  Iron pnictides are the only known family of unconventional high-temperature
superconductors besides cuprates. Until recently, it was widely accepted that
superconductivity is spin-fluctuation driven and intimately related to their
fermiology, specifically, hole and electron pockets separated by the same wave
vector that characterizes the dominant spin fluctuations, and supporting order
parameters (OP) of opposite signs. This picture was questioned after the
discovery of a new family, based on the FeSe layers, either intercalated or in
the monolayer form. The critical temperatures there reach ~40 K, the same as in
optimally doped bulk FeSe - despite the fact that intercalation removes the
hole pockets from the Fermi level and, seemingly, undermines the basis for the
spin-fluctuation theory and the idea of a sign-changing OP. In this paper,
using the recently proposed phase-sensitive quasiparticle interference
technique, we show that in LiOH intercalated FeSe compound the OP does change
sign, albeit within the electronic pockets, and not between the hole and
electron ones. This result unifies the pairing mechanism of iron based
superconductors with or without the hole Fermi pockets and supports the
conclusion that spin fluctuations play the key role in electron pairing.
