John Vinson

Bethe-Salpeter equation calculations of core excitation spectra

We present a hybrid approach for Bethe-Salpeter equation (BSE) calculations of core excitation spectra, including x-ray absorption (XAS), electron energy loss spectra (EELS), and nonresonant inelastic x-ray scattering (NRIXS). The method is based on ab initio wave functions from the plane-wave pseudopotential code ABINIT; atomic core-level states and projector augmented wave (PAW) transition matrix elements; the NIST core-level BSE solver; and a many-pole self-energy model to account for final-state broadening and self-energy shifts. Multiplet effects are also approximately accounted for. The approach is implemented using an interface dubbed OCEAN (Obtaining Core Excitations using ABINIT and NBSE). To demonstrate the utility of the code we present results for the K edges in LiF as probed by XAS and NRIXS, the K edges of KCl as probed by XAS, the Ti L2,3 edge in SrTiO3 as probed by XAS, and the Mg L2,3 edge in MgO as probed by XAS. These results are compared with experiment and with other theoretical approaches.

Accurate X-Ray Spectral Predictions: An Advanced Self-Consistent-Field Approach Inspired by Many-Body Perturbation Theory

Constrained-occupancy delta-self-consistent-field (ΔSCF) methods and many-body perturbation theories (MBPT) are two strategies for obtaining electronic excitations from first principles. Using the two distinct approaches, we study the O 1s core excitations that have become increasingly important for characterizing transition-metal oxides and understanding strong electronic correlation. The ΔSCF approach, in its current single-particle form, systematically underestimates the pre-edge intensity for chosen oxides, despite its success in weakly correlated systems. By contrast, the Bethe-Salpeter equation within MBPT predicts much better line shapes. This motivates one to reexamine the many-electron dynamics of x-ray excitations. We find that the single-particle ΔSCF approach can be rectified by explicitly calculating many-electron transition amplitudes, producing x-ray spectra in excellent agreement with experiments. This study paves the way to accurately predict x-ray near-edge spectral fingerprints for physics and materials science beyond the Bethe-Salpether equation.

Understanding and control of bipolar self-doping in copper nitride

Semiconductor materials that can be doped both n-type and p-type are desirable for diode-based applications and transistor technology. Copper nitride (Cu3N) is a metastable semiconductor with a solar-relevant bandgap that has been reported to exhibit bipolar doping behavior. However, deeper understanding and better control of the mechanism behind this behavior in Cu3N is currently lacking in the literature. In this work, we use combinatorial growth with a temperature gradient to demonstrate both conduction types of phase-pure, sputter-deposited Cu3N thin films. Room temperature Hall effect and Seebeck effect measurements show n-type Cu3N with an electron density of 1017 cm-3 for low growth temperature (≈ 35 °C) and p-type with a hole density between 1015 cm-3 and 1016 cm-3 for elevated growth temperatures (50 °C to 120 °C). Mobility for both types of Cu3N was ≈ 0.1 cm2/Vs to 1 cm2/V. Additionally, temperature-dependent Hall effect measurements indicate that ionized defects are an important scattering mechanism in p-type films. By combining X-ray absorption spectroscopy and first-principles defect theory, we determined that VCu defects form preferentially in p-type Cu3N while Cui defects form preferentially in n-type Cu3N; suggesting that Cu3N is a compensated semiconductor with conductivity type resulting from a balance between donor and acceptor defects. Based on these theoretical and experimental results, we propose a kinetic defect formation mechanism for bipolar doping in Cu3N, that is also supported by positron annihilation experiments. Overall, the results of this work highlight the importance of kinetic processes in the defect physics of metastable materials, and provide a framework that can be applied when considering the properties of such materials in general.

The rediscovery of the ‘French Blue’ diamond

l Francois Farges 1,2, John Vinson 3, John J. Rehr 3 and Jeffrey E. Post 4 DOI: 10.1051/epn/2012103 l 1 Laboratoire de mineralogie et de comsochimie du Museum (LMCM) and CNRS UMR 7160 Museum national d’Histoire naturelle Paris, France. l 2 Dept. of Environmental and Geological Sciences Stanford University, CA 94305-2115 USA. l 3 Dept. of Physics University of Washington Seattle, WA 98195-1560 USA. l 4 Department of Mineral Sciences Smithsonian Institution Washington, DC 20560 USA.

Spectroscopic Signature of Oxidized Oxygen States in Peroxides

Recent debates on the oxygen redox behaviors in battery electrodes have triggered a pressing demand for the reliable detection and understanding of nondivalent oxygen states beyond conventional absorption spectroscopy. Here, enabled by high-efficiency mapping of resonant inelastic X-ray scattering (mRIXS) coupled with first-principles calculations, we report distinct mRIXS features of the oxygen states in Li2O, Li2CO3, and especially, Li2O2, which are successfully reproduced and interpreted theoretically. mRIXS signals are dominated by valence-band decays in Li2O and Li2CO3. However, the oxidized oxygen in Li2O2 leads to partially unoccupied O-2p states that yield a specific intraband excitonic feature in mRIXS. Such a feature displays a specific emission energy in mRIXS, which disentangles the oxidized oxygen states from the dominating transition-metal/oxygen hybridization features in absorption spectroscopy, thus providing critical hints for both detecting and understanding the oxygen redox reactions in transition-metal oxide based battery materials.

Resonant x-ray emission of hexagonal boron nitride

The electronic structure of hexagonal boron nitride (h-BN) is explored using measurements of x-ray absorption and resonant inelastic x-ray scattering (RIXS) at the nitrogen K edge (1s) in tandem with calculations using many-body perturbation theory within the GW and Bethe-Salpeter equation (BSE) approximations. Our calculations include the effects of lattice disorder from phonons activated thermally and from zero point energy. They highlight the influence of disorder on near-edge x-ray spectra.

Predictive modeling of resonant inelastic X-ray scattering with OCEAN

Near-edge x-ray spectroscopies, including emission and resonant inelastic x-ray scattering (RIXS), are widely used to probe the local electronic and molecular structure of materials. The spectra, however, provide only an indirect measure of the interesting parameters of a system: a transition metal L edge might reveal charge state in a battery cathode whereas the oxygen K edge can reflect the hydrogen bond network in water. In both cases it is not the spectra themselves that are of interest, but the electronic and structural configurations that give rise to them. Accurate modeling provides an invaluable tool for not only the interpretation of measured results, but also the design of experiments. The OCEAN code simulates near-edge spectra by solving the Bethe-Salpeter equation on top of a density-functional theory foundation, without system-dependent fitting parameters [1]. By contrasting OCEAN results with high-resolution measurements we are able to identify discrepancies that arise from specific effects or physical processes originally neglected in the calculations. We present examples showcasing the importance of correctly accounting for valence-band quasiparticle lifetimes [2], intrinsic disorder including zero-point motion, and phonon scattering when modeling x-ray emission and RIXS. [1] J. Vinson, et al. (2011). Phys. Rev. B, 83, 115106 [2] J. Vinson, et al. (2016). Phys. Rev. B, 94, 035163