Quantum plasmons and intraband excitons in doped nanoparticles: Failure of the Tamm-Dancoff approximation and importance of electron-hole attraction

• DOI: 10.48550/arxiv.1907.11289
• Publication Date: 2019-01-01
• Authors: Lau, Bryan T. G.; Berkelbach, Timot...
• Keywords: Chemical Physics (physics.chem-ph); Condensed Matter - Mesoscale and Nanoscale Physics; Physics - Chemical Physics; 0103 physical sciences; Mesoscale and Nanoscale Physics (cond-mat.mes-hall); FOS: Physical sciences; 01 natural sciences

10 pages, 5 figures<br/>We use excited-state quantum chemistry techniques to investigate the intraband absorption of doped semiconductor nanoparticles as a function of doping density, nanoparticle radius, and material properties. The excess electrons are modeled as interacting particles confined in a sphere. We compare the predictions of various single-excitation theories, including time-dependent Hartree-Fock, the random-phase approximation, and configuration interaction with single excitations. We find that time-dependent Hartree-Fock most accurately describes the character of the excitation, as compared to equation-of-motion coupled-cluster theory with single and double excitations. The excitation evolves from confinement-dominated, to excitonic, to plasmonic with increasing number of electrons at fixed density, and the threshold number of electrons to produce a plasmon increases with density due to quantum confinement. Exchange integrals (attractive electron-hole interactions) are essential to properly describe excitons, and de-excitations (i.e.~avoidance of the Tamm-Dancoff approximation) are essential to properly describe plasmons. We propose a schematic model whose analytic solutions closely reproduce our numerical calculations. Our results are in good agreement with experimental spectra of doped ZnO nanoparticles at a doping density of $1.4\times 10^{20}$ cm$^{-3}$.<br/>