Bond-ordered states and $f$-wave pairing of spinless fermions on the honeycomb lattice

TL;DR

This paper investigates the phase diagram of spinless fermions on the honeycomb lattice beyond half-filling, revealing charge-density wave, bond-order, and novel $f$-wave superconducting phases through functional renormalization group analysis.

Contribution

It introduces a comprehensive analysis of many-body instabilities away from half-filling, identifying $f$-wave superconductivity as a new emergent phase.

Findings

Charge-density wave instability dominates at large interactions.

Bond-order phase appears at van-Hove filling due to Fermi surface nesting.

$f$-wave superconductivity emerges at lower fillings from competing interactions.

Abstract

Spinless fermions on the honeycomb lattice with repulsive nearest-neighbor interactions are known to harbour a quantum critical point at half-filling, with critical behaviour in the Gross-Neveu (chiral Ising) universality class. The critical interaction strength separates a weak-coupling semimetallic regime from a commensurate charge-density-wave phase. The phase diagram of this basic model of correlated fermions on the honeycomb lattice beyond half-filling is, however, less well established. Here, we perform an analysis of its many-body instabilities using the functional renormalization group method with a basic Fermi surface patching scheme, which allows us to treat instabilities in competing channels on equal footing also away from half-filling. Between half-filling and the van-Hove filling, the free Fermi surface is hole-like and we again find a charge-density wave instability to be dominant at large interactions. Moreover, its characteristics are those of the half-filled case. Directly at the van-Hove filling the nesting property of the free Fermi surface stabilizes a dimerized bond-order phase. At lower filling the free Fermi surface becomes electron-like and a superconducting instability with $f$-wave symmetry is found to emerge from the interplay of intra-unitcell repulsion and collective fluctuations in the proximity to the charge-density wave instability. We estimate the extent of the various phases and extract the corresponding order parameters from the effective low-energy Hamiltonians.