Magnetic interaction of Co ions near the {10\bar{1}0} ZnO surface

Co-doped ZnO is the prototypical dilute magnetic oxide showing many of the characteristics of ferromagnetism. The microscopic origin of the long range order however remains elusive, since the conventional mechanisms for the magnetic interaction, such as super-exchange and double exchange, fail either at the fundamental or at a quantitative level. Intriguingly, there is a growing evidence that defects both in point-like or extended form play a fundamental role in driving the magnetic order. Here we explore one of such possibilities by performing {\it ab initio} density functional theory calculations for the magnetic interaction of Co ions at or near a ZnO \{10$\bar{1}$0\} surface. We find that extended surface states can hybridize with the $e$-levels of Co and efficiently mediate the magnetic order, although such a mechanism is effective only for ions placed in the first few atomic planes near the surface. We also find that the magnetic anisotropy changes at the surface from an hard-axis easy-plane to an easy axis, with an associated increase of its magnitude. We then conclude that clusters with high densities of surfacial Co ions may display blocking temperatures much higher than in the bulk

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 non-divalent 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 intra-band 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.Comment: 25 pages, 4 figures, plus 11 pages of Supplementary Information with 4 figure

{\AA}ngstr\"om-resolved Interfacial Structure in Organic-Inorganic Junctions

Charge transport processes at interfaces which are governed by complex interfacial electronic structure play a crucial role in catalytic reactions, energy storage, photovoltaics, and many biological processes. Here, the first soft X-ray second harmonic generation (SXR-SHG) interfacial spectrum of a buried interface (boron/Parylene-N) is reported. SXR-SHG shows distinct spectral features that are not observed in X-ray absorption spectra, demonstrating its extraordinary interfacial sensitivity. Comparison to electronic structure calculations indicates a boron-organic separation distance of 1.9 {\AA}, wherein changes as small as 0.1 {\AA} result in easily detectable SXR-SHG spectral shifts (ca. 100s of meV). As SXR-SHG is inherently ultrafast and sensitive to individual atomic layers, it creates the possibility to study a variety of interfacial processes, e.g. catalysis, with ultrafast time resolution and bond specificity.Comment: 19 page

Implementation of self interaction free density functional methods and applications to dilute magnetic semiconductors

THESIS 8366The present thesis mainly deals with the implementation of self interaction free density functional methods in a numerically efficient density functional theory code SIESTA and the application of the said methods in ab-inito electronic structure calculations. We present calculations on the polarizability of molecular chains using explicity orbital dependent density functional methods viz., exact exchange and self interaction corrected (SIC) functionals, which address some long standing issues in the field. We show that SIC functionals in fact perform better than the exact exchange functional at the KLI level of approximation. We discuss the ASIC method which is an approximate self interaction correction scheme that is computationally in-expensive. Results for finite systems as well as extended systems are discussed. We then discuss the electronic and magnetic structure of defective Hafnium Oxide systems where in we show that native Hafnium vacancies exhibit spin polarized ground states with ferromagnetic inter-defect coupling. An extensive study on Co doped ZnO is then presented mainly focusing on the possibility for intrinsic ferromagnetism in the material arising from native defect induced perturbations to the electronic structure of doped Co. We show that a defect-dopant complex center formed form a substitutional Co ion and an Oxygen vacancy can mediate long range ferromagnetic interactions. However we attribute the experimentally observed signatures of ferromagnetism to blocked superiparamagnetic clusters. Finally we present ab-initio electronic transport calculations on Mn12 based single molecule magnets in a two terminal device set up. The current-voltage (I-V) characteristics of the molecule for two different spin configurations of the Mn12 center are studied. We show that a change in the local magnetic configuration on the molecule can lead to a detectable change in the I-V characteristics