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Browsing by Subject "iron pnictides"

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    Sign-changing s-wave symmetry in iron-based superconductors: Manifestations and extensions
    (2016-12) Hinojosa Alvarado, Alberto
    I perform theoretical studies of the family of iron-based superconductors, which are a group of materials that can achieve a relatively high critical temperature Tc. In most of these multi-band compounds the superconducting gap parameter has s-wave symmetry along the Fermi surfaces, but the sign of the gap can change between Fermi surfaces yielding the so-called s+- symmetry. In this dissertation I focus on the experimental consequences of this gap structure and later on two of its possible extensions. In the first part, I review how the resonance in inelastic neutron scattering can be explained as a pole in the spin susceptibility in an s+- superconductor, computed using the random phase approximation. Then I extend the analysis to include the effect of pairing fluctuations and show that except in special cases these fluctuations merely shift the frequency of the resonance by a few percentage points. I also consider Raman spectroscopy experiments that measured the susceptibility in the B1g symmetry channel and found a strong temperature dependence in the static part and a resonance below Tc in the dynamic part. I show how both of these can be explained through the coupling of fermions to spin fluctuations via the Aslamazov-Larkin process. In the second part, I study the gap structure when superconductivity develops from a preexisting antiferromagnetic state. I show that magnetism induces an additional spin-triplet pairing component in addition to the standard singlet pairing. This additional pairing state can coexist with the standard one and leads to superconductivity that breaks time-reversal symmetry. I also consider the case of materials whose gap structure has accidental nodes on the electron pockets. I analyze how two competing types of hybridization effects between the electron pockets shift the nodes in different directions and the consequences for the gap structure.
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    Static and Dynamic Electron Microscopy Investigations at the Atomic and Ultrafast Scales
    (2016-05) Suri, Pranav Kumar
    Advancements in the electron microscopy capabilities – aberration-corrected imaging, monochromatic spectroscopy, direct-electron detectors – have enabled routine visualization of atomic-scale processes with millisecond temporal resolutions in this decade. This, combined with progress in the transmission electron microscopy (TEM) specimen holder technology and nanofabrication techniques, allows comprehensive experiments on a wide range of materials in various phases via in situ methods. The development of ultrafast (sub-nanosecond) time-resolved TEM with ultrafast electron microscopy (UEM) has further pushed the envelope of in situ TEM to sub-nanosecond temporal resolution while maintaining sub-nanometer spatial resolution. A plethora of materials phenomena – including electron-phonon coupling, phonon transport, first-order phase transitions, bond rotation, plasmon dynamics, melting, and dopant atoms arrangement – are not yet clearly understood and could be benefitted with the current in situ TEM capabilities having atomic-level and ultrafast precision. Better understanding of these phenomena and intrinsic material dynamics (e.g. how phonons propagate in a material, what time-scales are involved in a first-order phase transition, how fast a material melts, where dopant atoms sit in a crystal) in new-generation and technologically important materials (e.g. two-dimensional layered materials, semiconductor and magnetic devices, rare-earth-element-free permanent magnets, unconventional superconductors) could bring a paradigm shift in their electronic, structural, magnetic, thermal and optical applications. Present research efforts, employing cutting-edge static and dynamic in situ electron microscopy resources at the University of Minnesota, are directed towards understanding the atomic-scale crystallographic structural transition and phonon transport in an iron-pnictide parent compound LaFeAsO, studying the mechanical stability of fast moving hard-drive heads in heat-assisted magnetic recording (HAMR) technology, exploring the possibility of ductile ceramics in magnesium oxide (MgO) nanomaterials, and revealing the atomic-structure of newly discovered rare-earth-element-free iron nitride (FeN) magnetic materials. Via atomic-resolution imaging and electron diffraction coupled with in situ TEM cooling on LaFeAsO, it was found that additional effects not related to the structural transition, namely dynamical scattering and electron channeling, can give signatures reminiscent of those typically associated with the symmetry change. UEM studies on LaFeAsO revealed direct, real-space imaging of the emergence and evolution of acoustic phonons and resolved dispersion behavior during propagation and scattering. Via UEM bright-field imaging, megahertz vibrational frequencies were observed upon laser-illumination in TEM specimens made out of HAMR devices which could be detrimental to their long-term thermal and structural reliability. Compression testing of 100-350 nm single-crystal MgO nanocubes shows size-dependent stresses and engineering strains of 4-13.8 GPa and 0.046-0.221 respectively at the first signs of yield accompanied by an absence of brittle fracture, which is a significant increase in plasticity of a brittle ceramic material. Atomic-scale characterization of FeN phases show that it is possible to detect interstitial locations of low atomic-number nitrogen atoms in iron crystal and hints at a development of novel routes (without involving rare-earth elements) for bulk permanent magnet synthesis.