Distinguishing Bulk and Surface Electron-Phonon Coupling in the Topological InsulatorBi2Se3Using Time-Resolved Photoemission Spectroscopy

Abstract

We report time-and angle-resolved photoemission spectroscopy measurements on the topological insulator Bi2Se3. We observe oscillatory modulations of the electronic structure of both the bulk and surface states at a frequency of 2.23 THz due to coherent excitation of an A1g phonon mode. A distinct, additional frequency of 2.05 THz is observed in the surface state only. The lower phonon frequency at the surface is attributed to the termination of the crystal and thus reduction of interlayer van der Waals forces, which serve as restorative forces for out-of-plane lattice distortions. DFT calculations quantitatively reproduce the magnitude of the surface phonon softening. These results represent the first band-resolved evidence of the A1g phonon mode coupling to the surface state in a topological insulator.PACS numbers: 78.47. J-, 73.20.-r, 63.20.kd, 79.60.-i Topological insulators (TIs) are materials that behave as electronic insulators in their bulks, but have robust surface states (SSs) which enable metallic conduction [1][2][3][4][5]. A particularly exciting property of the SS is that it is strongly spin-polarized, with the electrons' spinorientations locked perpendicular to their momenta [6,7]. While this novel spin texture greatly reduces the phase space for spin-conserving scattering events [8,9], there still remain scattering processes which give the SS electrons a finite lifetime and limit their ballistic transport, and thus must be considered for device applications [10].Among these scattering processes, those driven by electron-phonon coupling (EPC) in particular have been the subject of intense study because they affect any finitetemperature application of the TIs. The fundamental questions regarding EPC are: Which electronic states are involved, and to which phonon modes do they couple? A number of recent measurements including helium atom scattering [11][12][13] and inelastic transport [14] seem to be arriving at a consensus that scattering in Bi 2 Se 3 is dominated by a ∼ 7 − 8 meV optical A 1g phonon mode [15]. However, because of the coexistence of bulk and surface carriers in Bi 2 Se 3 [16], it is not clear from these experiments whether the measured A 1g mode coupling corresponds to EPC in the bulk or surface states. In principle, this could be investigated by angle-resolved photoemission spectroscopy (ARPES) because of its capability to measure the electron self-energy directly on the SS band. However, the ARPES results reported in the literature have been scattered: two works identified no particular mode coupling to the SS [17,18], one identified an ∼ 18 meV mode [19], while yet another identified modes at both ∼3 and ∼18 meV [20]. To-date no measurement has directly observed the A 1g mode coupling to the SS band.The discrepancy in ARPES measurements is likely attributed to the fact that the coupling occurs on a small energy scale accompanied by a weak spectral signature, so very high energy resolution is required to unambiguously detect it [13]. Here we circumvent this experimental difficul...

Watching ultrafast responses of structure and magnetism in condensed matter with momentum-resolved probes

“…Figure 6 shows a simplified sketch of the entire process under this interpretation, which is a particular case of a more general phenomenon known as displacive excitation of coherent phonons (DECP). Similar DECP-related phenomena have been observed in time-resolved ARPES experiments on other materials, 36–39 but in these cases, the time-dependent changes are interpreted as a consequence of periodic modulation of the band structure and not of the chemical potential.…”

Subpicosecond spin dynamics of excited states in the topological insulator Bi2Te3

“…In parallel with material efforts to obtain more bulkinsulating samples so that the bulk contribution to the conductivity can be completely suppressed [13][14][15], recent time-resolved (tr) experiments without spin resolution so far revealed the critical role of bulk-mediated electron-phonon scattering in the decay process of hot electrons across the linear energy-momentum dispersion of TSSs [16][17][18][19][20][21][22][23][24]. These findings indicate promising routes to overcome the problem of the bulk conductivity on ultrafast time scales, for example by generating low-energy excitations using fs-laser pulses, so that only electrons transiently occupying the TSS are excited within the bulk band gap [25].…”

Tunable Fano quantum-interference dynamics using a topological phase transition in(Bi1−xInx)Sim1, Koirala2, Brahlek3 et al. 2015Phys. Rev. B413270View full textAdd to dashboardCite

“…The Fano parameter 1/q is widely used to quantify the measure of asymmetry (see Appendix B), which is associated with the ~ 2 THz narrow phonon peak in the TIs [9,15,20,22].…”