Mini-Symposium: Material Dynamics

Schedule:

13:00 - 13:15 Introduction, Paul Erhart
13:15 - 13:40 Florian Libisch, Technical University of Vienna
13:40 - 14:05 Ion Errea, University of the Basque Country
14:05 - 14:30 Olle Hellman, Weizmann Institute

14:30 - 14:50 Coffee break

14:50 - 15:15 Ivana Savic, King’s College London
15:15 - 15:40 Jochen Rohrer, Technical University of Darmstadt
15:40 - 16:05 Erik Fransson, Chalmers

Abstracts:

Jochen Rohrer

Dynamics of and in Si-O-C-H-Na systems studied with machine-learned interatomic potentials

Jochen Rohrer, Linus Erhard, Niklas Leimeroth, Karsten Albe

Matierals Modelling Division, Technical University Darmstadt, Germany

In this contribution we present machine-learned potentials for silica (SiO2), siliconoxycarbides (SixOyCz(Hw)) and carbon-based polymer-electrolytes for sodium ion batteries (CxHyOzNaz+d). Starting from pristine SiO2 [1], a general-purpos potential for arbitrary mixtures of Si and O was developped [2], extended to ternary Si-O-C systems [3]  and finally augmented with training data to describe a large variety of quaternary and quinary Si-O-C-H-Na systems.  Using these potentials, we (i) address the dynamics of phase transitions in SiO2 during shock experiments, (ii) investigate the dependence of SiOC(H) microstructures on the choice of preceramic precursors and pyrolisis temperatures and (iii) eventually simulate the time evolution of Na-battery half-cell models employing SiOC and hard-carbon anodes. These simulations are all made possible with the aid of actively-learned atomic cluster expansion (ACE) potentials. For the case of silica, we also present a comparsion of computaional load vs. accuracy for various other flavours of machine-learning potentials.

[1] Erhard, L.C., Rohrer, J., Albe, K. et al. A machine-learned interatomic potential for silica and its relation to empirical models. npj Comput Mater 8, 90 (2022). https://doi.org/10.1038/s41524-022-00768-w

[2] Erhard, L.C., Rohrer, J., Albe, K. et al. Modelling atomic and nanoscale structure in the silicon–oxygen system through active machine learning. Nat Commun 15, 1927 (2024). https://doi.org/10.1038/s41467-024-45840-9

[3] Leimeroth, N., Rohrer, J., Albe, K. Structure-property relations of silicon oxycarbides studied using a machine learning interatomic potential, arxXiv:2403.10154v1 [cond-mat.mtrl.-sci]

Florian Libisch

Moire-phonons in twisted bilayers

Stacking two layers of two-dimensional materials slightly twisted relative to each other causes significant alternations of the physical properties of the resulting bilayer. For example, at the right twist angle, the electronic band structure of twisted bilayer graphene features a flat band at the Fermi level that gives rise to interesting many-body physics such as correlated insulators or superconducting states. Likewise, a finite twist angle modifies the phonon band structure. A reciprocal space continuum model including lattice reconstruction due to relaxation allows us to investigate the continuous evolution of the phonon band structure with twist angle [1]. At intermediate angles, we find a complicated structure of the phonon density of states around the frequency of the layer breathing mode, that is substantially broadened by the moiré-induced interaction with the acoustic phonon branches. For twisted MoS2, Raman measurements of the layer breathing mode validate our predictions [2]. At higher Raman shifts, splittings of the E2g mode provide details on the local strain modulations [3]. Our results suggest that suitably twisting structures may manipulate both phonon and electron properties of such a system, and thus set the stage to test electron-phonon contributions to the observed correlated states.

[1] N. Girotto, L. Linhart, and F. Libisch, Phys. Rev. B 108, 155415 (2023)

[2] J. Quan et al., Nature Materials 20,  1100–1105 (2021)

[3] J. Quan et al., Nano Letters 23, 11510 (2023)

Ivana Savic

Coupling with phonons and conductivity of electronic surface states in topological insulators

Topological insulators are a new form of quantum matter whose surface states are protected from backscattering due to disorder. Despite this protection, topological surface states may be scattered by phonons. Recent developments in ultrafast measurements and first principles computational methods have created opportunities for detailed characterisation of electron-phonon coupling in various materials. In this talk, I will present our experimental and computational efforts to understand coupling between the electronic surface states in Bi2Te3 and Bi2Se3 and coherent phonon modes driven by photoexcitation [1,2]. I will also discuss our efforts to develop accurate models of phonon limited conductivity of surface states of these materials.

[1] J. A. Sobota et al., Phys. Rev. B 107, 014307 (2023)

[2] Y. Huang et al., Phys. Rev. X 13, 041050 (2023)

Ion Errea

Bending rigidity, sound propagation and ripples in flat graphene

Many of the applications of graphene rely on its uneven stiffness and high thermal conductivity, but the mechanical properties of graphene—and, in general, of all two-dimensional materials—are still not fully understood. Harmonic theory predicts a quadratic dispersion for the out-of-plane flexural acoustic vibrational mode, which leads to the unphysical result that long-wavelength in-plane acoustic modes decay before vibrating for one period, preventing the propagation of sound. The robustness of quadratic dispersion has been questioned by arguing that the anharmonic phonon–phonon interaction linearizes it. However, this implies a divergent bending rigidity in the long-wavelength regime. Here we show that rotational invariance protects the quadratic flexural dispersion against phonon–phonon interactions, and consequently, the bending stiffness is non-divergent irrespective of the temperature. By including non-perturbative anharmonic effects in our calculations, we find that sound propagation coexists with a quadratic dispersion. We also show that the temperature dependence of the height fluctuations of the membrane, known as ripples, is fully determined by thermal or quantum fluctuations, but without the anharmonic suppression of their amplitude previously assumed. These conclusions should hold for all two-dimensional materials.