# Chiral second-sound collective mode at the edge of 2D systems with nontrivial Berry curvature

## TL;DR

This paper predicts the existence of chiral second-sound edge modes in 2D systems with nontrivial Berry curvature, such as gapped graphene and topological insulators, leading to ballistic heat transport along edges.

## Contribution

It introduces the concept of chiral second-sound collective modes localized at the edges of 2D systems with Berry curvature, highlighting their topological origin and thermal transport properties.

## Key findings

- Chiral second-sound modes propagate along system edges.
- Edge mode localization length has topological origin.
- Thermal Hall conductance shows a sharp transition indicating ballistic heat transport.

## Abstract

We study the thermal transport in two-dimensional systems with a nontrivial Berry curvature texture. The physical realizations are many: for a sake of definiteness we consider undoped graphene gapped by the presence of an aligned hexagonal-Boron-Nitride substrate. The same phenomenology applies, i.e., to surface states of 3D topological insulators in the presence of a uniform magnetization. We find that chiral valley-polarized second-sound collective modes propagate along the edges of the system. The localization length of the edge modes has topological origin stemming from the anomalous velocity term in the quasiparticle current. At low temperature, the single-particle contribution to the transverse thermal conductance is exponentially suppressed and only second-sound modes carry heat along the boundary. A sharp change in the behavior of the thermal Hall conductance, extracted from nonlocal measurements of the temperature along the edge, marks the onset of ballistic heat transport due to second-sound edge modes.