical correlation between paramagnon
energy and maximal Tc is further found to extend to other cuprate families with relatively high
Tc’s, hinting at a fundamental connection between them.