Anomalous nodal count and singularities in the dispersion relation of honeycomb graphs

TL;DR

This paper investigates the anomalous nodal count in honeycomb and bi-dendral graphs, revealing that their dispersion relations have singularities and conical points, which are linked to the graphs' spectral properties and symmetry features.

Contribution

It establishes a connection between anomalous nodal counts and singularities in the dispersion relation, identifying conditions for critical points and the presence of conical points in less symmetric graphs.

Findings

Nodal surplus is never 0 or Betti number, indicating an anomaly.

Extrema in the dispersion relation correspond to singularities in the maximal abelian cover.

Conical points are present even in graphs with less symmetry than previously known.

Abstract

We study the nodal count of the so-called bi-dendral graphs and show that it exhibits an anomaly: the nodal surplus is never equal to 0 or $\beta$, the first Betti number of the graph. According to the nodal-magnetic theorem, this means that bands of the magnetic spectrum (dispersion relation) of such graphs do not have maxima or minima at the "usual" symmetry points of the fundamental domain of the reciprocal space of magnetic parameters. In search of the missing extrema we prove a necessary condition for a smooth critical point to happen inside the reciprocal fundamental domain. Using this condition, we identify the extrema as the singularities in the dispersion relation of the maximal abelian cover of the graph (the honeycomb graph being an important example). In particular, our results show that the anomalous nodal count is an indication of the presence of the conical points in the dispersion relation of the maximal universal cover. Also, we discover that the conical points are present in the dispersion relation of graphs with much less symmetry than was required in previous investigations.