Granular Superconductor in a honeycomb lattice as a realization of Bosonic Dirac Material

TL;DR

This paper demonstrates that a two-dimensional granular superconductor arranged in a honeycomb lattice hosts bosonic Dirac nodes in its collective excitation spectrum, expanding the concept of Dirac materials to bosonic systems.

Contribution

It introduces the concept of Bosonic Dirac Materials by showing Dirac nodes in bosonic excitations of a granular superconductor on a honeycomb lattice, with analysis of mode behavior and symmetry breaking effects.

Findings

Dirac nodes appear in bosonic collective modes at K and K points.

Dirac nodes are preserved despite inter-grain interactions.

Breaking sublattice symmetry opens a gap at the Dirac point.

Abstract

We examine the low energy effective theory of phase oscillations in a two-dimensional granular superconducting sheet where the grains are arranged in honeycomb lattice structure. Using the example of graphene we present the evidence for the engineered Dirac nodes in the bosonic excitations: the spectra of the collective bosonic modes cross at the K and K points in the Brillouin zone and form Dirac nodes. We show how two different types of collective phase oscillations are obtained and that they are analogous to the Leggett and the Bogoliubov-Anderson-Gorkov modes in a two-band superconductor. We show that the Dirac node is preserved in the presence of an inter-grain interaction, despite induced changes of the qualitative features of the two collective modes. Finally, breaking the sublattice symmetry by choosing different on-site potentials for the two sublattices leads to a gap opening near the Dirac node, in analogy with Fermionic Dirac materials. Dirac node dispersion of bosonic excitations is thus expanding the discussion of the conventional Dirac cone excitations to the case of bosons. We call this case as a representative of Bosonic Dirac Materials (BDM), similar to the case of Fermionic Dirac materials extensively discussed in the literature.