Abstract:

Excitations of a material using visible light pulses generate electronic heating, which transient electronic temperatures largely exceeding the one of the underlying atomic lattice.
In the case of metals, Allen showed [1] that the subsequent equilibration between the "hot" electrons and "cold" lattice vibrations can be understood with a two-temperature (2T) picture, in which electrons and phonons remain in distinct thermal equilibria. In this talk, I will show the limitations of this physical picture when applied to semiconductors and low-dimensional materials -- materials with reduced dielectric screening, anisotropy, and, in some cases, higher lattice thermal conductivity.  Based on first-principles calculations and the semiclassical Boltzmann transport equations for electrons and phonons,  I will propose a generalized 2T model which captures the full thermal relaxation of hot electrons and holes, and discuss its consequences on measuring electron-phonon and phonon-phonon couplings from time-resolved spectroscopy experiments [2]. Finally, I will show how such findings can be used to generate non-classical electron-induced heat which can be observed in experiments [3,4]. [1] P. B. Allen, Phys. Rev. Lett. 59, 1460 (1987), [2] S. Sadasivam, et al. Phys. Rev. Lett. 119, 136602 (2017), [3] I.C. Tung, et al. Nat. Photonics, (2019), [4] P. Guo et al. Nat. Comm. 9 (1), 2019 (2018)

Bio: Pierre Darancet is an Assistant Scientist at the Center for Nanoscale Materials at Argonne National Laboratory and a fellow at the Northwestern-Argonne Institute for Science and Engineering. He obtained his PhD in 2008 at the Institut Néel in Grenoble before working as a postdoctoral fellow at Lawrence Berkeley National Lab and at Columbia University. His research focuses on the first-principles modeling of charge and energy transport in nanoscale materials and interfaces.