Surface Collective Modes in the Topological Insulators Bi_{2}Se_{3} and Bi_{0.5}Sb_{1.5}Te_{3-x}Se_{x}

Area of Science:

• Condensed Matter Physics
• Materials Science
• Quantum Materials

Background:

• Topological insulators (TIs) possess unique electronic properties.
• The existence of a "spin plasmon" in TIs has been theoretically predicted.
• Understanding surface collective modes is crucial for TI applications.

Purpose of the Study:

• To investigate surface collective modes in 3D topological insulators Bi$_{2}$Se$_{3}$ and Bi$_{0.5}$Sb$_{1.5}$Te$_{3-x}$Se$_{x}$.
• To experimentally identify the predicted spin plasmon excitation.
• To clarify the origin of spectral weight features in TIs.

Main Methods:

• Low-energy, momentum-resolved inelastic electron scattering.
• Spectroscopic analysis of the bosonic surface function $\chi^{''}(q, \omega)$.
• Terahertz (THz) energy scale investigations.

Main Results:

• The dominant collective mode observed was a surface plasmon, not the spin plasmon.
• This surface plasmon originates from bulk free carriers.
• The surface plasmon accounts for spectral weight at THz frequencies and explains quasiparticle dispersion kinks.

Conclusions:

• The primary surface collective mode in these TIs is a bulk-derived surface plasmon.
• The spin plasmon may interact with or be masked by this dominant surface plasmon.
• A revised understanding of optical experiments involving spin plasmons is warranted.