Physics
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Item The Development of Tunnel Diode Oscillators And High Magnetic Field Studies of Unconventional Superconductor UTe2: Unveiling the Phase Diagram And Unconventional Hall Effect(2023) Lin, Wen-Chen; Paglione, Johnpierre J; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This dissertation presents an extensive investigation of UTe2 under various magnetic field and pressure conditions by utilizing the Tunnel Diode Oscillator (TDO) and electrical transport. The study is primarily divided into two parts. The first part focuses on the utilization of the TDO to establish a magnetic susceptibility measurement device. The TDO's frequency, highly sensitive to inductance changes due to the negative I-V curve regime of the tunnel diode, is harnessed to create a measurement circuit compatible with low-temperature refrigerators. The design and development process of these devices are thoroughly detailed. The second part unveils the comprehensive (H, T, P) phase diagram of UTe2 under magnetic fields reaching 41 T along the crystallographic b-axis, combined with applied pressures of up to 18.8 kbar. Utilizing magnetoresistance and tunnel diode oscillator measurements, we investigated the pressure-induced evolutions of multiple phases. By monitoring the field-induced transition between superconducting and magnetic field-polarized phases across various pressures (up to 18.8 kbar), we track the suppression of this transition with increasing pressure. This suppression culminates in the disappearance of superconductivity near 16 kbar, concurrently leading to a distinct, pressure-induced magnetic ordered state that is stable at zero field. The evolution of a second superconducting phase and its upper critical field under pressure are also investigated, leading to insights into the confinement of superconductivity by magnetic phases and the boundaries of triplet superconductivity.Item Photoexcitation of graphene in the quantum Hall regime(2021) Cao, Bin; Hafezi, Mohammad; Solomon, Glenn; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Multipole transitions beyond the dipole approximation apply when the Bohr radius of the quantum state is larger or comparable to the excitation wavelength. This is rarely the case for atoms or quantum dots. However, in the quantum Hall regime, wave functions can be extended to a length scale comparable to optical wavelengths, and the coherence is topologically protected against dephasing. Consequently, multipole transitions become possible. Motivated by this, we study the light-matter interaction in graphene in the quantum Hall regime, manifested as the photocurrent (PC). In the first part of the thesis, we experimentally study the PC in graphene in the quantum Hall regime. Prominent PC oscillations as a function of gate voltage on samples’ edges are observed with minimal obscurations and noise. These oscillation amplitudes form an envelope which depends on the strength of the magnetic field, as does the PCs’ power dependence and their saturation behavior. We explain these experimental observations through a model using optical Bloch equations, incorporating relaxations through acoustic-, optical-phonons and Coulomb interactions. The simulated PC agrees with our experimental results, leading to a unified understanding of the chiral PC in graphene at various magnetic field strengths, and providing hints for the occurrence of a sizable carrier multiplication. In the second part, we theoretically study the light-matter interaction beyond dipole, manifested as a PC. Inspired by the seminal gedankenexperiment by Laughlin which describes the charge transport in quantum Hall systems via the pumping of flux, we propose an optical scheme which probes and manipulates quantum Hall systems in a similar way: When light containing orbital angular momentum interacts with electronic Landau levels, it acts as a flux pump which radially moves the electrons through the sample. We investigate this effect for a graphene system with Corbino geometry, and calculate the radial current in the absence of any electric potential bias. Remarkably, the current is robust against the disorder, and in the weak excitation limit, the current shows a power-law scaling with intensity characterized by the novel exponent 2/3.Item RAPID HEATING AND CHEMICAL SPECIATION CHARACTERIZATION FOR COMBUSTION PERFORMANCE ANALYSIS OF METALLIZED, NANOSCALE THERMITES AND PVDF BOUND SOLID PROPELLANT COMPOSITIONS(2021) Rehwoldt, Miles Christian; Rodriguez, Efrain; Zachariah, Michael R; Chemical Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Energetic materials research focuses on performance analysis of cost-effective solid materials which safely, precisely, and efficiently transitions stored chemical potential energy to kinetic energy at a rate throttled through chemical or architectural means. Heterogenous compositions of metal fuels and solid materials with a high storage capacity of condensed oxidizing elements, such as oxygen and/or fluorine, is a class of energetic material of interest given its relatively high reaction enthalpies and adiabatic flame temperatures. In the wake of the earliest instances of metal fuels being used as a high energy additive during World War II, characterizing the reaction mechanisms of micron and nanoparticle aluminum fuels with various oxidizer sources has been a primary subject of research within the solid energetics community. The advent of nanotechnologies within the past two decades brought with it the promise of a prospective revolution within the energetics community to expand the utility and characterization of metallized energetic materials in solid propellants and pyrotechnics. Significant prior research has mapped reactivity advantages, as well as the many short comings of aluminum-based nanoscale energetic formulations. Examples of short comings include difficulties of materials processing, relative increase in native oxide shell thickness, and particle aggregate sintering before primary reaction. The less than flaw-less promises of nanoscale aluminum fuels have thus become the impetus for the development of novel architectural solutions and material formulations to eliminate drawbacks of nanomaterial energetics while maintaining and improving the benefits. This dissertation focuses on further understanding reaction mechanisms and overall combustion behavior of nanoscale solid energetic composite materials and their potential future applications. My research branches out from the heavy research involved in binary, aluminum centric systems by developing generalized intuition of reaction and combustion behaviors through modeling efforts and coupling time-of-flight mass spectrometry to rapid heating techniques and novel modes of product sampling. The studies emphasize reaction mechanisms and microwave sensitivities of under-utilized compositions using metal fuels such as titanium, generalize the understanding of the interaction of fluoropolymer binders with metal fuels and oxidizer particles, and characterize how multi-scale architectural structure-function relations of materials effect ignition properties and energy release rates.Item SUPERCONDUCTING RADIO FREQUENCY MATERIALS SCIENCE THROUGH NEAR-FIELD MAGNETIC MICROSCOPY(2020) Oripov, Bakhrom Gafurovich; Anlage, Steven M; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Superconducing Radio-Frequency (SRF) cavities are the backbone of a new generation of particle accelerators used by the high energy physics community. Nowadays, the applications of SRF cavities have expanded far beyond the needs of basic science. The proposed usages include waste treatment, water disinfection, material strengthening, medical applications and even use as high-Q resonators in quantum computers. A practical SRF cavity needs to operate at extremely high rf fields while remaining in the low-loss superconducting state. State of the art Nb cavities can easily reach quality factors Q>2x10^10 at 1.3 GHz. Currently, the performance of the SRF cavities is limited by surface defects which lead to cavity breakdown at high accelerating gradients. Also, there are efforts to reduce the cost of manufacturing SRF cavities, and the cost of operation. This will require an R&D effort to go beyond bulk Nb cavities. Alternatives to bulk Nb are Nb-coated Copper and Nb3Sn cavities. When a new SRF surface treatment, coating technique, or surface optimization method is being tested, it is usually very costly and time consuming to fabricate a full cavity. A rapid rf characterization technique is needed to identify deleterious defects on Nb surfaces and to compare the surface response of materials fabricated by different surface treatments. In this thesis a local rf characterization technique that could fulfill this requirement is presented. First, a scanning magnetic microwave microscopy technique was used to study SRF grade Nb samples. Using this novel microscope the existence of surface weak-links was confirmed through their local nonlinear response. Time-Dependent Ginzburg-Landau (TDGL) simulations were used to reveal that vortex semiloops are created by the inhomogenious magnetic field of the magnetic probe, and contribute to the measured response. Also, a system was put in place to measure the surface resistance of SRF cavities at extremely low temperatures, down to T=70 mK, where the predictions for the surface resistance from various theoretical models diverge. SRF cavities require special treatment during the cooldown and measurement. This includes cooling the cavity down at a rate greater than 1K/minute, and very low ambient magnetic field B<50 nT. I present solutions to both of these challenges.Item Magnetism and superconductivity in topotactically modified transition metal chalcogenides(2020) Wilfong, Brandon Cody; Rodriguez, Efrain E; Paglione, Johnpierre; Chemical Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Inspired by the structure of the simplest iron-based chalcogenide superconductor, FeSe, the class of tetrahedral transition metal chalcogenides (TTMCs) exhibit interesting chemical and physical properties due to its structure. This structure consists of tetrahedrally coordinated transition metal chalcogenides stacked to form two dimensional layers held together by van der Waals forces. This structure and its associated tetrahedral coordination of transition metal to chalcogenide, square transition metal sublattice, van der Waals layered structure, and d-electron filling at the Fermi level yields interesting properties from superconductivity to frustrated itinerant magnetism. In this dissertation work, we demonstrate that the anti-PbO type FeCh (Ch = S, Se, Te) structure offers a perfect platform for the study of superconductivity in the iron-based system as well as new physics as the class is expanded to different transition metals. Prior to this work, the binaries of the TTMC family was limited to iron, but has been expanded to cobalt. In the cobalt compound, CoSe, superconductivity in the FeSe binary is suppressed and a frustrated spin glass like magnetic state emerges. Beyond the binaries, we have shown that topotactic hydrothermal synthetic routes on the iron chalcogenide system can lead to novel intercalated phases where long range magnetic order can co-exist with superconductivity in the (LiOH)FeSe system. This synthetic scheme also allows the intercalation of organic molecules, specifically ethylenediamine, to form organic-inorganic hybrids which can offer a new avenue for designing heterolayer compounds with complex interlayer interactions and bonding.Item Electronic and Magnetic Properties of MnP-Type Binary Compounds(2019) Campbell, Daniel James; Paglione, Johnpierre; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The interactions between electrons, and the resulting impact on physical properties, are at the heart of present-day materials science. This thesis looks at this idea through the lens of several compounds from a single family: the MnP-type transition metal pnictides. FeAs and FeP show long range magnetic order with some similarities to the high temperature, unconventional iron-based superconductors. CoAs lies on the border of magnetism, with strong fluctuations but no stable ordered state. CoP, in contrast, shows no strong magnetic fluctuations but serves as a useful baseline in determining the origin (from composition, structure, or magnetic order) of behavior in the other materials. For this work, single crystals were grown with two different techniques: solvent flux and chemical vapor transport. In the case of FeAs the flux method resulted in the highest quality crystals yet produced. Extensive work was then performed on these samples at the University of Maryland and the National High Magnetic Field Laboratory. Quantum oscillations observed in high magnetic fields, in combination with density functional theory calculations, give insight into the Fermi surfaces of these materials. Large magnetoresistance in the phosphides, but not the arsenides, demonstrates differences in the choice of pnictogen atom that cannot be simply a product of electron count. Angle-dependent linear magnetoresistance in FeP is a sign of a possible Dirac dispersion and topological physics, as has been hinted at in other MnP-type materials. Ultimately, it is possible to examine results for all four compounds and draw conclusions on the role of each of the two elements in the formula, which can be extended to other members of this family.Item Novel Approaches to Control Surface Reactions in Plasma Etching of Electronic Materials(2019) Li, Chen; Oehrlein, Gottlieb S; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Advanced semiconductor manufacturing requires precise plasma etching control for patterning complex semiconductor device structures. Pattern transfer into dielectric materials is one of the most frequently performed operation and traditionally done using continuous wave (CW) plasma etching processes based on fluorocarbon (FC) chemistries. Such etching methods are facing challenges when the critical dimension (CD) approach 10 nm. Issues include low materials etching selectivity, surface damage, roughness, and poor etching profile control. In this work, various aspects of low temperature plasma-based etching approaches are tailored for optimal plasma etching performance, including novel gaseous precursors for better control of gas phase and surface processes, tailoring the relative importance of radicals and ion bombardment at surface by sequential processes, and a new way to input energy to surfaces to stimulate etching reactions. We systematically studied the impact of molecular structure parameters of hydrofluorocarbon (HFC) precursors on plasma deposition of fluorocarbon (FC) and material etching performance. The HFC chemical composition and molecular structure such as ring structure, C=C, C≡C, C-O, C-H and degree of unsaturation have dramatic impacts on FC surface polymerization and material etching performance. Further, we report a new atomic layer etching (ALE) technique which temporally separates chemical reactant supply to a surface from ion bombardment induced etching. By this ALE method, the ion bombardment energy can be reduced to ensure low substrate damage and extremely high etching selectivity of two materials. Finally, we developed a hollow cathode electron beam etching system to reduce the energy and momentum input on the material surface by utilizing an electron-radical synergy effect. This present work has unveiled highly promising elements of a new roadmap of next generation semiconductor etching approaches and is expected to impact multiple areas of nanoscience and technology, including plasma etching of post-silicon materials. The use of specially selected gaseous precursor chemistry, temporal separation of radical exposure and energy-induced etching, and finally using electron bombardment for activation of surface etching, challenge our current understanding of semiconductor plasma processing and presents an important step forward in terms of the further industrial development of these approaches.Item INNOVATIVE SCANNING PROBE METHODS FOR ENERGY STORAGE SCIENCE: ELUCIDATING THE PHYSICS OF BATTERY MATERIALS AT THE NANO-TO-MICROSCALE(2017) Larson, Jonathan; Reutt-Robey, Janice E; Einstein, Theodore L; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In recent decades, approaches to generate electrical energy through renewable means has greatly benefited from technological advancements. However, the need for robust schemes to store that energy in safe and cost-effective manners persists. Thus, there is a shared global call to advance electrical energy storage science and technology. Breakthroughs in the field stand to impact humans, ecosystems, environments, economies, and even international security. Currently, many innovative routes rooted in basic science are being taken to develop novel concepts, chemistries, electrolytes, and geometries for electrical energy storage. Many of these approaches make use of nano-to-mesoscale structures and technologies which increases the demand for new methods of characterization and scientific discovery at those scales. Still, progress to address this demand is stymied by practical scientific and technological challenges associated with the buried interfaces in battery systems. In this dissertation, I present how my PhD work has precisely targeted this need within the energy storage community, and made lasting impact. I detail why, and how, I have pioneered scanning-probe based technologies and techniques that make use of “battery probes” consisting of electrochemically active materials. A suite of techniques is developed and leveraged for basic electrical energy storage science: scanning nanopipette and probe microscopy, pascalammetry with microbattery probes, inverted scanning tunneling spectroscopy, and nanoscale solid-state electrochemistry with nanobattery probes. The use of these techniques motivated finite-element numerical simulations of electrostatic potentials, and electric fields, at play during field-driven lithiation of multi-walled carbon nanotubes. Also motivated were analytical models for surface diffusion and diffusion through a stressed electrolyte simultaneously experiencing latent-species activation.Item The effects of geometry and patch potentials on Casimir force measurements(2017) Garrett, Joseph L.; Munday, Jeremy N.; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Electromagnetic fluctuations of the quantum vacuum cause an attractive force between surfaces, called the Casimir force. In this dissertation, the first Casimir force measurements between two gold-coated spheres are presented. The proximity force approximation (PFA) is typically used to compare experiment to theory, but it is known to deviate from the exact calculation far from the surface. Bounds are put on the size of possible deviations from the PFA by combining several sphere-sphere and sphere-plate measurements. Electrostatic patch potentials have been postulated as a possible source of error since the first Casimir force measurements sixty years ago. Over the past decade, several theoretical models have been developed to characterize how the patch potentials contribute an additional force to the measurements. In this dissertation, Kelvin probe force microscopy (KPFM) is used to determine the effect of patch potentials on both the sphere and the plate. Patch potentials are indeed present on both surfaces, but the force calculated from the patch potentials is found to be much less than the measured force. In order to better understand how KPFM resolves patch potentials, the artifacts and sensitivities of several different KPFM implementations are tested and characterized. In addition, we introduce a new technique, called tunable spatial resolution (TSR) KPFM, to control resolution by altering the power-law separation dependence of the KPFM signal.Item Investigation of Graphene and other Low Dimensional Materials(2017) Tosado, Jacob Alexander; Fuhrer, Michael S; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This thesis describes experiments to characterize defects in two-dimensional materials and understand their effect on electrical conductivity. Defects limit the electrical conductivity through a material by scattering electrons. Understanding the physics of defects is therefore essential to building materials and structures with novel electronic properties. This dissertation has focused on low dimensional materials because they are simple thereby allowing for more advanced theory and they will act as a foundation for understanding higher dimensional systems. High resolution x-ray photoelectron spectroscopy (XPS) and near edge x-ray absorption fine structure spectroscopy (NEXAFS) were used to determine the character of vacancy defects in graphene. Vacancies were induced in graphene on a thermally oxidized silicon substrate using argon ion bombardment. XPS of the carbon 1s core level of pristine graphene shows a C 1s spectrum consistent with a single C 1s peak broadened both instrumentally and by a Doniach-Sunjic type effect. As defects are created, the resulting spectrum is deconvolved into two peaks. The first retains the same spectral width as that for the pristine graphene but with a reduced intensity. The broader second peak at higher binding energy (~200 meV), increases in intensity with increasing defect concentration. This second peak is identified as the experimental XPS signature of defective graphene. The observation is somewhat at odds with theoretical calculations of XPS spectra for graphene with various vacancy arrangements, which generally produce C 1s peaks shifted to lower binding energy. Instead, the emergence of this second peak, together with the emergence of a single sharp resonance seen near the vacuum level in the NEXAFS spectra, is interpreted as a distribution of molecular-like states forming on the surface. Preliminary efforts were made to characterize defects in semiconducting monolayer MoS2 using scanning tunneling microscopy (STM) and spectroscopy (STS). Techniques for obtaining a clean MoS2 surface suitable for ultra-high vacuum STM were developed, and preliminary characterization of the single layer tungsten disulfide surface by STM and STS was carried out. The local density of states of MoS2, as measured by STS, shows the semiconducting bandgap as well as signatures of donor and acceptor states within the gap.