Quantum Phase Transition of 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 (dmit = 1,3-dithiole-2-thione-4,5-dithiolate), 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.