  A quarter of a century after their discovery the mechanism that pairs
carriers in the cuprate high-Tc superconductors (HTS) still remains uncertain.
Despite this the general consensus is that it is probably magnetic in origin
[1] so that the energy scale for the pairing boson is governed by J, the
antiferromagnetic exchange interaction. Recent studies using resonant inelastic
X-ray scattering strongly support these ideas [2]. Here as a further test we
vary J (as measured by two-magnon Raman scattering) by more than 60% by
changing ion sizes in the model HTS system LnA2Cu3O7-{\delta} where A=(Ba,Sr)
and Ln=(La, Nd, Sm, Eu, Gd, Dy, Yb, Lu). Such changes are often referred to as
"internal" pressure. Surprisingly, we find Tcmax anticorrelates with J where
internal pressure is the implicit variable. This is the opposite to the effect
of external pressure and suggests that J is not the dominant energy scale
governing Tcmax.
