Charge and spin interplay in a molecular-dimer-based organic Mott insulator

Triangular lattice quasi-two-dimensional Mott insulators based on the bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) molecule and its analogies present a possibility to produce exotic phases by coupling charge and spin degrees of freedom. In this work we discuss magnetic properties of one such material, kappa-(BEDT-TTF)(2)Hg(SCN)(2)Cl, which is found at the border of the phase transition between a Mott insulator into a charge ordered state. Our magnetic susceptibility and cantilever magnetization measurements demonstrate how the charge degree of freedom defines magnetic properties for few different charge phases observed in this material as a function of temperature. Between T-CO = 30 K and T-S = 24 K we observe charge and spin separation due to one-dimensional charge stripes formed in this material below T-CO = 30 K. Below T-S = 24 K charge and spin degrees of freedom demonstrate coupling. Spin-singlet correlations develop below 24 K, however, the melting of charge order below 15 K prevents the spin-singlet-state formation, leaving the system in the inhomogeneous state with charge ordered spin-singlet domains and charge and spin fluctuating ones.