Dimerization-induced enhancement of the spin gap in the quarter-filled two-leg rectangular ladder

TL;DR

This study uses density-matrix renormalization group calculations to show that bond-dimerization and electron interactions significantly enhance the spin gap in a quarter-filled two-leg ladder, with implications for organic charge-transfer solids.

Contribution

It demonstrates how bond-dimerization and electron-electron interactions influence the spin gap in a quarter-filled ladder, highlighting conditions for large gaps similar to experimental observations.

Findings

Dimerization along ladder legs enhances the spin gap.

Electron-electron interactions further increase the spin gap.

Large spin gaps occur at high dimerization and rung hopping levels.

Abstract

We report density-matrix renormalization group calculations of spin gaps in the quarter-filled correlated two-leg rectangular ladder with bond-dimerization along the legs of the ladder. In the small rung-coupling region, dimerization along the leg bonds can lead to large enhancement of the spin gap. Electron-electron interactions further enhance the spin gap, which is nonzero for all values of the rung electron hopping and for arbitrarily small bond-dimerization. Very large spin gaps, as are found experimentally in quarter-filled band organic charge-transfer solids with coupled pairs of quasi-one-dimensional stacks, however, occur within the model only for large dimerization and rung electron hopping that are nearly equal to the hopping along the legs. Coexistence of charge order and spin gap is also possible within the model for not too large intersite Coulomb interaction.