# Non-linear phononics in THz excited Bi2Te3 nanofilms

R Busselez, A Levchuk, P Ruello, V Juvé, B Arnaud

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R Busselez, A Levchuk, P Ruello, V Juvé, B Arnaud. Non-linear phononics in THz excited Bi2Te3 nanofilms. 27th ETSF Workshop on Electronic Excitations: Shining light on matter through inter-disciplinary theoretical approaches, Jun 2024, Marseille, France. hal-05031512

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Submitted on 11 Apr 2025

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Non-linear phononics in THz excited Bi₂Te₃ nanofilms

R. Busselez<sup>a</sup>, A. Levchuk<sup>b</sup>, P. Ruello<sup>a</sup>, V. Juvé<sup>a</sup> and B. Arnaud<sup>a</sup>

<sup>a</sup> Institut des Molécules et Matériaux du Mans, UMR 6283 CNRS, Le Mans université, France

<sup>b</sup> Laboratoire des Solides Irradiés, UMR 7642 CEA-CNRS, Ecole Polytechnique, France

Density Functional Theory (DFT) calculations not only allow to predict the vibrational and optical properties of solids[1] but also to understand and disentangle the mechanisms playing a key role in the generation of acoustic and optical coherent phonons[2-5]. Recent experiments performed on a Bi₂Te₃ nanofilm have shown that a THz pulse launches a coherent A₁g phonon[5]. Such an observation can be explained by invoking either a sum frequency process[6] or non linear phonon-phonon couplings[7] . By resorting to group theory and calculating energy surfaces from first-principles, the main phonon-phonon couplings can be identified. Furthermore, a minimal model can be built to explain qualitatively pump-probe experiments. This model is validated by simulating the detection process as the modification of the dielectric function arising from the generation of optical or acoustic phonons can easily be evaluated at the random phase (RPA) approximation level[8,9]. Therefore, ab-initio calculations when combined with models are invaluable tools to shed lights on the complex mechanisms at the heart of ultrafast physics.

References

[1] R. Busselez, A. Levchuk, P. Ruello, V. Juvé and B. Arnaud, "Anisotropy in the dielectric function of Bi₂Te₃ from first principles: From the UV-visible to the infrared range", Phys. Rev. B **107**, 174305 (2023).

[2] Y. Giret, A. Gell and B. Arnaud, "Entropy driven atomic motion in laser-excited bismuth", Phys. Rev. Lett. **106**, 155503 (2011).

[3] E. Papalazarou, J. Faure, J. Mauchain, M. Marsi, A. Taleb, I. Reshetnyak, A. van Roekeghem, I. Timrov, N. Vast, B. Arnaud, and L. Perfetti, "Coherent phonon coupling to individual bloch states in photoexcited bismuth", Phys. Rev. Lett. **108**, 256808 (2012).

[4] B. Arnaud, and Y. Giret, "Electron Cooling and Debye-Waller Effect in Photoexcited Bismuth", Phys. Rev. Lett. **110**, 016405 (2013).

[5] A. Levchuk, Wilk, G. Vaudel, F. Labbé, B. Arnaud, K. Balin, J. Szade, P. Ruello, and V. Juvé, "Coherent acoustic phonons generated by ultrashort terahertz pulses in nanofilms of metals and topological insulators", Phys. Rev. B **101**, 180102(R) (2020).

[6] S. Maehrlein, A. Paarmann, M. Wolf, and T. Kampfrath, "Terahertz Sum-Frequency Excitation of a Raman-Active Phonon", Phys. Rev. Lett. **119**, 127402 (2017).

[7] R. Mankowsky, M. Först and A. Cavalleri, "Non-equilibrium control of complex solids by nonlinear phononics", Rep. Prog. Phys. **79**, 064503 (2016).

[8] B. Arnaud and M. Alouani, "Local-field effects and excitonic effects in the calculated optical properties of semiconductors from first principles", Phys. Rev. B **63**, 085208 (2001).

[9] A. Marini, C. Hogan, M. Grüning, and D. Varsano, "Yambo: An ab-initio tool for excited state calculations", Comput. Phys. Commun. **180**, 1392 (2009).