Plasmons in Topological Insulators

Details Plasmons in Topological Insulators Plasmons in Topological Insulators

2012-01-11

arxiv.org/abs/1201.2359v1

Physics

Condensed Matter

Materials Science

Scientific paper

We calculated the edge-plasmon excitation spectrum within the anti-crossing bulk bandgap region for an inverted $\texttt{HgTe/CdTe}$ quantum well by employing the Bernevig, Hugues and Zhang (BHZ) model within the random-phase approximation (RPA). Our proposed model system consists of a single component electron helical liquid (HL) in a semi-infinite quantum well, in which a topological state that is localized around the system edge can exist. With linearly-polarized incident light as a perturbation to such a single component HL, the unique charge dynamics for collective excitation of these edge-bound electrons with broken time-reversal symmetry is investigated. The plasmon dispersion $\omega_p(q)$ of the single component HL has the form $\omega_p(q)\sim-\omega_0q\,\texttt{ln} (qa)$ in the long-wavelength limit, which is in sharp contrast with $\omega_p(q)\sim-\omega_0 \sqrt{- \texttt{ln} (q W)}$ for a one-dimensional electron gas (1DEG) in a quantum-wire system. Here, the plasmon wave number in a conventional 1DEG is scaled with a characteristic width $W$, while that in our model system is scaled with a lattice constant $a$. Similar to interband plasmons in a metallic armchair graphene nanoribbon, $\omega_0$ is found to be independent of the linear electron density for the intraband plasmon in our system. Besides the spin factor of two and $W$ scaling for the wave number, the dispersion relation for the collective excitation of the two component HL is the same as that in an armchair graphene nanoribbon. The particle-hole excitation region in our system is found to be collapsed into a straight line, instead of a wide region for a conventional 1DEG. The plasmon energies of the two and single-component HL are spectrally separated although the same particle-hole excitation region is shared by both of them.

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