# Plasmons in topological insulator cylindrical nanowires

## TL;DR

This paper provides a theoretical analysis of Dirac magneto-plasmons in topological insulator nanowires, revealing how geometry, doping, and magnetic fields influence plasmon behavior and spectral properties.

## Contribution

It offers an analytical model of surface plasmons in topological insulator nanowires considering Berry phase effects and external magnetic fields, advancing understanding of their dynamic structure factors.

## Key findings

- Surface plasmons exist at low doping levels and are sensitive to magnetic field-induced gap closure.
- Magnetic fields induce a shift to lower energies and sharper peaks in plasmon spectra.
- Increased doping causes a transition from damped inter-band to sharp intra-band magneto-plasmons.

## Abstract

We present a theoretical analysis of Dirac magneto-plasmons in topological insulator nanowires. We discuss a cylindrical geometry where Berry phase effects induce the opening of a gap at the neutrality point. By taking into account surface electron wave functions introduced in previous papers and within the random phase approximation, we provide an analytical form of the dynamic structure factor. Dispersions and spectral weights of Dirac plasmons are studied with varying the radius of the cylinder, the surface doping, and the strength of an external magnetic field. We show that, at zero surface doping, inter-band damped plasmon-like excitations form at the surface and survive at low electron surface dopings ($\sim 10^{10} cm^{-2} $). Then, we point out that the plasmon excitations are sensitive to the Berry phase gap closure when an external magnetic field close to half quantum flux is introduced. Indeed, a well-defined magneto-plasmon peak is observed at lower energies upon the application of the magnetic field. Finally, the increase of the surface doping induces a crossover from damped inter-band to sharp intra-band magneto-plasmons which, as expected for large radii and dopings ($\sim 10^{12} cm^{-2}$), approach the proper limit of a two-dimensional surface.