Paired electron crystal: Order from frustration in the quarter-filled band

Abstract

We present a study of the effects of simultaneous charge-and spin-frustration on the twodimensional strongly correlated quarter-filled band on an anisotropic triangular lattice. Our conclusions are based on exact diagonalization studies that include electron-electron interactions as well as adiabatic electron-phonon coupling terms treated self-consistently. The broken-symmetry states that dominate in the weakly frustrated region near the rectangular lattice limit are the well known antiferromagnetic state with in-phase lattice dimerization along one direction, and the Wigner crystal state with the checkerboard charge order. For moderate to strong frustration, however, the dominant phase is a novel spin-singlet paired-electron crystal (PEC), consisting of pairs of charge-rich sites separated by pairs of charge-poor sites. The PEC, with coexisting charge-order and spin-gap in two dimension, is the quarter-filled band equivalent of the valence bond solid (VBS) that can appear in the frustrated half-filled band within antiferromagnetic spin Hamiltonians. We discuss the phase diagram as a function of on-site and intersite Coulomb interactions as well as electronphonon coupling strength. We speculate that the spin-bonded pairs of the PEC can become mobile for even stronger frustration, giving rise to a paired-electron liquid. We discuss the implications of the PEC concept for understanding several classes of quarter-filled band materials that display unconventional superconductivity, focusing in particular on organic charge transfer solids. Our work points out the need to go beyond quantum spin liquid (QSL) concepts for highly frustrated organic charge-transfer solids such as κ-(BEDT-TTF)2Cu2(CN)3 and EtMe3Sb[Pd(dmit)2]2, which we believe show frustration-induced charge disproportionation at low temperatures. We discuss possible application to layered cobaltates and 1 4 -filled band spinels.

Coulomb enhancement of superconducting pair-pair correlations in a34-filled model forκ−(BEDT-TTF)2X

“…Enhanced pair-pair correlations originate from the strong tendency to spin-singlet coupling at ρ = 0.5 and 1.5, both because of the existence of a commensurate PEC at these densities 33,34 , and because the stabilization due to the kinetic energy gain from pair motion is highest at these carrier concentrations. Consideration of nearest neighbor spin-bonded sites as effective "negative U" centers has a long history, especially in the context of bipolaron theories of SC 70 .…”

“…Several of the more complicated κ materials such as κ-(ET) 4 69 (M = Co, Fe and Cr) are charge-ordered. Charge-ordering in the last group of materials is accompanied by spin-singlet formation and is apparently driven by strong interdimer coupling 67,68 , which is also the condition for PEC formation 33,34 . Taken together, these results suggest that even as AFM spin-spin correlations in κ-ET are significant, these systems are at the threshold of transitions to proximate broken symmetries that include the PEC as well as SC.…”

“…As mentioned above, previous works have shown (i) suppression of pair-pair correlations within the effective ρ = 1 model [17][18][19][20] , and (ii) the possibility of a fluctuating CO within models focusing on the monomer molecules 33,34,[36][37][38] . We therefore construct pair creation operators that allow unequal charge densities on the monomer molecules that constitute a dimer in Fig.…”

“…Given that SC proximate to CO is also seen in (ET) 2 X with crystal structures different from κ [29][30][31][32] , it is natural to ask whether the mechanisms of SC in all (ET) 2 X are actually related, and whether even in the κ-ET family there exists a hidden competing CO state. Uniquely for the case of carrier density precisely 0.5 or 1.5 per site, we have shown that there can indeed occur a spin-paired CO state, a paired-electron crystal (PEC), in frustrated lattices 33,34 . The PEC is a Wigner crystal of spin-bonded pairs, rather than of single electrons 35 .…”

“…The PEC is a Wigner crystal of spin-bonded pairs, rather than of single electrons 35 . In the κ materials, the PEC would require unequal charge densities on the monomer molecules that constitute each ET dimer, with interdimer spin-pairing between monomers with large charge densities 33,34 . Other groups have also proposed related theories of fluctuating intradimer charge disproportionation [36][37][38] , in order to explain the peculiar dielectric responses of κ-CN 39 and κ-Cl 40 .…”