WorldCat Identities

Shen, Zhi-Xun

Works: 39 works in 46 publications in 2 languages and 55 library holdings
Genres: Conference papers and proceedings 
Roles: Thesis advisor, Author, Editor
Publication Timeline
Most widely held works by Zhi-Xun Shen
Proceedings of the Conference on Spectroscopies in Novel Superconductors : Stanford Linear Accelerator Center, Stanford, California, March 15-18, 1995 by Conference on Spectroscopies in Novel Superconductors( Book )

4 editions published in 1995 in English and held by 10 WorldCat member libraries worldwide

Ru he bi mian 10 ge zui da de gou fang xian jing( Book )

2 editions published in 1998 in Chinese and held by 4 WorldCat member libraries worldwide

Photoemission studies of high temperature superconductors and related materials by Zhi-Xun Shen( )

2 editions published in 1989 in English and held by 3 WorldCat member libraries worldwide

Spectroscopies in novel superconductors : [proceedings of the Conference on Spectroscopies in Novel Superconductors, Picacho Plaza Hotel, Santa Fe, New Mexico, March 17-19, 1993]( Book )

2 editions published between 1993 and 1995 in Undetermined and English and held by 2 WorldCat member libraries worldwide

Spectroscopies in novel superconductors : [proceedings of the Conference on Spectroscopies in Novel Superconductors, September 14-18, 1997, Sea Crest Conference Center, Falmouth, Cape Cod, Massachusetts by Conference on Spectroscopies in Novel Superconductors( Book )

2 editions published in 1995 in Undetermined and English and held by 2 WorldCat member libraries worldwide

Spin-resolved photoemission study of epitaxially grown MoSe2 and WSe2 thin films( )

1 edition published in 2016 in English and held by 1 WorldCat member library worldwide

Abstract: Few-layer thick MoSe2 and WSe2 possess non-trivial spin textures with sizable spin splitting due to the inversion symmetry breaking embedded in the crystal structure and strong spin–orbit coupling. We report a spin-resolved photoemission study of MoSe2 and WSe2 thin film samples epitaxially grown on a bilayer graphene substrate. We only found spin polarization in the single- and trilayer samples—not in the bilayer sample—mostly along the out-of-plane direction of the sample surface. The measured spin polarization is found to be strongly dependent on the light polarization as well as the measurement geometry, which reveals intricate coupling between the spin and orbital degrees of freedom in this class of material
Molecular diamonds enabled synthesis and growth by Hitoshi Ishiwata( )

1 edition published in 2015 in English and held by 1 WorldCat member library worldwide

Diamond has amazing bulk properties including extreme hardness, the highest thermal conductivity of any solid and high refractive index. Furthermore, wide band gap with the possibility of doping offer potential for power electronics. Hydrogen-terminated diamond surface exhibits negative electron affinity (NEA), which enables its use as a cathode for energy conversion or for electron emission devices. Recent developments in diamond research have focused on diamond nanoparticles which have shown a wide range of new applications including quantum computing, drug delivery, biomarkers and biosensors. Molecular diamonds, also known as "diamondoids" are the smallest possible forms of hydrogen-terminated diamond. The smallest diamondoid is an adamantane which consists of a single diamond cage composed of ten carbons terminated with hydrogen at the surface. Diamondoids offers a unique research platform in which diamond and diamond-surface properties can be explored at the molecular level. Fundamental properties of diamondoids, such as photoluminescence of the solid and gas phases, dielectric constants, ionization potentials, band structure etc. have been discussed in previous published papers. This thesis focuses primarily on three topics: 1) The use of diamondoid monolayers to obtain 10nm resolution on X-ray Photoemission Electron Microscope (XPEEM) images. 2) Identification of critical nucleation size for CVD diamond nucleation using diamondoid seeds of various sizes and shapes. 3) Symmetrical structure offered by SiV- color center was used for production of photonic nanocrystal structure with SiV- center to obtain identical emission spectrum. The first part of this thesis discusses applications of diamondoids for X-ray imaging. X-ray Photoemission Electron Microscope (XPEEM) is extremely useful for obtaining chemical and magnetic images. Typical XPEEM resolution is limited due the wide range of electron energies emitted from the target. To overcome this limitation, expensive focusing apparatus are generally necessary for best resolution. Taking advantage of the monochromatic nature of electrons photo-emitted through a diamondoid monolayer (FWHM of 0.3eV), we were able to overcome XPEEM resolution limitations by simply coating sample with a diamondoid monolayer. Using this technique, we were able to obtain the 10nm resolution XPEEM image. We found diamondoid monolayers minimized chromatic aberration by a factor of 10 and increased contrast by a factor of two for samples ranging from 100nm to 10nm. We achieved this resolution by simply dipping the sample in a solution containing diamondoids with functional groups (linkers) designed to form Self Assembled Monolayers on the sample surface. The second part of this thesis describes use of diamondoid seeds of various sizes and shapes can lead to a better understanding of the critical nucleation size of diamond nanoparticles in CVD diamond growth. By comparing diamond growth from various pure seeds including diamantane of different chemical funcationalization configuration, two different tetramantane isomers and two pentamantane isomers, a critical nucleation size at sub 1nm, smaller than previously thought, was determined. Critical nucleation size was determined by an exponential increase in nucleation rate as thermal activation is no longer required for a post-critical size particle. In addition, this study found the critical nuclei depend on both size and structure of the molecule. The third topic of this thesis revolves around the development of a bottom-up method for growing diamond nanoparticles with Silicon-Vacancy (SiV)- color centers. SiV- centers have emerged as promising candidates for quantum information processing applications and possibly as biosensors. The inversion symmetry of SiV- centers provide superior spectral stability and narrow inhomogeneous broadening with linewidths of a few nanometers and nearly transform-limited linewidths at low temperature. We demonstrate stability of SiV- color center by forming nanophotonic crystal structure on SiV film and SiC using diamondoids as seeds for diamond nucleation. The resulting nanopillars contain high quality SiV- centers with very narrow linewidths and very low inhomogeneous broadening, thereby enabling implementation of quantum photonic devices containing identical quantum emitters, as needed for quantum simulations and quantum network
Numerical studies of cuprate high-temperature superconductors by Yvonne Kung( )

1 edition published in 2015 in English and held by 1 WorldCat member library worldwide

Strongly correlated electron systems exhibit a panoply of fascinating phases, such as antiferromagnetism, charge order, and unconventional superconductivity, that compete or cooperate with one another. Yet the intertwined degrees of freedom that create this complexity also obscure the underlying mechanisms, so that numerous experimental and theoretical studies have been brought to bear on these materials. This thesis provides a window on the field from the perspective of phenomenological and microscopic studies of the cuprates and nickelates. The first part presents a phenomenological model of strongly coupled orders that explains recent time-resolved x-ray diffraction experiments on the nickelates. The new experimental technique enables us to disentangle degrees of freedom that are intertwined in thermal equilibrium, motivating a need for theoretical developments. Our time-dependent Ginzburg-Landau theory facilitates an understanding of how coupled orders behave when driven out of equilibrium. The second part of the thesis explores numerical simulations of microscopic models to answer open questions in the cuprates. A comparison of spin and charge susceptibilities computed in the single-band Hubbard model via determinant quantum Monte Carlo (DQMC) to those calculated using the random phase approximation (RPA) elucidates how electronic correlations evolve throughout the phase diagram. Combined DQMC and exact diagonalization (ED) studies of the three-orbital Hubbard model systematically evaluate broken-symmetry states that have been proposed to explain the pseudogap regime. Thus the use of complementary techniques, both analytical and computational, sheds light on different ordered phases and excitations in strongly correlated systems and points the way to further studies
Numerical studies of inelastic light scattering in correlated materials by Chunjing Jia( )

1 edition published in 2014 in English and held by 1 WorldCat member library worldwide

The mechanism of high temperature superconductivity has been a puzzle for almost three decades. With the rapid development of synchrotron light sources, photon spectroscopies such as x-ray absorption (XAS), inelastic x-ray scattering (IXS) and resonant inelastic x-ray scattering (RIXS), have shown to provide more and more useful information for the study of elementary excitations in strongly correlated ma- terials. In particular, RIXS at the Cu K-edge has been shown to be a probe of charge excitations. RIXS at the Cu L-edge in hole-doped cuprates has been shown to measure high energy collective spin excitations which persist well into the overdoped regime. We perform numerical simulations to study light scattering cross-sections using the single and multi-orbital Hubbard models. Our calculations highlight the ability to obtain spin and charge dynamical structure factors using Cu L or K-edge RIXS, respectively, and the importance of incident energy dependence (or the res- onant pro le) and the full in uence of light polarization. Our results ll in the gap between theoretical understanding and inelastic light scattering experiments on correlated materials, and further provides useful information for understanding the superconducting mechanism
Yi ge quan li shi dai de jie shu( Book )

1 edition published in 1994 in Chinese and held by 1 WorldCat member library worldwide

Electronic phases in iron-based high temperature superconductors by Ming Yi( )

1 edition published in 2014 in English and held by 1 WorldCat member library worldwide

High temperature superconductivity (HTSC) is a topic that has fascinated physicists for decades not only due to the intricate underlying properties of nature that it reflects, but also to the challenging nature of the physics problem it presents. For over two decades, condensed matter physicists have wrestled the cuprate problem as the only material family of HTSC. The discovery of the iron-based superconductors (FeSCs) in 2008 brought a fresh perspective to the field, as now comparison and contrast can be made between the two families in the hope of identifying the bare minimal ingredients for the HTSC phenomenon. In this regard, one of the key findings early on was that the FeSC share a phase diagram that is arguably similar to the cuprates, that is, the parent compound is a magnetically ordered state, which is suppressed with doping before superconductivity emerges in a dome-like fashion. In this dissertation, I approach the FeSC problem from two major perspectives using the technique of angle-resolved photoemission spectroscopy: i) understanding the electronic correlation strength in the normal state, and ii) understanding the nature of the electronic phases in proximity to superconductivity. Through a comprehensive and systematic study of many FeSC families, I find that FeSCs are not Fermi liquids, nor are they strongly correlated like the doped Mott insulators of the cuprates, but rather moderately correlated, for which theoretical models of both itinerant and localized components are necessary for capturing the essential physics. For the second perspective, I find direct evidence for an electronic nematic phase bounded by the structural transition and Fermi surface reconstruction associated with the spin density wave order. Furthermore, I present direct spectroscopic evidence that these two orders compete with superconductivity in an underdoped FeSC. These results are similar to the competing nature of the pseudogap to superoconductivity in the cuprates, suggesting that phase competition may play a non-trivial role for HTSC
Unexpected edge conduction in mercury telluride quantum wells under broken time-reversal symmetry by Eric Yue Ma( )

1 edition published in 2017 in English and held by 1 WorldCat member library worldwide

Operando X-ray photoelectron spectroscopy investigation of ceria/gas electrochemical interfaces by Zhuoluo Albert Feng( )

1 edition published in 2015 in English and held by 1 WorldCat member library worldwide

At solid-gas electrochemical interfaces, gas molecules interact dynamically with surface ions and electrons. A fundamental understanding of the technologically important interfaces can lead to better fuel cells and electrolyzers. In the bulk of typical oxygen-ion-conducting solids, oxygen vacancies and mobile electrons migrate under the influence of concentration and electrostatic potential gradients. Similarly, at gas-solid interfaces, these charge carriers migrate across an electrochemical double layer. The two-way traffic of ions and electrons contrasts sharply with conventional metal-based electrocatalysis, in which only electrons are transferred. This type of ion insertion reaction is ubiquitous in energy conversion and storage devices, such as lithium ion batteries, water-splitting membranes and solid oxide fuel cells. CeO2-[delta] (ceria) is a model oxygen-ion-conducting electrode, which is commonly employed to catalyze H2 oxidation and H2O dissociation reactions, as well as CO oxidation and CO2 dissociation reactions. In my thesis studies, I developed synchrotron-based ambient pressure X-ray photoelectron spectroscopy to characterize the electrochemical double layer under reaction conditions. Concentrations and binding energy of oxygen ions, localized electrons, and surface reaction intermediates were quantified using core level and valence band X-ray photoelectron spectroscopy as a function of electrochemical overpotentials. These measurements reveal that localized electrons and oxygen vacancies segregate persistently from the bulk to the surface, resulting in concentrations up to four orders of magnitude greater on the surface than in the bulk. Under water splitting conditions, H2O molecules incorporate rapidly into surface oxygen vacancies. Spectroscopy and electrochemistry results suggest that the electron transfer between Ce 4f states and OH adsorbates is rate determining. Under CO oxidation and CO2 dissociation conditions, on the other hand, carbonate is the stable adsorbate. The larger footprint of carbonate relative to hydroxyl adsorbate gives rise to adsorbate-adsorbate interactions, resulting in a coverage-dependent reaction pathway. Lastly, measurement of surface dipole potential energy in both cases reveals intrinsic dipole moments of adsorbates as the origin of electrostatic potential gradient near the surface. Combined, these in-situ investigations unravel the electrochemical reaction pathway, particularly the role of point defects at ceria/gas interfaces, and establish a rational path towards enhancing the efficacy of oxide electrocatalysts
Experimental studies of high-Tc cuprate superconductors with density wave correlations by He Ruihua( )

1 edition published in 2010 in English and held by 1 WorldCat member library worldwide

High-Tc cuprate superconductors remain the source of central intellectual challenges for condensed matter physicists two decades after their discovery. Accumulating evidence suggests possible ubiquity of coexisting superconducting and density wave correlations in the ground state of these materials. As both correlations in their spatiotemporally incoherent forms can in principle produce a pseudogap in the excitation spectrum, the nature of the pseudogap widely seen in cuprates above Tc comes to forefront of debate. I have been trying to address the relationship triangle between the pseudogap, superconductivity and density waves, by mainly using angle-resolved photoemission spectroscopy (ARPES) to study the momentum spaces of single-layer La-based (La214) and Bi-based (Bi2201) cuprates. In this thesis, I first talk about our ARPES-neutron scattering joint effort (in collaboration with Tohoku University) in the stripe-ordered state of La214. We found around the 1/8 doping level a dual nature of the pseudogap (in LBCO) and a dual nature of the spin stripe ordering ground state. The latter will be discussed in the context of a doping-dependent crossover between localized- and itinerant-spin ground-state physics, as demonstrated for the first time in a condensed matter system (1% Fe-doped LSCO). I then focus on our ARPES findings in both the pseudogap and superconducting states of nearly optimally-doped Pb-Bi2201 obtained over unprecedented momentum, energy and temperature ranges. They suggest that the pseudogap is a broken-symmetry state that is density-wave like and distinct from homogeneous superconductivity; it explicitly coexists with coherent superconductivity below Tc, causing a striking, momentum-dependent distortion of the high-Tc ground state. I also include in the appendix our recent observation of symmetry-distinct states (likely derived from the Cu dz orbital) in proximity to the Zhang-Rice singlet that has so far been generally taken as the only relevant electronic building block for cuprates. This finding might profoundly transform our upper-level understanding of high-Tc superconductivity as discussed above. This thesis ends with a chapter which describes my experimental philosophy, speculations and outlook for the near future of this field
Microfabricated thermionic energy converters by Jae Hyung Lee( )

1 edition published in 2013 in English and held by 1 WorldCat member library worldwide

Thermionic energy converters (TECs) are heat engines that convert heat directly to electricity at very high temperatures. This energy conversion process is based on thermionic emission--the evaporation of electrons from conductors at high temperatures. In its simplest form, the converter consists of two electrodes in the parallel plate capacitor geometry, and it uses the thermionically emitted current to drive an electrical load. This dissertation presents research on five key areas of microfabricated thermionic energy converters ([mu]-TECs). First, the numerical calculation of the emitter-collector gap that maximizes the power conversion efficiency of thermionic energy converters (TECs) is discussed. Thermionic energy converters require emitter and collector work-functions that are relatively low, to reach useful efficiencies at typical operating temperatures of 1000 - 1500 ºC. The optimum arises because efficiency drops both at very large gaps, due to space-charge limitations on the TEC current, and at very small gaps, due to the increased heat loss via near-field radiative heat transfer. The numerical calculation results show that, for typical TECs made with cesiated tungsten electrodes, the optimal gaps range from 900 nm to 3 [micrometers]. I then discuss several prototypes of mechanically and thermally robust [mu]-TECs, including the stress-relieved emitter design, emitter-collector structural design, as well as a recent approach for the stand-alone (encapsulated) [mu]-TECs. Thermionic emission from the SiC emitter was demonstrated for the first time. The stress-relieved design emitters were analyzed, and the work-function of the SiC emitter was estimated at temperatures of up to 2900K. Also described are both the planar and the U-shaped suspension for microfabricated TECs ([mu]-TECs). Our initial planar [mu]-TECs achieved emitter temperatures of over 2000 K with incident optical intensity of approximately 1 W/mm2 (equivalent to 1000 Suns), remained structurally stable under thermal cycling, and maintained a temperature difference between the emitter and the collector of over 1000 K. Conformal sidewall deposition of poly-SiC on a sacrificial mold is used to fabricate stiff suspension legs with U-shaped cross sections, which increases the out-of-plane rigidity and prevents contact with the substrate during the heating of the suspended emitter. By extending the conventional technique of cesium coatings to SiC, we reduce the work-function from 4 eV to 1.65 eV at room temperature. Subsequently, we tested [mu]-TECs with both barium and barium oxide coatings. The coatings reduced the work-function of the SiC emitter to as low as ~2.14 eV and increased the thermionic current by 5-6 orders of magnitude, which is a key step toward realizing a efficient thermionic energy converter. Encapsulation of [mu]-TEC was achieved by an anodic bond between pyrex and the silicon substrate with via feedthroughs. Last, I introduce the photon-enhanced thermionic emission (PETE) concept, and show why a single crystal photo-emitter is needed. I cover my recent fabrication development of smart-cut layer transfer using Spin-on-Glass (SoG). In addition, a novel layer transfer technology that can transfer any device materials onto the glass substrate, which I call "Anything on Glass, " is briefly described. I, then, describe how the first demonstration of the photon-enhanced thermionic emission (PETE) from the microfabricated emitter was achieved. The p-type SiC emitter was used to demonstrate PETE in an uncesiated and microfabricated sample, bringing this energy conversion approach closer to practical applications
Proceedings of the Conference on Spectroscopies in Novel Superconductors Held in Stanford, California on 15-18 March 1995( )

1 edition published in 1996 in English and held by 1 WorldCat member library worldwide

A copy of the proceedings published in the Journal of Physics and Chemistry of Solids is included as a part of the report. The Conferences on Spectroscopies in Novel Superconductors started with the Argonne meeting in 1991, where the k-resolved spectroscopies of electronic states at and near the Fermi energy were discussed. The spectroscopies included in the Argonne conference were photoemission, de Haas-van Alphen, positron 2D-ACAR and tunnelling. In the following conferences in Sendai, July, 1992 and in Santa Fe, March, 1993, the scope the conference expanded to include optical, microwave, transport, NMR and NQR measurements. The Stanford conference further broadened the conference scope by including neutron, quantum interference and thermodynamics experiments. This evolution reflects the growing sophistication of the field and the increasing importance of spectroscopies to understanding the novel superconductors. p4 & 5
Charge transport at the nanoscale : new materials and probes by Jason David Fabbri( )

1 edition published in 2012 in English and held by 1 WorldCat member library worldwide

Many biological phenomena and energy harvesting devices involve electron transport through organic molecules. Conductance measurements at the interface between inorganic electrodes and a single organic molecule or molecular monolayer, when combined with structural characterization, can yield precise understanding of molecular charge transport. However, difficulties such as interface instability, molecular structure fluctuations, and limited in-situ probe access have hampered progress. One of the major challenges has been ambiguity in the electron distribution and electrostatic potential within a molecular junction. The charge transport is known to be critically dependent on these parameters, yet experimental measurements have been lacking. We have developed an experimental method to measure these parameters using synchrotron X-ray reflectivity (XRR) combined with a soft lithographic technique to form robust large-area molecular junctions. High resolution electron distribution plots of a chlorophyll monolayer between two macroscopic electrodes were obtained. Using a lock-in technique to detect small changes in reflected intensity as a function of applied voltage, the electrostatic potential profile within the junction was measured. Many studies involving systematic variations of molecular length have yielded important insights into charge transport. More elaborate structural variations have not been as thoroughly explored due to lack of suitable materials. Diamondoids are a new class of carbon nanomaterial with rigid, well-defined sizes and shapes making them an attractive platform to explore the relationship between molecular structure and charge transport. We deposited a series of diamondoid thiol monolayers on gold and measured current-voltage (I-V) tunneling curves using conducting atomic force microscopy (AFM). One of the diamondoid isomers showed surprisingly efficient charge transport, making it appear more like a conjugated molecule despite its being a fully saturated hydrocarbon. Using ultraviolet photoelectron spectroscopy (UPS) and density functional theory (DFT) computations, along with in-depth structural characterization of the monolayers, we are able to explain this finding by enhanced intermolecular electronic coupling. Reinterpretations of certain results in the field of molecular electronics are suggested by our results. We also lay the groundwork for future electron tunneling studies through diamondoid molecular assemblies by characterizing the first diamondoid Langmuir films. Isothermal data, AFM, grazing incidence X-ray diffraction (GIXD), and interfacial stress rheometry (ISR) were used to characterize the thermodynamic, morphological, structural, and mechanical properties of diamondoid Langmuir and Langmuir-Blodgett films. This is the first study of a pure nanodiamond film at the air/water interface. Finally, we fabricate and characterize the performance and stability of a molecular electronic device. This device consists of a diamondoid siloxane monolayer deposited via solution or vapor phase on a silicon substrate. This composite material shows intense, stable monochromatic electron photoemission. Contact angle measurements, Fourier transform infrared (FTIR) spectroscopy, near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, and X-ray photoelectron spectroscopy (XPS) are used to characterize the structure and stability of the monolayers. Appendices discuss diamondoid molecular crystal growth and mechanical properties, a new method for X-ray reflectivity data analysis, air-free chemical attachment of monolayers, and the relevance of Landauer transport modeling to intermolecular charge transport as measure by transition voltage spectroscopy
Magnetic phases of the frustrated spin dimer compound Ba3Mn2O8 by Eric Cohen Samulon( )

1 edition published in 2010 in English and held by 1 WorldCat member library worldwide

Spin dimer compounds are based on pairs of spins with antiferromagnetic exchange. At low fields the ground state is a product of singlets, with excited triplet states at higher energies. Application of a magnetic field closes the spin gap between the excited triplet state and singlet state. Interactions between dimers broaden the triplet bands, such that above a critical field where the minimum of the triplet band crosses the singlet, long range magnetic order (LRMO) can arise. The ordered states can take several novel forms, including a spin superlattice or a Bose-Einstein condensate of magnons, depending upon the spin Hamiltonian describing the system. Ba3Mn2O8 is a spin dimer system based on dimers of S = 1, 3d2, Mn5+ ions arranged on a triangular lattice. A pair of antiferromagnetically linked S = 1 ions has total spin 0, 1 or 2, leading to, in zero field, excited quintuplet states in addition to the excited triplet states above the singlet ground state. The triangular lattice is composed of vertical dimers on hexagonal layers which are stacked according to an 'ABC' structure. In this thesis, I describe the results of experiments which probed this system via different thermodynamic measurements of single crystals, revealing at least three novel ordered states. Measurements of heat capacity, magnetocaloric effect, torque magnetometry and magnetostriction revealed significant anisotropy in the singlet-triplet regime, with a single ordered state observed for fields along the easy c axis and two states observed for fields away from that direction. Analysis of the minimal spin Hamiltonian yields candidate phases for the canted antiferromagnetic order observed, including incommensurate order close to the archetypal 120° order for triangular systems as well as modulated order for fields away from the c axis. The triplet-quintuplet regime was probed via heat capacity, magnetocaloric effect and magnetization measurements, the first experiments to probe such ordered states of a spin dimer compound. A significant asymmetry in the quintuplet condensate was revealed in both the magnetization and the phase boundary. This asymmetry is understood as a consequence of zero point phase fluctuations, which are absent at the saturation field but present everywhere else. Finally, the effect of disorder in this spin dimer compound was studied by substitution of non-magnetic S = 0 3d0, V5+ ions for the S = 1, 3d2, Mn5+ ions in Ba3(Mn1-xVx)2O8. This work was motivated in part by theoretical predictions that substitution of non-magnetic species on a square lattice of dimers would result in a low-temperature ordered magnetic state, for which interactions between the unpaired magnetic moments is mediated by short range correlations of the background singlet ground state. We do not find any evidence for such a state down to 50 mK. Rather, the magnetic entropy is progressively removed over an extended range of temperature, from [approximately equal to]2 K down. The temperature and doping dependence of the heat capacity do not conform to expectations for a spin glass, leading us to suggest that Ba3(Mn1-xVx)2O8 manifests a random singlet state for the range of compositions and temperatures studied
Improving the cathode/electrolyte interface : from alloying electrodes to refining grain boundaries by Xu Tian( )

1 edition published in 2012 in English and held by 1 WorldCat member library worldwide

Solid oxide fuel cells (SOFCs) have been demonstrated to have great potential for efficiently converting chemical energy to electrical energy. Recent studies have focused on reducing the operating temperature of SOFCs to below 500°C. At such low temperatures, the cathode/electrolyte interface is one of the bottlenecks for better fuel cell performance. In this work, improving the cathode/electrolyte interface was first attempted by replacing pure platinum electrodes with platinum alloy electrodes. Pure platinum electrodes suffer degradation of microstructure over time at elevated temperatures due to The Ostwald ripening. The results show, that in solid oxide fuel cells employing nanoporous Pt-Ni cathodic catalysts instead of pure Pt, better microstructural stabilities, lower electrode impedances, and higher power densities can be achieved at elevated operating temperatures (350-550°C). Independently, improving the cathode/electrolyte interface was also attempted by improving surface structures of the electrolyte. Grain boundary regions of electrolytes were postulated to enhance the rate of oxide ion incorporations. A continuum model was developed to simulate oxide ion diffusion under the influence of electrical fields in polycrystalline oxides. Based on experimental O-18 tracer diffusion data, the model is consistent with enhanced oxide ion incorporation rates in close proximity of the grain boundaries. The results suggest that employing electrolytes with high densities of surface grain boundaries may enhance the performance of low-temperature SOFCs
Broken rotational symmetry in iron based superconductors by Jiun-Haw Chu( )

1 edition published in 2012 in English and held by 1 WorldCat member library worldwide

High-temperature superconductivity often emerges in the proximity of a symmetry-breaking ground state in strongly interacting electronic materials. In the case of the superconducting iron pnictides, in addition to the antiferromagnetic ground state of the parent compounds, a ubiquitous but small structural distortion breaks the crystal C4 rotational symmetry in the underdoped part of the phase diagram. It has been proposed that this structural transition is driven by an electronic nematic phase transition, below which the electronic system spontaneously organizes with an orientational order without developing additional spatial periodic order. In this thesis I show how the effects of this electronic nematic order can be explicitly revealed by observing the response of the system to in-plane uniaxial stress. I present transport measurements of single crystal samples of various iron-based superconductors held under an in-situ tunable strain at temperatures above the phase transition, which explicitly confirm that the structural transition is fundamentally driven by a thermodynamic instability in the electronic part of the free energy. I will also discuss the nematic fluctuations prevailing throughout the overdoped part of the phase diagram, which suggests that a quantum phase transition occurs at the optimal doped composition. Finally, I will describe the in-plane resistivity anisotropy measurements of detwinned samples at temperatures below phase transition, which potentially reveals an intriguing interplay between topological protected Dirac pockets and spin/orbital ordering
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