Robust quantum spin liquid state in hydrogen-bonded organic Mott insulators

κ-H₃(Cat-EDT-TTF)₂ is a hydrogen-bonded organic Mott insulator that provides a new class of quantum spin liquids (QSLs), where the strong coupling between the localized spins and the hydrogen atoms leads to a quantum paramagnetic and quantum paraelectric (QPE) state. Although this material has a 2D spin-1/2 triangular lattice, its anisotropy parameter t'/t = 1.25 is far from unity. This raises a question as to whether the geometric frustration of the tranigular lattice is an important factor for realizing the QSL state in this system. Here, we investigate a series of κ-H₃(Cat-X)₂ (X = EDT-TTF, EDT-ST, EDT-d4-TTF, and EDSe-TTF), where the substitution of X affects the anisotropy of the triangular lattice as well as the hydrogen-bond dynamics. Our dielectric and thermal-transport measurements reveal that all the materials exhibit a QSL and QPE state in spite of the large t'/t (for instance, t'/t = 1.84 for X = EDSe-TTF), indicating that the coupling between the π electrons and the hydrogen atoms plays an important role for stabilizing the QSL state. We also find that the QPE behavior is strongly enhaced in X = EDT-ST that is located near a regime where the hydrogen atoms are localized at low temperatures, suggesting the presence of a QCP related to the hydrogen-bond dynamics.