# A novel undamped gapless plasmon mode in tilted type-II Dirac semimetal

## TL;DR

This paper predicts a new, long-lived gapless plasmon mode in tilted type-II Dirac semimetals, arising from out-of-phase density oscillations, protected from damping by the material's unique Fermi surface structure.

## Contribution

It introduces a novel gapless plasmon mode in type-II Dirac semimetals, which is protected from damping due to the open Fermi surface topology.

## Key findings

- Existence of a long-lived gapless plasmon mode in type-II DSMs.
- The mode arises from out-of-phase oscillations of electron and hole pockets.
- The mode is protected from Landau damping by the particle-hole continuum gap.

## Abstract

We predict the existence of a novel long-lived gapless plasmon mode in a type-II Dirac semimetal (DSM). This gapless mode arises from the out-of-phase oscillations of the density fluctuations in the electron and the hole pockets of a type-II DSM. It originates beyond a critical wave-vector along the direction of the tilt axis, owing to the momentum separation of the electron and hole pockets. A similar out-of-phase plasmon mode arises in other multi-component charged fluids as well, but generally it is Landau damped and lies within the particle-hole continuum. In the case of a type-II DSM, the open Fermi surface prohibits low-energy finite momentum single-particle excitations, creating a `gap' in the particle-hole continuum. The gapless plasmon mode lies within this particle-hole continuum gap and, thus, it is protected from Landau damping.