LAUSR

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 κ-[(BEDSeTTF)x(BEDT-TTF)1−x]2Cu[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.