  We investigate the continuous variation of electronic states from a Fermi
liquid to a quantum spin liquid induced by chemical substitution in the organic
dimer-Mott system of
$\kappa$-[(BEDSe-TTF)$_x$(BEDT-TTF)$_{1-x}$]$_2$Cu[N(CN)$_2$]Br. Electrical
transport measurements reveal that the mixing of BEDSe-TTF into the BEDT-TTF
layers induces a quantum Mott transition around $x$=0.10. Although a charge gap
disappears at this point, magnetic susceptibility and heat capacity
measurements indicate that fermionic low-energy excitations remain in the
insulating salts, suggesting that fermionic spin excitations persist. We
propose that the transition can be classified into a genuine Mott transition.
