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

The occurrence of antiferromagnetism in kappa-(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 antiferromagnetism, 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 state coexisting 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 kappa-(ET)_2Cu_2(CN)_3 and beta'-EtMe_3Sb[Pd(dmit)_2]_2 which we believe are PECs at low temperatures.