Browsing by Author "Wu, Yiming"
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Item Austenite-Martensite phase transformation and magneto-mechanical studies on nickel manganese gallium single crystal alloy.(2010-05) Wu, YimingThe results of experiments on austenite-martensite phase transformation and magneto- mechanical behavior of a NiMnGa ferromagnetic shape-memory alloy are reported. This alloy undergoes a cubic to tetragonal martensitic phase transformation at about 5 oC and both phases are ferromagnetic. These experiments were conducted using a Magneto- Mechanical Testing Machine (MMTM) that is capable of simultaneously applying a uni- axial load and a magnetic ¯eld that can be varied in a plane containing the mechanical loading axis. One of the main accomplishments of this work was the extension of these measurements to tensile loads. Two types of experiments were conducted. First a set of experiments were performed to determine the e®ects of applied loads and magnetic ¯elds on the transformation temperature of the alloy. The e®ect of compression on the phase transformation is about 0.2 to 0.4 K/MPa, while the e®ect of tension is only 0.1 to 0.2 K/MPa. These results agree with predictions made using the Clausius-Clapeyron equation. The e®ects of applied magnetic ¯eld are more complicated and do not follow the simple tends predicted by this equation. Observations of the microstructure that forms during transformation agree reasonably well with the predictions of the crystal- lographic theory of martensite. The second set of experiments was conducted to measure the behavior of the alloy un- der a constant load and applied magnetic ¯elds. These experimental results are directly applicable to using this material in an actuator. Several di®erent magnetic ¯elds paths were used to determine if this has an e®ect on the strains observed. Measurements of the specimen average magnetization where also made using Hall probes to measure the stray ¯eld produced by the specimen during these experiments. These results have some portions of the strain-magnetization curves that are linear, which is what is pre- dicted by models that have both of these quantities directly related to variant volume fractions. Two of the key parameters for actuators that were measured are blocking stress and work output. It was found that the maximum work output occurs for a small tensile bias load due to the reduced e®ect of specimen demagnetization in this case. The blocking stress in tension appears to be just above the largest tensile stress of 3 MPa that could be applied, which is similar to the value in compression. The values of these two parameters compare well to values in the literature for compression, while the tensile results are the results to be reported.Item Effect of lattice mismatch strain on Fe8N-Fe16N2 phase transformation and aging process.(2010-12) Wu, YimingIt has been debated for decades whether ”-Fe16N2 has a giant saturation magnetization, which couldn’t be predicted by the traditional band magnetic theory. Because of a lot of previous inconsistent research results in the past 40 years, ”-Fe16N2 has been regarded as a debatable mystery material in magnetic research community. Recently, Wang’s group has successfully rationalized a partially localized 3d electron model based on the first principles calculation and predicted the existence of the giant saturation magnetization in ”-Fe16N2. Furthermore, we have synthesized FeN thin films with partially ordered ”-Fe16N2 phase on GaAs substrate with Fe as the underlayer. Our repeatable experimental results proved the existence of giant saturation magnetization of partially ordered ”-Fe16N2 phase. However, there has been a critical question on this topic for any experimentalist to answer, which has actually bothered magnetic researchers for almost four decades. Why did most experimental research groups not succeed to report the giant saturation magnetization value even with their fabricated FeN films with observed ”-Fe16N2 phase? This critical question has been answered in this MS thesis work. The effect of the initial strain on the phase transformation between Fe8N and Fe16N2, which is caused by the lattice mismatch between the FeN layer and its growth template, was investigated. A model was proposed, based on the Johnson-Mehl-Avrami equation, to describe the phase transformation process between Fe8N and Fe16N2 phases. Aging effect of the partially ordered Fe16N2 phase, e.g. degradation of giant magnetization behavior vs. time, was analyzed semi-quantitatively. Based on this proposed model, we have successfully rationalized the inconsistency of the fabricated ”-Fe16N2 films by different research groups using different growth technologies. An integrated facing-target sputtering system has been set up during my MSEE thesis work, which has three pairs of facing targets and ultra high vacuum capability. Note: Please see PDF abstract for correct chemical symbols that are not properly copiedItem Superconductivity at a quantum critical point: A theoretical approach to the understanding of unconventional superconductors in strongly correlated systems.(2021-08) Wu, YimingThe complexity of strongly correlated electronic system is manifested by the interplay of multiple electronic orders, among which superconductivity is one of the most interesting phases. In experimentally observed phase diagrams for materials such as heavy fermion compounds, cuprates and iron based superconductors, superconductivity is close to other electronic orders such as ferromagnetism, anti-ferromagnetism and nematicity etc. This fact brings about interests of studying the role of a possible underlying quantum critical point(QCP) in determining the unusual properties of these materials. Here we consider an itinerant fermion system which is close to a QCP. Because of the closeness, the collective boson mode due to the order parameter fluctuations will couple to low energy fermions and mediate the fermion-fermion interaction. This effective interaction simultaneously gives rise to two competing fate for the fermions: On one hand it can lead to SC if the there is any pairing instability in at least one pairing channel. On the other hand, the same interaction also diminish fermion coherence and results in non Fermi liquid behavior. These two tendencies compete with each other, in a sense that SC gaps out low energy fermions and reduces the self energy , while non Fermi liquid tends to destroy fermion coherence and is detrimental to SC. In order to capture this story we adopt the approach that Eliashberg first used when he studied the electron-phonon coupling system, i.e. we approximate the fermion self energy by neglecting the vertex corrections, which is controllable when the vertex is parametrically smaller. We further assume the interaction depends only on frequency via a dynamical exponent $\gamma$, namely $V(\Omega_m)\propto 1/|\Omega_m|^\gamma$. Based on this model, we unveil many special properties of SC state on both imaginary and real frequency axis, including the ‘gap closing’ behavior observed in cuprates. As a unique feature of pairing at a QCP, we find there exists an infinite set of solutions to the gap equation, corresponding to different local minima in free energy. This set becomes a continuous one at a special case $\gamma=2$, which corresponds to phonon-mediated pairing interaction with a vanishingly small phonon frequency. We also studied the odd-frequency pairing state from this model, and find there is no zero bias peak in the quasiparticle density of states which was considered as an evidence of odd- frequency pairing. At last, in addition to mean field analysis, the superconducting phase fluctuation is also discussed.