  Although recent experiments and theories have shown a variety of exotic
transport properties of non-equilibrium quasiparticles (QPs) in superconductor
(SC)-based devices with either Zeeman or exchange spin-splitting, how QP
interplays with magnon spin currents remains elusive. Here, using non-local
magnon spin-transport devices where a singlet SC (Nb) on top of a ferrimagnetic
insulator (Y3Fe5O12) serves as a magnon spin detector, we demonstrate that the
conversion efficiency of magnon spin to QP charge via inverse spin-Hall effect
(iSHE) in such an exchange-spin-split SC can be greatly enhanced by up to 3
orders of magnitude compared with that in the normal state, particularly when
its interface superconducting gap matches the magnon spin accumulation. Through
systematic measurements with varying the current density and SC thickness, we
identify that superconducting coherence peaks and exchange spin-splitting of
the QP density-of-states, yielding a larger spin excitation while retaining a
modest QP charge-imbalance relaxation, are responsible for the giant QP iSHE.
The latter exchange-field-modified QP relaxation is experimentally proved by
spatially resolved measurements with varying the separation of electrical
contacts on the spin-split Nb.
