Surface Collective Modes in the Topological InsulatorsBi2Se3and

Abstract

We used low-energy, momentum-resolved inelastic electron scattering to study surface collective modes of the three-dimensional topological insulators Bi_{2}Se_{3} and Bi_{0.5}Sb_{1.5}Te_{3-x}Se_{x}. Our goal was to identify the "spin plasmon" predicted by Raghu and co-workers [Phys. Rev. Lett. 104, 116401 (2010)]. Instead, we found that the primary collective mode is a surface plasmon arising from the bulk, free carriers in these materials. This excitation dominates the spectral weight in the bosonic function of the surface χ^{"}(q,ω) at THz energy scales, and is the most likely origin of a quasiparticle dispersion kink observed in previous photoemission experiments. Our study suggests that the spin plasmon may mix with this other surface mode, calling for a more nuanced understanding of optical experiments in which the spin plasmon is reported to play a role.

Chiral Spin Mode on the Surface of a Topological Insulator

“…An essential aspect of this physics is an interplay between the Coulomb interaction and SOC, which is expected to give rise to a new type of collective spin excitations -chiral spin waves [21][22][23][24][25][26]. In the long wavelength (q = 0) limit, these modes are completely decoupled from the charge channel and thus distinct from spin-plasmons [27,28], Dirac plasmons [29,30], and surface plasmons in TIs [30,31].In this Letter, we employ polarization-resolved resonant Raman spectroscopy, a technique of choice for probing the collective charge [32,33], spin [34][35][36][37] and orbital excitations [38], to study collective spin excitations of the chiral surface states in Bi 2 Se 3 . To enhance the signal from the surface states, we tune the energy of incoming photons in resonance with a transition between two sets of chiral surface states: near the Fermi energy and about 1.8 eV above it [ Fig.…”

Plasmons in topological insulator cylindrical nanowires

“…We have also studied the effects of q || = 0 analyzing charge excitations where the angular degrees of freedom are more involved. It could be interesting to consider not only the presence of Dirac surface plasmons but also of other plasmon excitations 42 , in particular of surface plasmons 43 related to bulk charge carriers which have been observed very recently in EELS experiments 44,45 . Finally, it could be useful to study plasmon features at finite temperatures analyzing the effects of electronphonon couplings 46,47 , in particular, in the case of wires freely suspended or grown on a substrate 48 .…”

Optoelectronic devices, plasmonics, and photonics with topological insulators

“…The influence of spin-plasmons has been invoked to be crucial in the coupling of Dirac-cone electrons of TIs with phonons. 76 However, attempts to directly probe spin-plasmons by means of electron energy loss spectroscopy have been unsuccessful, 60 since the predominating mode in the plasmonic spectrum is a surface plasmon arising from the bulk, free carriers. 52,60 This mode hybridizes with the spin-plasmon for momenta far from the Γ point of the Brillouin zone.…”

“…76 However, attempts to directly probe spin-plasmons by means of electron energy loss spectroscopy have been unsuccessful, 60 since the predominating mode in the plasmonic spectrum is a surface plasmon arising from the bulk, free carriers. 52,60 This mode hybridizes with the spin-plasmon for momenta far from the Γ point of the Brillouin zone. 52 To selectively excite the spin-plasmon, a mechanism exploiting optical spin injection has been proposed by Raghu et al 74 (Figure 2).…”

“…An extension of the momentum range up to ∼0.3 Å 1 has been achieved by exciting the Dirac plasmon with probing electrons. 52,60 Successively, the plasmonic response of ring structures patterned in Bi 2 Se 3 films has been studied by THz spectroscopy. 61 The rings exhibit a bonding plasmon mode and an antibonding plasmon mode, whose frequency can be tuned by changing their diameter.…”