Beyond the quantum spin liquid concept in frustrated two dimensional organic superconductors

The occurrence of antiferromagnetism (AFM) in κ-(ET)₂X can be understood within an effective 1/2-filled band with dimers of ET molecules containing one hole each. We argue that while this effective model can describe the presence of AFM, a complete description for these materials requires the correct carrier density of one-half per molecule. For dimerized and strongly frustrated 1/4-filled lattices we show that a singlet-paired statecoexisting with charge ordering occurs that we have termed the paired electron crystal (PEC). Here we investigate the 1/4-filled model on a dimerized lattice, showing regions where AFM, PEC, and the Wigner-crystal occur. We point out the need to go beyond quantum spin liquid concepts for highly frustrated materials such as κ-(ET)₂Cu₂(CN)₃ and β′-EtMe₃Sb[Pd(dmit)₂]₂ which we believe are PECs at low temperatures.