Plasmons in topological insulator cylindrical nanowires

We present a theoretical analysis of Dirac magnetoplasmons 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, interband damped plasmonlike excitations form at the surface and survive at low electron surface dopings (~1010cm-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 magnetoplasmon 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 interband to sharp intraband magnetoplasmons, which, as expected for large radii and dopings (~1012cm-2), approach the proper limit of a two-dimensional surface.