23 research outputs found
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 \{100\} surface. We
find that extended surface states can hybridize with the -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
Coordination Characteristics of Uranyl BBP Complexes: Insights from an Electronic Structure Analysis
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