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Seminario: "Helium atom scattering, a suitable tool for studying electron-phonon coupling, bending rigidity and thermal expansion of 2D materials"

Helium atom scattering, a suitable tool for studying electron-phonon coupling, bending rigidity and thermal expansion of 2D materials
Profesor Daniel Farías
Institución de origen
Departamento de Física de la Materia Condensada
12:00 h
Evento online, MS-TEAMS, “Seminarios del Departamento”

Helium atom scattering (HAS) is a powerful, well established technique for investigating the structural and dynamical properties of crystalline solid surfaces. Because of the low energies used (10 200 meV ), monoenergetic beams of neutral He atoms probe the topmost surface layer of any material in an inert, completely nondestructive manner. Diffraction measurements allow the determination of the surface unit cells, whereas time of flight spectr a provide information on the surface phonon dynamics [1]. The energy resolution in the acoustic phonons region (up to  50 meV ) is 0.5 meV , whereas the angular resolution is typically 0.1 deg. I will present an overview of a range of material properties particularly suited to be measured with HAS, with selected examples from 2D materials.

The observation of diffraction features along the two main high symmetry directions allows determining the in plane surface lattice constant with high accuracy. Within experimental error, the in plane lattice parameter of 1T PdTe 2 , 1T PtTe 2 and 2H MoS 2 was found to remain constant in a broad temperature range [2]. Measurements of the surface phonon dispersion curves with HAS at low energies allows determining the substrate bond strength as well as th e b ending rigidity, a piece of information difficult to get with other experimental techniques. I will present results obtained for graphene grown on different metal substrates and sapphire. Finally, I will show how the thermal attenuation of the He specular peak from metal surfaces, described by the Debye Waller exponent, can be directly related to the electron phonon coupling constant λ, also known as the mass correction factor of superconductivity [3]. This makes HAS an ideally suited technique to measure λ in the low energy range (< 0.1 eV) for 2D materials and van der Waals heterostructures . Results obtained recently in our laboratory for 1T PdTe 2 , 1T PtTe 2 and 2H MoS 2 will be presented [ 4].

[1] D. Farías and K. H. Rieder , Rep. Prog . Phys. 61, (1998) 1575; A . Al Taleb and D. Farías , J. Phys.: Condens . Matter 28, (2016) 103005
[2]G. Anemone et al . 2D Materials 5 (3), 035015 (2018 ); G. Anemone et al. 2D Materials 7 (2020) 025007 .
[3] G. Benedek et al. J. Phys. Chem. Lett. 9 (2018) 76; Advanced Materials 32 (2020) 2002072
[4]G. Anemone et al. The Journal of Physical Chemistry C 123 (2019) 3682; G . Anemone et al. npj 2D Materials and Applications 5 (2021) 1 .


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