Helium–Surface Interaction and Electronic Corrugation of Bi₂Se₃(111)

Abstract

We present a study of the atom–surface interaction potential for the He–Bi2Se3(111) system. Using selective adsorption resonances, we are able to obtain the complete experimental band structure of atoms in the corrugated surface potential of the topological insulator Bi2Se3. He atom scattering spectra show several selective adsorption resonance features that are analyzed, starting with the free-atom approximation and a laterally averaged atom–surface interaction potential. Based on quantum mechanical calculations of the He–surface scattering intensities and resonance processes, we are then considering the three-dimensional atom–surface interaction potential, which is further refined to reproduce the experimental data. Following this analysis, the He–Bi2Se3(111) interaction potential is best represented by a corrugated Morse potential with a well depth of D = (6.54 ± 0.05) meV, a stiffness of κ = (0.58 ± 0.02) Å–1, and a surface electronic corrugation of (5.8 ± 0.2)% of the lattice constant. The experimental data may also be used as a challenging benchmark system to analyze the suitability of several van der Waals approaches: the He–Bi2Se3(111) interaction captures the fundamentals of weak adsorption systems where the binding is governed by long-range electronic correlations.

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

“…[151][152][153] Hence, HAS provides a sensitive probe to determine the surface phonon dispersion and energy dissipation processes in terms of the e-ph coupling constant, l, on the surfaces of these materials. The obtained experimental data of various TIs 92,145,[154][155][156][157][158][159] promise to evolve a more general picture about the surface dynamics and the atom-surface interaction of these peculiar surfaces. The e-ph coupling, as determined for several topological insulators belonging to the class of bismuth chalcogenides, suggests a dominant contribution of the surface quantum well states over the Dirac electrons in terms of l. 145 Investigations of the archetypal TI which is Bi 2 Te 3 (111) 155 show a prominent surface acoustic mode that may have important implications in layered and nanoscale devices.…”

THz Surface Modes and Electron-Phonon Coupling in Bi$_2$Se$_3$(111)

“…24 ), applicable to highly corrugated surfaces and successfully used to detect with HAS high-energy optical surface modes in ionic crystals 24,51 . The complete set of He-surface bound states has been measured previously 40 . Bound-state inelastic resonances occur in the HAS-TOF spectrum, with a possibly large enhancement of the inelastic intensities, at the locus of intersections of the scan curve (1) 23 with the inelastic bound-state resonance condition (2) (see Chap.…”

“…In Equation 2 the vector G has been conveniently expressed via its components parallel and orthogonal to the scattering plane, respectively. In Bi 2 Se 3 (111) the measured He-surface boundstate energies 40 are E 0n = −5.6, −3.8, −2.3, −1.2, −0.5 meV for n = 0, 1, 2, 3, 4, respectively. For a fixed scattering geometry ϑ i + ϑ f = ϑ SD (here ϑ SD = 91.5 • ), Eqs.…”

“…where φ = 4.9 eV is the work function 53 , A c = 14.92 Å2 the unit cell area, I(T S ) the He-beam specular intensity, T S the surface temperature, k iz = 3.18 Å−1 the normal component of the incident wavevector, and n s the number of conducting layers which contribute to the phononinduced modulation of the surface charge density 54 . The latter is estimated to be n s = 2λ T F /c 0 , where λ T F is the Thomas-Fermi screening length characterising the surface band-bending region (here ≈ 6 nm) 19 , c 0 = 9.6 Å the quintuple layers (QL) thickness, and the factor 2 indicates the 2DEG multiplicity as observed with ARPES It should be noted that, unlike in the case of low-index metal surfaces, characterised by a soft-wall repulsive potential and negligible corrugation, here the large electronic corrugation 40 implies a hard-wall potential. In this case one needs to correct k 2 iz so as to account for the acceleration impressed by the attractive part of the potential on the He atom when approaching the surface turning point (Beeby correction 28 ).…”

“…, where m is the He mass and D = 6.54 meV the He-surface potential well depth 40 . With the Beeby correction it is λ = 0.23.…”