The dynamics at surfaces underpin many processes and phenomena in science and technology. For example, low-frequency phonons considerably contribute to the electron-phonon (e-ph) coupling constant, ?, which is key in the working of emerging technologies such as quantum devices, and phenomena such as room-temperature superconductivity. As another example, the diffusion of particles facilitates the growth of 2D materials, which are an important part of the quantum materials family. Diffusion is important not only in growth processes, but underpins chemical reactions, battery technology, and more.

Diffusion is important not only for systems where diffusion plays a major role. Since a diffusing molecule experiences the entire landscape of interactions, measuring it can provide direct information on the friction, and on interaction potentials. Such studies can be useful, for example, to benchmark ab-initio quantum computational methods, and to reveal the energy dissipation mechanisms in molecule-surface systems which are relevant in fields ranging from carbon capture/storage to spintronics.

The task of studying low-energy processes in the range of << 0.1 eV and with sufficient surface sensitivity, is challenging. This is evident, for example, from the intense debate about the value of ? for magic-angle graphene [1], or from the inability to measure the phonon mode which participates in the exciton condensation in quantum materials such as 1T-TiSe2 [2].

In the talk, I will explain how low-energy dynamics at surfaces can be studied using helium spin-echo (HeSE) spectroscopy [1]. I will survey the strength of HeSE in studying vibrations, and recent breakthroughs in studying 2D growth [3], and energy dissipation [4] at the low-energy regime. I will also give examples for when helium scattering is crucial for studies of surface structures [5-6], and will point on some potential applications, including in metrology of surface quality and disorder, and in studying chemical reactions at surfaces [7].

References:

[1] Holst, B. et. al, 2021. Material properties particularly suited to be measured with helium scattering: selected examples from 2D materials, van der Waals heterostructures, … PCCP.

[2] Kogar, A. et. al., 2017. Signatures of exciton condensation in a transition metal dichalcogenide. Science, 358(6368), pp.1314-1317.

[3] Kelsall, J. et. al., 2021. Ultrafast Diffusion at the Onset of Growth: O/Ru (0001). PRL, 126(15), p.155901.

[4] Raghavan, A. et. al., 2020. Alkali metal adsorption on metal surfaces: new insights from new tools. PCCP.

[5] Avidor, N. and Allison, W., 2016. Helium diffraction as a probe of structure and proton order on model ice surfaces. J.Phys.Chem. Lett., 7(22), pp.4520-4523.

[6] Lin, C. et. al., 2018. Two-dimensional wetting of a stepped copper surface. PRL, 120(7), p.076101.

[7] Irmejs, R. and Avidor, N., 2020. Atomic Scattering For Chemical Analysis Of Surfaces. arXiv:2012.01897.