Static Density Response of the Warm Dense Electron Gas beyond Linear Response Theory: Excitation of Harmonics.*

Experimental setups as well as theoretical modeling of Warm Dense Matter (WDM) heavily rely on linear response theory. However, Dornheim et. al.  [Phys. Rev. Lett.125, 085001 (2020)] showed that assuming the linear regime is not always justified for WDM. We use the ab initio Path-Integral Monte-Carlo (PIMC) technique to obtain exact results for the harmonically perturbed homogeneous electron gas. A thorough analysis for different perturbation amplitudes is carried out. The corresponding density response reveals resonances at higher harmonics of the perturbation frequency. Furthermore, the induced density response as a function of the perturbation strength unveils that the dominant term beyond linear response is the second harmonic. We show that the nonlinear density response is highly sensitive to exchange–correlation effects, rendering it a potentially valuable new diagnostic tool. The results signify the importance of response contributions beyond the linear regime to accurately model WDM.