Correlations and Renormalization of the Electron-Phonon Coupling in the Honeycomb Hubbard Ladder and Superconductivity in Polyacene

TL;DR

This study uses DMRG to analyze the Hubbard model on a honeycomb ladder, revealing a quadratic band touching and suggesting that lightly doped polyacenes could exhibit high-temperature superconductivity driven by electron-phonon interactions.

Contribution

It provides new insights into the electron-phonon coupling and superconductivity potential in honeycomb ladder systems and polyacenes, highlighting the role of band structure and doping.

Findings

Quadratic band touching at half filling in the Hubbard model.

No pair-binding suggests limited superconductivity from electron-electron interactions.

Enhanced electron-phonon coupling indicates potential for high-temperature superconductivity in doped polyacenes.

Abstract

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. %SAK as a function of a third-neighbor hopping parameter. For one electron per site, non-zero $U$ 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.