Connecting electron and phonon spectroscopy data to consistently determine quasiparticle-phonon coupling on the surface of topological insulators

Abstract

Photoemission and phonon spectroscopies have yielded widely varying estimates of the electronphonon coupling constant λ on the surfaces of topological insulators, even for a particular material and technique. We connect the results of these experiments by determining the Dirac fermion quasiparticle spectral function using information from measured spectra of a strongly-interacting, low-lying optical surface phonon band. The manifest spectral features resulting from the coupling are found to vary on energy scales < 1 meV, and are distinct from those traditionally observed in the case of acoustic phonons in metals. We explore different means of determining λ from the electron perspective and identify definitions that yield values consistent with phonon spectroscopy.

Material properties particularly suited to be measured with helium scattering: selected examples from 2D materials, van der Waals heterostructures, glassy materials, catalytic substrates, topological insulators and superconducting radio frequency materials

“…144,160,161 Whether Kohn anomalies at a phonon momentum that connects opposite sides of a topological Dirac cone are possible, is still an open question as it would require a phonon-induced transition involving a spinflip. 144,160,162,163 The latter may become possible by creating or annihilating a phonon which carries an angular momentum of 1 quantum number. While Kohn anomalies have been reported in the lower part of the phonon spectrum of TIs, 155,160,162 recent studies have shown that the major contribution to e-ph coupling in these materials comes from polar optical modes.…”

A supervised learning algorithm for interacting topological insulators based on local curvature

“…As electron-phonon interactions are ubiquitous in real materials and can affect properties such as transport of surface states, we now consider the impact of such interactions on the Chern insulator [40][41][42][43][44][45][46][47][48][49][50][51][52][53][54][55][56]. In particular, we consider the deformation potential coupling between an acoustic phonon mode and spinless fermions of the form [57]…”

The anomalous spin texture as the probe for interactions in $Bi_{2-y} Sb_y Se_x Te_{3-x}$

“…Furthermore, we apply the emergent theory to the specific EPI with the 7meV optical phonon mode [21,22]. Our work therefore builts a theoretical bridge between the electron-optical phonon coupling experiments in Ref's [21,22] and the in-plane spin anomaly experiments in Ref. [34].…”

“…The electron-disorder interaction remains the dominant mechanism affecting the low energies and low temperatures where the e-ph interaction is suppressed. The coherent helium-atom-scattering experiments [21][22][23]28] in a wide range of temperatures (100 < T (K) < 300) indicated the presence of the optical EPI pointing at a strong softening of the 7meV phonon mode. The position of the softening at the phonon momentum near 2k F points at a Kohn anomaly and thus a strong coupling of the optical branch to the electronic states [28] with a less significant role played by the acoustic phonons [12,29,30].…”

“…In this context, coupling of the surface electrons to a number of excitations such as, spin density waves at the Fermi surface [10], spin plasmons [11,18] have been considered. The EPI has been heavily studied by a number of experimental [17,[19][20][21][22][23] and theoretical [24][25][26] works where the Dirac quasiparticle lifetime, the self-energy corrections to the energy bands and the phonon frequencies have been estimated. The estimates of the coupling constants vary in a large range making difficult to extract the individual modespecific phonons.…”