Thermal displacements in microscopies and spectroscopies

*Matthieu J. Verstraete (1,2)
(1) nanomat/QMAT/CESAM University of Liege, Belgium; (2) European Theoretical Spectroscopy Facility

Thermal mean square displacements have the wonderful musty smell of the 1910s, when Debye showed a simple classical harmonic model could explain the form factor for the scattering of x-rays (or neutrons or anything small) from a crystalline lattice. These effects have had a resurgence in the past few years, as the precision and power of ab initio methods has grown, and meeting experiment now requires taking realistic environmental constraints into account. Vibrational modes are routinely available for complex materials, through density functional perturbation theory, and even beyond with anharmonic fitting methods.

I will show examples of the consequences of thermal vibrations on the electronic and magnetic properties of bulk and nanostructured materials. The interaction of vibration waves with magnons is an important ingredient in modern spin(calori)tronics devices: we estimate thermal effects on magnons in a renormalized atomistic spin dynamics framework, and show its strong interference of with disorder in bulk Permalloy [1]. In nanostructured materials, thermal displacements can become more strongly anisotropic, with important effects both in microscopy and spectroscopy. We quantify their impact on the movement of defects in graphene in a TEM [2], and the x-ray photoemission spectrum of noble metal surfaces [3].

[1] M. Di Gennaro, et al., Physical Review B 97 (21), 214417 (2018).

[2] M. Tripathi, et al., Nano Letters 18 (8), 5319-5323 (2018).

[3] L. Nicolai et al., unpublished