We have performed extensive density matrix renormalization group (DMRG) studies of the Hubbard model on a honeycomb ladder. The band structure (with Hubbard $U=0$) exhibits an unusual quadratic band touching at half-filling, which is associated with a quantum Lifshitz transition from a band insulator to a metal. For one electron per site, nonzero $U$ drives the system into an insulating state in which there is no pair-binding between added electrons; this implies that superconductivity driven directly by the repulsive electron-electron interactions is unlikely in the regime of small doping, $x\ll 1$. However, the divergent density of states as $x\to 0$, the large values of the phonon frequencies, and an unusual correlation induced enhancement of the electron-phonon coupling imply that lightly doped polyacenes, which approximately realize this structure, are good candidates for high-temperature electron-phonon driven superconductivity.

• Received 29 July 2013

DOI:https://doi.org/10.1103/PhysRevB.88.224512