  The pairing of interacting fermions leading to superfluidity has two limiting
regimes: the Bardeen-Cooper-Schrieffer (BCS) scheme for weakly interacting
degenerate fermions and the Bose-Einstein condensation (BEC) of bosonic pairs
of strongly interacting fermions. While the superconductivity that emerges in
most metallic systems is the BCS-like electron pairing, strongly correlated
electrons with poor Fermi liquidity can condense into the unconventional
BEC-like pairs. Quantum spin liquids harbor extraordinary spin correlation free
from order and the superconductivity that possibly emerges by carrier doping of
the spin liquids is expected to have a peculiar pairing nature. The present
study experimentally explores the nature of the pairing condensate in a doped
spin-liquid candidate material and under varying pressure, which changes the
electron-electron Coulombic interactions across the Mott critical value in the
system. The transport measurements reveal that the superconductivity at low
pressures is a BEC-like condensate from a non-Fermi liquid and crosses over to
a BCS-like condensate from a Fermi liquid at high pressures. The Nernst-effect
measurements distinctively illustrate the two regimes of the pairing in terms
of its robustness to the magnetic field. The present Mott tuning of the BEC-BCS
crossover can be compared to the Feshbach tuning of the BEC-BCS crossover of
fermionic cold atoms.
