Seminars, SNBNCBS

Antiferromagnetism proximate to superconductivity is common between the high T_c cuprates and superconducting organic charge-transfer solids. In both cases this is considered to be a signature of strong Coulomb repulsion between the carriers. In the case of the charge-transfer solids it has been shown that the antiferromagnetism coexists with other spatial broken symmetries including charge-ordering. We develop a theory of these coexisting broken symmetries, and show that the coexistence is a consequence of the co-operative effects of electron-electron and electron-phonon interactions in these systems. The charge-ordered state is similar to the Wigner crystal, except that lattice sites of the Wigner crystal are occupied pairs of electrons. Whether or not this paired nature of the semiconducting state is a precursor to superconductivity is an intriguing question.