The Paired Electron Crystal: order from frustration in the quarter-filled band

TL;DR

This paper investigates how charge and spin frustration in a quarter-filled band on an anisotropic triangular lattice lead to a novel paired-electron crystal phase, with implications for understanding unconventional superconductivity in organic and other correlated materials.

Contribution

It introduces the spin-singlet paired-electron crystal (PEC) as a new phase arising from frustration, extending the understanding of charge and spin order in quarter-filled systems.

Findings

Identification of the PEC as a dominant phase under strong frustration.

Phase diagram showing transitions between antiferromagnetic, Wigner crystal, and PEC states.

Discussion of potential mobility of paired electrons leading to a liquid state.

Abstract

We present a study of the effects of simultaneous charge- and spin-frustration on the two-dimensional strongly correlated quarter-filled band on an anisotropic triangular lattice. 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 electron-phonon 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 kappa-(BEDT-TTF)_2Cu_2(CN)_3 and EtMe_3Sb[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.