Recently, Shih et al. [Phys. Rev. Lett. 105, 146401 (2010)] published a theoretical band gap for wurtzite ZnO, calculated with the non-self-consistent GW approximation, that agreed surprisingly well with experiment while deviating strongly from previous studies. They showed that a very large number of empty bands is necessary to converge the gap. We reexamine the GW calculation with the full-potential linearized augmented-plane-wave method and find that even with 3000 bands the band gap is not completely converged. A hyperbolical fit is used to extrapolate to infinite bands. Furthermore, we eliminate the linearization error for high-lying states with local orbitals. In fact, our calculated band gap is considerably larger than in previous studies, but somewhat (4)] is constructed from the Kohn-Sham Green function taken from a densityfunctional theory calculation with the local-density approximation (LDA) for the exchange-correlation energy functional. The quasiparticle energy E σ nq with band index n, Bloch vector q, and spin σ is then obtained from the nonlinear equation 5-8 invoking the one-shot LDA + GW approach showed that the band gap of wurtzite ZnO is underestimated with respect to the experimental value by more than 1 eV. They fall in the range 2.12-2.6 eV while the experimental gap amounts to 3.6 eV, 9 after correction for vibrational effects. This large underestimation is untypical for GW calculations of sp-bound systems.The Letter of Shih et al. addressed two issues: first, the erroneous hybridization effects between Zn 3d and O 2p states that results from the self-interaction error within the LDA, 10 and second, the band convergence in the correlation part of the self-energy. The first problem was tackled with the LDA + U approach, 11 in which an orbital-dependent potential corrects the position of the 3d bands and, thus, reduces hybridization effects with the O 2p states. However, the combination LDA + U and GW yields a band gap that is still well below the experimental value. Therefore, the authors investigated the second issue by carefully converging the correlation self-energy and the dielectric matrix with respect to the number of bands. They performed calculations with up to 3000 bands corresponding to a maximal band energy of 67 Ry as well as a cutoff for the dielectric matrix of up to 80 Ry and showed that the resulting GW band gaps, 3.4 eV for LDA + GW and 3.6 eV for LDA + U + GW , turned out to be in very good agreement with experiment. They also demonstrated that a too small energy cutoff for the dielectric matrix can lead to a false convergence behavior: the band gap seems to converge with many fewer bands, but toward a value that is too small.These new results for ZnO are in striking contrast to previous studies. If they are correct, they cast doubt on all GW calculations published so far, not only for ZnO but also for other materials, especially for systems with localized states. In fact, Shih et al. point out at the end of their paper that "many of the previous quasiparticle calculat...

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Collective spin excitations in magnetic materials arise from the correlated motion of electron-hole pairs with opposite spins. The pair propagation is described by the transverse magnetic susceptibility, which we calculate within many-body perturbation theory from first principles employing the full-potential linearized augmentedplane-wave formalism. Ferromagnetic materials exhibit a spontaneously broken global rotation symmetry in spin space leading to the appearance of acoustic magnons (zero gap) in the long-wavelength limit. However, due to approximations used in the numerical scheme, the acoustic magnon dispersion exhibits a small but finite gap at. We analyze this violation of the Goldstone mode and present an approach that implements the magnetic susceptibility using a renormalized Green function instead of the Kohn-Sham one. This much more expensive approach shows substantial improvement of the Goldstone-mode condition. In addition, we discuss a possible correction scheme, which involves an adjustment of the Kohn-Sham exchange splitting, which is motivated by the spin-wave solution of the one-band Hubbard model. The new exchange splittings turn out to be closer to experiment. We present corrected magnon spectra for the elementary ferromagnets Fe, Co, and Ni.

Located beyond the resolution limit of nanoindentation, contact resonance atomic force microscopy (CR-AFM) is employed for nano-mechanical surface characterization of single crystalline 14M modulated martensitic Ni-Mn-Ga (NMG) thin films grown by magnetron sputter deposition on (001) MgO substrates. Comparing experimental indentation moduli-obtained with CR-AFM-with theoretical predictions based on density functional theory (DFT) indicates the central role of pseudo plasticity and inter-martensitic phase transitions. Spatially highly resolved mechanical imaging enables the visualization of twin boundaries and allows for the assessment of their impact on mechanical behavior at the nanoscale. The CR-AFM technique is also briefly reviewed. Its advantages and drawbacks are carefully addressed.

Polar oxide interfaces are an important focus of research due to their novel functionality which is not available in the bulk constituents. So far, research has focused mainly on heterointerfaces derived from the perovskite structure. It is important to extend our understanding of electronic reconstruction phenomena to a broader class of materials and structure types. Here we report from high-resolution transmission electron microscopy and quantitative magnetometry a robust – above room temperature (Curie temperature TC ≫ 300 K) – environmentally stable- ferromagnetically coupled interface layer between the antiferromagnetic rocksalt CoO core and a 2–4 nm thick antiferromagnetic spinel Co3O4 surface layer in octahedron-shaped nanocrystals. Density functional theory calculations with an on-site Coulomb repulsion parameter identify the origin of the experimentally observed ferromagnetic phase as a charge transfer process (partial reduction) of Co3+ to Co2+ at the CoO/Co3O4 interface, with Co2+ being in the low spin state, unlike the high spin state of its counterpart in CoO. This finding may serve as a guideline for designing new functional nanomagnets based on oxidation resistant antiferromagnetic transition metal oxides.