Defect Physics without the Band-Gap Problem: Combining DFT and \textit{GW}
COFFEE_KLATCH · Invited
Abstract
In computational defect physics and chemistry the local-density and generalized gradient approximations (LDA/GGA) to density functional theory (DFT) are widely applied due to their computational efficiency. However, their predictive power is limited by intrinsic deficiencies like artificial self-interaction and the absence of the derivative discontinuity in the exchange-correlation potential (that give rise to the so called band-gap problem). We present a new formalism that combines DFT with many-body perturbation theory (MBPT) in the $G_0W_0$ approximation to overcome these deficiencies [1,2]. The formation energy of a defect is expressed as successive charging of a lower charge state, for which the defect level is unoccupied, permitting a decomposition into a lattice (DFT) and an electron addition part ($G_0W_0$) [2]. For the self-interstitial in silicon the approach increases the LDA formation energy of the neutral state by $\sim$1.1~eV in good agreement with diffusion Monte Carlo calculations [2,3,4]. For the anion vacancy in bulk MgO (also called F- or color center), which can probably be regarded as \emph{the} classic intrinsic point defect in compound insulators, it proves to be necessary to go one step further in the hierarchy of MBPT. After including the electron-hole and electron-phonon interaction the absorption energies of the neutral and the positively charged F-center become practically identical -- a fact that has impeded the F-center's characterization for decades -- in good agreement with optical absorption studies [5].\\[4pt] [1] Hedstr\"om {\it et al.} PRL {\bf 97}, 226401 (2006)\\[0pt] [2] Rinke {\it et al.} PRL {\bf 102}, 026402 (2009)\\[0pt] [3] Batista {\it et al.} PRB {\bf 74}, 121102(R) (2006)\\[0pt] [4] Leung {\it et al.} PRL {\bf 83}, 2351 (1999)\\[0pt] [5] Kappers {\it et al.} PRB {\bf 1}, 4151(1970)
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Authors
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Patrick Rinke
University of California at Santa Barbara, Materials Department, UC Santa Barbara