Charge excitations across a superconductor-insulator transition

TL;DR

This study investigates the charge excitations and universality class of the superconductor-insulator transition in an attractive honeycomb Hubbard model, revealing a crossover from bosonic to fermionic excitations and confirming the 3D-XY universality class.

Contribution

It demonstrates the nature of charge excitations across the SIT in a honeycomb lattice and shows the universality of the transition class across different lattice geometries.

Findings

Charge excitations are bosonic across the transition.

Increasing staggered potential causes a crossover to fermionic excitations.

The transition belongs to the 3D-XY universality class.

Abstract

We study the superconductor-insulator transition (SIT) in the ground state of the attractive honeycomb Hubbard model in the presence of a staggered potential (a mass term), using a combination of unbiased computational methods, namely, exact diagonalization and quantum Monte Carlo simulations. We probe the nature of the lowest-energy charge excitations across the SIT and show that they are bosonic, as inferred (and shown in the strongly interacting regime) in a previous study of the same model in the square lattice. Increasing the strength of the staggered potential leads to a crossover in which bosonic low-energy excitations give way to fermionic ones within the insulating phase. We also show that the SIT belongs to the 3$d$-XY universality class, like in its square lattice counterpart. The robustness of our results in these two lattice geometries supports the expectation that our findings are universal for SITs in clean systems.