515 research outputs found
Magnetism and Transport Properties of Transition Metal Oxides and Nanoparticles
This dissertation is devoted to the study of the properties of transition metal oxides in both thin film and nanocrystalline forms.
The first section is devoted to the transport properties of manganese oxide thin film samples. The colossal magnetoresistance in these materials is usually explained using double-exchange, but this explanation is only partially correct. Recent theoretical and experimental work has shown that these compounds have a strong tendency towards phase-separation. The impact of strain on phase separation has been investigated by growing films of La5/8-0.3Pr0.3Ca3/8MnO3 on a variety of substrates. Very small changes in the induced strain cause large changes in the magnetic and magnetotransport properties of the film.
The second section is devoted to scanning tunneling microscopy studies of these same films at room temperature. In this case, small localized holes are observed at the Mn4+ ion sites. The distribution of holes is non-random and shows strong short range correlation.
The final section of this thesis is devoted to small particles of binary transition metal oxides. These oxides are antiferromagnetic in bulk samples, but when the dimensions of the particle are small (\u3c50nm), they show an enhanced magnetic moment. Nanoparticles of a-Fe2O3 have been prepared and their magnetic properties have been measured, and an increase in particle moment versus temperature has been observed using standard analysis techniques. More advanced analysis techniques indicate that this increase is a result of a transition specific to the a-Fe2O3 system, and not the result of thermoinduced magnetizatio
Nanoscale Voltage Enhancement at Cathode Interfaces in Li-ion Batteries
Interfaces are ubiquitous in Li-ion battery electrodes, occurring across
compositional gradients, regions of multiphase intergrowths, and between
electrodes and solid electrolyte interphases or protective coatings. However,
the impact of these interfaces on Li energetics remains largely unknown. In
this work, we calculated Li intercalation-site energetics across cathode
interfaces and demonstrated the physics governing these energetics on both
sides of the interface. We studied the olivine/olivine-structured
LixFePO4/LixMPO4 (x=0 and 1, M=Co, Ti, Mn) and layered/layered-structured
LiNiO2/TiO2 interfaces to explore different material structures and transition
metal elements. We found that across an interface from a high- to low-voltage
material the Li voltage remains constant in the high-voltage material and
decays approximately linearly in the low-voltage region, approaching the Li
voltage of the low-voltage material. This effect ranges from 0.5-9nm depending
on the interfacial dipole screening. This effect provides a mechanism for a
high-voltage material at an interface to significantly enhance the Li
intercalation voltage in a low-voltage material over nanometer scale. We showed
that this voltage enhancement is governed by a combination of electron transfer
(from low- to high-voltage regions), strain and interfacial dipole screening.
We explored the implications of this voltage enhancement for a novel
heterostructured-cathode design and redox pseudocapacitors
Transition state redox during dynamical processes in semiconductors and insulators
Activation barriers associated with ion diffusion and chemical reactions are
vital to understand and predict a wide range of phenomena, such as material
growth, ion transport, and catalysis. In the calculation of activation barriers
for non-redox processes in semiconductors and insulators, it has been widely
assumed that the charge state remains fixed to that of the initial electronic
ground state throughout a dynamical process. In this work, we demonstrate that
this assumption is generally inaccurate and that a rate-limiting transition
state can have a different charge state from the initial ground state. This
phenomenon can significantly lower the activation barrier of dynamical process
that depends strongly on charge state, such as carbon vacancy diffusion in
4H-SiC. With inclusion of such transition state redox, the activation barrier
varies continuously with Fermi level, in contrast to the step-line feature
predicted by the traditional fixed-charge assumption. In this study, a
straightforward approach to include the transition state redox effect is
provided, the typical situations where the effect plays a significant role are
identified, and the relevant electron dynamics are discussed
Singlet fission spin dynamics from molecular structure: a modular computational pipeline
Singlet fission, which has applications in areas ranging form solar energy to
quantum information, relies critically on transitions within a multi-spin
manifold. These transitions are driven by fluctuations in the spin-spin
exchange interaction, which have been linked to changes in nuclear geometry or
exciton migration. Whilst simple calculations have supported this mechanism, to
date little effort has been made to model realistic fluctuations which are
informed by the actual structure and properties of physical materials. In this
paper, we develop a modular computational pipeline for calculating singlet
fission spin dynamics by way of electronic structural calculations, molecular
dynamics, and numerical models of spin dynamics. The outputs of this pipeline
aid in the interpretation of measured spin dynamics and allow us to place
constraints on geometric fluctuations which are consistent with these
observations.Comment: 23 pages (including SI), 7 Figure
Budd-Chiari syndrome recurring in a transplanted liver
A patient with Budd-Chiari syndrome who underwent orthotopic liver transplantation and developed recurrent disease is described. The immediate postoperative period was complicated by multiple thrombotic episodes, followed by a period of apparent remission associated with the initiation of coumadin and persantine therapy. After discontinuation of such antithrombotic therapy in order to biopsy the liver, the patient experienced another series of clinically overt vascular thromboses and ultimately died of sepsis 15 mo posttransplantation after a prolonged and complicated terminal hospital course. At autopsy, recurrent Budd-Chiari syndrome as well as thromboses in numerous other organs was demonstrated. © 1983
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