Condensed Matter Seminar - "Hot, disordered, and quantum: Electrons in materials from first principles"

Most first-principles electronic-structure calculations assume a perfect crystal at zero temperature, probed by classical fields. However, atoms in real materials vibrate and their magnetic moments fluctuate, as electrons image and modify materials. In this talk I will discuss two recent results from my group in this context:

First, I will show how thermal lattice and magnetic disorder determine optical spectra. We combine large supercells containing thermally disordered atomic or spin configurations with Williams–Lax theory, and use this to compute temperature-dependent optical and magneto-optical spectra of ferromagnetic bcc Fe. Phonon- and magnon-assisted intraband transitions produce large optical signals below 1 eV. The prominent interband peak near 2.7 eV red-shifts with magnetic temperature but depends only weakly on the lattice temperature. Unfolding the disordered band structure reveals band kinks, a direct fingerprint of electron–magnon coupling. I will connect these results to zero-point renormalization and to lattice-disorder effects in wide-gap oxides such as SnO₂ and SrTiO₃.

Second, I will discuss the regime, in which an electron as a projectile has to be treated quantum mechanically rather than a classical point charge: Real-time time-dependent density functional theory lets us propagate a Gaussian wave packet through graphene together with all of the target's electrons. We then compare it with an ensemble of classical trajectories sampled from the same distribution. Energy loss, secondary-electron emission and backscattering reveal a window around 400 eV in which only the wave packet backscatters. Above about 600 eV these quantum effects fade. In addition, a hot lattice rather than hot electrons controls the emission yield. Together, these results show where temperature, disorder and quantum mechanics affect the electronic response of materials.