Surface properties of 1T-TaS and contrasting its electron-phonon coupling with TlBiTe from helium atom scattering.

We present a detailed helium atom scattering study of the charge-density wave (CDW) system and transition metal dichalcogenide 1T-TaS. In terms of energy dissipation, we determine the electron-phonon (e-ph) coupling, a quantity that is at the heart of conventional superconductivity and may even "drive" phase transitions such as CDWs. The e-ph coupling of TaS in the commensurate CDW phase ( = 0.

Observation of Dirac Charge-Density Waves in BiTeSe.

While parallel segments in the Fermi level contours, often found at the surfaces of topological insulators (TIs), would imply "strong" nesting conditions, the existence of charge-density waves (CDWs)-periodic modulations of the electron density-has not been verified up to now. Here, we report the observation of a CDW at the surface of the TI Bi2Te2Se(111), below ≈350K, by helium-atom scattering and, thus, experimental evidence for a CDW involving Dirac topological electrons. Deviations of the order parameter observed below 180K, and a low-temperature break of time reversal symmetry, suggest the onset of a spin-density wave with the same period as the CDW in the presence of a prominent electron-phonon interaction, originating from Rashba spin-orbit coupling.

Evolution of ordered nanoporous phases during h-BN growth: controlling the route from gas-phase precursor to 2D material by monitoring.

Large-area single-crystal monolayers of two-dimensional (2D) materials such as graphene and hexagonal boron nitride (h-BN) can be grown by chemical vapour deposition (CVD). However, the high temperatures and fast timescales at which the conversion from a gas-phase precursor to the 2D material appears, make it extremely challenging to simultaneously follow the atomic arrangements. We utilise helium atom scattering to discover and control the growth of novel 2D h-BN nanoporous phases during the CVD process.

Surface electronic corrugation of a one-dimensional topological metal: Bi(114).

The surface of Bi(114) is a striking example where the reduced dimensionality gives rise to structural rearrangement and new states at the surface. Here, we present a study of the surface structure and electronic corrugation of this quasi one-dimensional topological metal based on helium atom scattering (HAS) measurements. In contrast to low-index metal surfaces, upon scattering from the stepped (114) truncation of Bi, a large proportion of the incident beam is scattered into higher order diffraction channels which in combination with the large surface unit cell makes an analysis challenging.

Atom-surface van der Waals potentials of topological insulators and semimetals from scattering measurements.

The phenomenology of resonant scattering has been known since the earliest experiments upon scattering of atomic beams from surfaces and is a means of obtaining experimental information about the fundamentals of weak adsorption systems in the van der Waals regime. We provide an overview of the experimental approach based on new experimental data for the He-SbTe(111) system, followed by a comparative overview and perspective of recent results for topological semimetal and insulator surfaces. Moreover, we shortly discuss the perspectives of calculating helium-surface interaction potentials from ab initio calculations.

Inelastic helium atom scattering from SbTe(111): phonon dispersion, focusing effects and surfing.

We present an experimental study of inelastic scattering processes on the (111) surface of the topological insulator SbTe using helium atom scattering. In contrast to other binary topological insulators such as BiSe and BiTe, SbTe is much less studied and the as-grown SbTe sample turns out to be p-doped, with the Fermi-level located below the Dirac-point as confirmed by angle-resolved photoemission spectroscopy. We report the surface phonon dispersion along both high symmetry directions in the energy region below 11 meV, where the Rayleigh mode exhibits the strongest intensity.

Origin of the Electron-Phonon Interaction of Topological Semimetal Surfaces Measured with Helium Atom Scattering.

He atom scattering has been demonstrated to be a sensitive probe of the electron-phonon interaction parameter λ at metal and metal-overlayer surfaces. Here it is shown that the theory linking λ to the thermal attenuation of atom scattering spectra (the Debye-Waller factor) can be applied to topological semimetal surfaces, such as the quasi-one-dimensional charge-density-wave system Bi(114) and the layered pnictogen chalcogenides. The electron-phonon 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 λ.

Helium-Surface Interaction and Electronic Corrugation of BiSe(111).

We present a study of the atom-surface interaction potential for the He-BiSe(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 BiSe. 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.