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Experimental Study of Magnetic and Transport Properties of Antiferromagnet Y2Co3 and Superconductor LaNiGa2
No full textIn this dissertation, I present experimental investigations on two materials of interest to the condensed matter community: the antiferromagnetic properties of Y2Co3 and the superconductivity and normal state properties of LaNiGa2. Both studies involve the development of single crystal growth methods and bulk magnetic and physical transport property measurements and analysis.The interest in Y2Co3 originates from its relatively high Néel temperature among the Co-based antiferromagnetic compounds, despite its high cobalt content. I developed, for the first time, the single-crystal growth method and reported its crystal structure belongs to the Cmce (No. 64) orthorhombic space group. The magnetic structure was also determined for the first time with single-crystal neutron diffraction to be an A-type antiferromagnet with almost collinear magnetic moments alignment, despite a seemingly non-collinear behavior observed in bulk magnetization measurements. Such discrepancy is attributed to the considerable temperature dependence of itinerant antiferromagnetic (AFM) exchange interactions, induced by thermal contraction along the b axis. Pressure study and high-field study revealed robust AMF ordering due to the compensating effect of lattice contraction on the ferromagnetic and AMF interactions. The high-field study revealed a spin-flop phase transition, offering further insight into the magnetocrystalline anisotropy.LaNiGa2 has attracted attention due to the evidence of time-reversal symmetry breaking in the superconducting state, alongside symmetry-enforced Dirac band crossings and anomalous pressure-dependent superconducting transition temperature. In this dissertation, I present a summary of the superconducting gap structure discussion based on recent works by our collaborators using our single-crystal samples, as well as the pressure studies. Furthermore, I discuss the normal state properties based on magnetization measurement and physical transport property measurement. The magnetization measurement indicates weak electron-electron interactions, making them unlikely to play a role in the unusual superconducting pairing. Transport measurements demonstrate multi-band behavior with both electron and hole carriers. The carrier densities remain nearly constant across the measured temperature range, with no evidence of sudden changes that might suggest electronic phase transitions. In particular, the magnetoresistance exhibits quadratic to linear field dependence, which deviates from conventional semiclassical transport behavior. Possible explanations including quantum transport effects resulting from small Fermi surface pockets and impeded circular motions are discussed. 
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Structural Characterization and Physical Properties of Superconducting LaNiGa2 and Antiferromagnetic CeIn3
Full text linkIn this dissertation, I will describe two experimental studies on two different condensed matter systems. The first study originates from our crystallographic findings on LaNiGa2. After developing a single crystal synthesis method, single-crystal X-ray diffraction results improve upon previous studies by showing that LaNiGa2 crystallizes in a Cmcm unit cell instead of a Cmmm one. As a result of uncovering nonsymmorphic symmetry operations, we show that these directly result in two topological features precisely at the Fermi level. With the previous knowledge that LaNiGa2 breaks time-reversal symmetry within its superconducting state and shows evidence for multi-gap superconducting behavior, we can then directly connect the new crystallographic results, the topological features, and the unique superconducting state. In the second study, we synthesize and then characterize single crystals of Nd substituted CeIn3. This well-studied heavy-fermion system has previously exhibited fascinating phenomena as its antiferromagnetic ordering is systematically suppressed. In Ce1-xNdxIn3 we reveal an interesting interplay between the Kondo lattice coherence and crystal electric field depopulation effects. Wherein we can separate the two features in electrical resistivity measurements in the most disordered substitution range of x=0.4-0.5. We also reveal a comprehensive phase diagram between the two antiferromagnetic ordering.In addition to these two studies, I also provide an overview of relevant concepts and experimental techniques. The purpose of this overview is to provide incoming graduate students a starting point to begin to understand what principles are relevant to the condensed matter systems discussed in this dissertation and how to perform specific experimental techniques to study said principles. Within the experimental section, I also provide some ``experimentalist's insights'' on the techniques I have used extensively throughout my graduate work
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Molten Hydroxide Flux Synthesis of KNi4S2 and its Magnetic and Physical Properties
Full text linkThis thesis describes how I used the molten-hydroxide flux method of single crystalsynthesis to synthesize some of the largest reported single crystals of KNi4S2 (142),
KNi2S2 (122), and K2Ni3S4 (234). This molten-hydroxide flux method had previously
never been implemented in our lab and, with my optimizations, could open up a previously inaccessible phase-space of meta-stable compounds for our group to explore. I also
report the first-known magnetization, resistivity, and heat-capacity data for KNi4S2. Although the KNi4S2 phase is isostructural to cuprate superconductors, we do not observe
signs of superconductivity nor antiferromagnetic order down to T = 2 K. Instead, our
magnetization measurements suggest that KNi4S2 may be a weak itinerant ferromagnet (TC ≈ 25 K). Growing large single crystals allowed us to study magnetic anisotropy,
and I found that with KNi4S2, the a and c appear to be isotropic while b seems to
be the easy magnetic direction. Resistivity measurements from T = 1.8 − 300 K show
the 142 phase has a resistivity similar to a bad metal. While I measure a relatively
high residual resistivity ratio (RRR) of 44 demonstrating good sample quality, no phase
transition is observed in either resistivity or heat capacity measurements. This may be
consistent with the weak itinerant nature of the magnetism, although we cannot exclude the possibility of an impurity origin of the ferromagnetic transition. In addition
to the 142 phase, I discuss the synthesis and present x-ray diffraction (XRD) patterns
and magnetization data for the 122 and 234 phases
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Experimental Studies of the Unusual and Fragile Ferromagnetism of LaCrGe3 and LaCrSb3
Full text linkThis dissertation details my exploration of two compounds: LaCrGe3 and LaCrSb3. While their names differ by only two letters, and therefore their compositions by only one element, they are quite different. The relevant question to ask about these two ferromagnetic systems is: “what happens when their ferromagnetism is suppressed?" Although the magnetic phase diagram of LaCrGe3 under pressure has previously been charted, there are aspects of its rich temperature-pressure-magnetic field phase space that are contested. In particular, through a meticulous study of its magnetic domain behavior, I provide evidence in favor of the existence of multiple ferromagnetic states in LaCrGe3. We find that investigating domain behavior can lead to a more accurate way of characterizing new materials and perhaps a method of probing crossovers between ferromagnetic states. On the other hand, the ferromagnetism of LaCrSb3 is relatively robust to pressure. Therefore, I use Fe substitution to suppress its Curie temperature and discover the first reported magnetic phase diagram of its kind—one with an avoided quantum tricritical point. The LaCr1-xFexSb3 system has a temperature-chemical substitution-magnetic field phase diagram that is ripe with magnetic features for closer examination. Further study of both materials will likely lead to additional discoveries in the world of magnetism
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Magnetic and Electric Properties of Topological materials Co3Sn2S2 and CeCoGe3
No full textTopology is a study of continuous deformation of objects. Of particular interest to the physicists is the topological invariant, a quantity which is preserved under a continuous deformation. For example, a coffee cup (with a handle) can be deformed continuously into a donut, and the number of holes (which is called genus in mathematical language) remains untouched during the process. It would require a discontinuous deformation to create or remove a hole, i.e., to deform a solid sphere to a donut. In physics, certain physical observables can be represented mathematically by topological invariants. These topological invariants are preserved under continuous changes in the Hamiltonian, unless the system undergoes a topological phase transition that causes a discontinuous change of the corresponding topological invariant, which is the so-called ``topological protection''.Topological materials represent a class of materials with unique electronic properties due to the unusual surface and edge states. Those states are protected by the topological invariants, making them robust against impurities and defects in the materials. The study of topological materials traces back to 1980s where people found quantum Hall effect, where the Hall conductance is quantized in units of e^2/h, regardless of the details of the Hamiltonian, such as strength of magnetic field and random disorder potential. Now it is well known that the quantized Hall conductance is related to a topological invariant called Chern number, which is the intrinsic nature of the electronic band structures. Later in 2000s, the discovery of topological insulator Bi2Se3, which exhibits dissipationless conduction of electricity on the surfuce while being insulating in the bulk, verifies the early theory of topological materials. Nowadays, the field of topological materials expands to topological superconductors, Weyl semimetals and Dirac semimetals. The topological nature in these materials gives rise to the exotic phenomena, for example, Weyl semimetals are characterized with Fermi arcs on the surface and exhibit unique electric transport properties such as Chiral anomaly and large anomalous Hall effect. Such unusual electronic properties offer promising applications in quantum computing and energy-efficient electronics.This dissertation describes my research on two topological materials---Weyl semimetal Co3Sn2S2 and Kondo-Weyl compound CeCoGe3. Previous work on Co3Sn2S2 has revealed its interesting electric properties such as Chiral anomaly, anomalous Hall effect and a breaking-down of Ohm's Law. Our work shows it also exhibits unusual domain wall motion due to its magnetic structure. The second project is about CeCoGe3, which is a potential Weyl metal with the interplay of Kondo effect. Our detailed study on the electric properties of this compound reveals a giant intrinsic anomalous Hall conductance, which is the largest among all compounds according to our best knowledge. This manuscript is currently under review for publication.This dissertation starts with an overview of fundamental physical principles related these compounds in chapter I, followed by a description of experimental techniques in chapter II. The detailed studies of Co3Sn2S2 and CeCoGe3 will be discussed in chapter III and chapter IV respectively
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Single Crystal Synthesis, Quantum Criticality , and Crystal Electric Field of Ce-based Kondo Lattice Compounds: Experimental Studies of CeTiVGe and CeCuSi
No full textThis dissertation presents four experimental studies on Ce-based compounds.The first study investigates quantum criticality in CeTi1−xVxGe3 single crystals. The magnetic phase diagram, constructed from magnetization and resistivity measurements, reveals the presence of a quantum critical point near x = 0.4. A detailed crystal electric field (CEF) analysis suggests a gradual change in the ground state from |±5/2⟩ in CeTiGe3 to |±1/2⟩ in CeVGe3, along with a suppression of CEF splitting energies near the quantum critical region. This study further compares the magnetic phase diagram of CeTi1−xVxGe3 with the pressure-driven phase transition observed in CeTiGe3, highlighting differences in lattice constant responses that indicate substitution effects cannot solely be attributed to chemical pressure. Notably, a common critical lattice constant of c = 5.78 Å was identified, at which ferromagnetic ordering is suppressed in both cases.The second study investigates the quantum phase transition in CeVGe3 under extreme magnetic field conditions. The magnetic phase diagram is constructed using magnetization and resistivity measurements, while neutron scattering experiments are used to determine the magnetic structure. Our measurements reveal that CeVGe3 undergoes a magnetic transition from an incommensurate AFM state to an up-up-down-down commensurate AFM structure, followed by a transition to a novel phase at higher fields. A quantum phase transition occurs around 21.3 T. The analysis reveals similarities between CeVGe3 and the well-studied heavy fermion system CeRhIn5.The third study investigates the physical properties of CeCuSi single crystals, a rare ferromagnetic Ce-based compounds that exhibits the highest Curie temperature (TC = 15.5 K) among the CeTX family. The synthesis of CeCuSi single crystals was achieved for the first time using the metallic flux method in our study. Magnetization, heat capacity, Raman spectroscopy, and resistivity measurements provide insights into the anisotropic properties of CeCuSi, along with a comprehensive CEF analysis.The fourth study investigates the unusual hard-axis ordering phenomena observed in some of the heavy-fermion systems, where the moments align along the CEF hard-axis at the ground state. An extended survey on Ce-based ferromagnetic Kondo lattices with available singlecrystal magnetization data reveals that such hard-axis ordering is less common than previously reported. However, our survey of Ce-based FM compounds reveals that such hard-axis ordering is less common than previously reported. Our analysis finds no clear correlation between the strength of the Kondo effect and whether a compound exhibits hard-axis or hard-plane ordering. Instead, the interplay between CEF anisotropy and exchange interaction anisotropy appears critical. Direct comparison between CeCuSi and CeAgSb2 further demonstrates that antiferromagnetic interactions along the CEF easy-axis can significantly stabilize hard-axis ordering. The survey also emphasizes the necessity of incorporating anisotropic RKKY models when analyzing Ce-based and other heavy fermion systems to fully capture their complex magnetic behaviors
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Structural and Ferromagnetic Properties of an Orthorhombic Phase of MnBi Stabilized with Rh Additions
Full text linkThe article addresses the possibility of alloy elements in MnBi which may modify the thermodynamic stability of the NiAs-type structure without significantly degrading the magnetic properties. The addition of small amounts of Rh and Mn provides an improvement in the thermal stability with some degradation of the magnetic properties. The small amounts of Rh and Mn additions in MnBi stabilize an orthorhombic phase whose structural and magnetic properties are closely related to the ones of the previously reported high-temperature phase of MnBi (HT MnBi). To date, the properties of the HT MnBi, which is stable between 613 and 719 K, have not been studied in detail because of its transformation to the stable low-temperature MnBi (LT MnBi), making measurements near and below its Curie temperature difficult. The Rh-stabilized MnBi with chemical formula Mn1.0625−xRhxBi [x=0.02(1)] adopts a new superstructure of the NiAs/Ni2In structure family. It is ferromagnetic below a Curie temperature of 416 K. The critical exponents of the ferromagnetic transition are not of the mean-field type but are closer to those associated with the Ising model in three dimensions. The magnetic anisotropy is uniaxial; the anisotropy energy is rather large, and it does not increase when raising the temperature, contrary to what happens in LT MnBi. The saturation magnetization is approximately 3μB/f.u. at low temperatures. While this exact composition may not be application ready, it does show that alloying is a viable route to modifying the stability of this class of rare-earth-free magnet alloys.This article is published as Taufour, Valentin, Srinivasa Thimmaiah, Stephen March, Scott Saunders, Kewei Sun, Tej Nath Lamichhane, Matthew J. Kramer, Sergey L. Bud’ko, and Paul C. Canfield. "Structural and ferromagnetic properties of an orthorhombic phase of MnBi stabilized with Rh additions." Physical Review Applied 4, no. 1 (2015): 014021. DOI: 10.1103/PhysRevApplied.4.014021. Posted with permission.</p
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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