Quantum phase transition of an organic spin liquid tuned by mixing counterions

We found a plateau in the magnitude of the isolated magnetic moments as a function of the anisotropy of the transfer integral (t^'/t) in the gapless quantum spin liquid (QSL) phase of an $S=1/2$ triangular lattice molecular solid X[Pd(dmit)₂]₂, accomplished by a fine tuning of t^'/t through the mixing of cations, X. In contrast, the magnetic susceptibility at the lowest temperature in the QSL phase parametrized by t^'/t evinces an unconventional suppression approaching the quantum phase transition, implying significant critical fluctuations.