Polaronic distortion and vacancy-induced magnetism in MgO

The electronic structure of the neutral and singly charged Mg vacancy in MgO is investigated using density functional theory. For both defects, semilocal exchange correlation functionals such as the local spin density approximation incorrectly predict a delocalized degenerate ground state. In contrast, functionals that take strong correlation effects into account predict a localized solution, in agreement with spin resonance experiments. Our results, obtained with the HSE hybrid, atomic self-interaction corrected and $\text{LSDA}+U$ functionals, provide a number of constraints to the possibility of ferromagnetism in hole doped MgO.

Figure 1

(Color online) LSDA electronic structure of $V_{\text{MgO}}$. (Left) DOS of a MgO supercell containing either $V^0{}_{\text{Mg}}$ or $V_{\text{Mg}}^{-}$. (Right) Charge density corresponding to the minority spin holes at the top of the valence band for $V^0{}_{\text{Mg}}$. Note that the hole density is spread uniformly over the O octahedron.

Figure 2

(Color online) ASIC electronic structure of $V_{\text{MgO}}$. (Left) DOS of a MgO supercell containing either $V^0{}_{\text{Mg}}$ or $V_{\text{Mg}}^{-}$. (Top right) Charge density corresponding to the holes of $V^0{}_{\text{Mg}}$. Note that the two holes are completely localized on two opposing O ions. (Bottom right) Charge density corresponding to the single hole of $V^0{}_{\text{Mg}}$. The hole density is localized on one O ion.

Figure 3

(Color online) DOS calculated with HSE for a MgO supercell containing either $V^0{}_{\text{Mg}}$ or $V_{\text{Mg}}^{-}$. The top two panels show the DOS for the polaronic ground state geometry (labeled GS) of the defects. The bottom two panels show the DOS corresponding to local minima solutions (labeled LM), which preserve the octahedral symmetry around the vacancy sites.

Figure 4

(Color online) (Left) $\text{LSDA}+U$ energy difference between the LSDA and ASIC geometries for $V^0{}_{\text{Mg}}$ and $V_{\text{Mg}}^{-}$ as a function of $U$. Note the crossover between the two geometries at $U\sim 4\text{eV}$, when the polaronic ASIC geometry becomes the most stable. (Right) Ground state DOS calculated with $\text{LSDA}+U$ as a function of $U$.