The Paired-Electron Crystal in the Two-Dimensional Frustrated Quarter-Filled Band

Clay, R. T., Dayal, S., Li, H., & Mazumdar, S. (2010). The Paired-Electron Crystal in the Two-Dimensional Frustrated Quarter-Filled Band. Bulletin of the American Physical Society. 2010 APS March Meeting, Portland, OR: American Physical Society.

Abstract

The competition between antiferromagnetic (AFM) and spin-singlet ground states within quantum spin models and the 1/2-filled band Hubbard model has received intense scrutiny. Using exact diagonalization and path integral renormalization group (PIRG) calculations we demonstrate a frustration-induced transition from Neel AFM to spin-singlet in the interacting 1/4-filled band on an anisotropic triangular lattice. While the AFM state has equal charge densities 0.5 on all sites, the spin-singlet state is a paired-electron crystal (PEC), with pairs of charge-rich sites separated by pairs of charge-poor sites. The PEC provides a natural description of the spin-gapped state proximate to superconductivity (SC) in many organic charge-transfer solids. Our theory explains the semiconducting behavior of Na_xCoO_2 at x=0.5, and also applies to spinels isoelectronic with superconducting LiTi_2O_4 and CuRh_2S_4. We discuss recent experimental results in the light of our theory. Pressure-induced SC in these correlated-electron systems is likely a transition from the 1/4-filled band valence bond solid to a valence bond liquid.