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The electronic nematic phase—in which electronic degrees of freedom lower the crystal rotational symmetry—is commonly observed in high-temperature superconductors. However, understanding the role of nematicity and nematic fluctuations in Cooper pairing is often made more complicated by the coexistence of other orders, particularly long-range magnetic order. Here we report the enhancement of superconductivity in a model electronic nematic system that is not magnetic, and show that the enhancement is directly born out of strong nematic fluctuations associated with a quantum phase transition. We present measurements of the resistance as a function of strain in Ba 1− x Sr x Ni 2 As 2 to show that strontium substitution promotes an electronically driven nematic order in this system. In addition, the complete suppression of that order to absolute zero temperature leads to an enhancement of the pairing strength, as evidenced by a sixfold increase in the superconducting transition temperature. The direct relation between enhanced pairing and nematic fluctuations in this model system, as well as the interplay with a unidirectional charge-density-wave order comparable to that found in the cuprates, offers a means to investigate the role of nematicity in strengthening superconductivity. Transport measurements show that nematic fluctuations near a phase transition increase the temperature at which superconductivity occurs by a factor of nearly six. This happens in a non-magnetic nickel-based compound.

The desire for a complete understanding of high temperature unconventional superconductivity has illustrated a necessity for the study of non-magnetic sources of superconducting enchantment, such as nematically driven fluctuations and charge order fluctuations. BaNi2As2, a non-magnetic counterpart to high Tc superconductor BaFe2As2, shows a six-fold superconducting enhancement neighboring charge and nematic orders, positioning it as an excellent candidate for studying the interactions between charge order, nematic order, and enhanced superconductivity. In this thesis, I will present X-ray diffraction and electrical transport evidence for the development of complex charge order within the system as functions of isovalent chemical substitution via Ba1−xSrxNi2As2 and applied hydrostatic pressure. The discovery of three separate charge orders will be detailed: an incommensurate charge order at Q = 0.28 and two commensurate charge orders at Q = 0.33 and Q = 0.5. X-ray diffraction measurements of the Q = 0.28 charge order will be used to show a strong correlation between it and a previously established nematic order for Ba1−xSrxNi2As2. Applied pressure of BaNi2As2 up to 10.4 GPa will detail the development of all three charge orders and be used to show a correlation between pressure and isovalvent substitution in BaNi2As2. The critical substitution of 71% Sr and the critical pressure of 9 ± 0.5 GPa will be directly compared by X-ray measurements of their lattice parameters, revealing a collapsed tetragonal phase. This phase is shown to be analogous to the collapsed tetragonal phase of the Fe-pnictide superconductors, likely playing a key role seen at the critical substitution and pressure of BaNi2As2.

Tungsten monophosphide (WP) has been reported to superconduct below 0.8 K, and theoretical work has predicted an unconventional Cooper pairing mechanism. Here we present data for WP single crystals grown by means of chemical vapor transport (CVT) of WO3, P, and I2. In comparison to synthesis using WP powder as a starting material, this technique results in samples with substantially decreased low-temperature scattering and favors a more three-dimensional morphology. We also find that the resistive superconducting transitions in these samples begin above 1 K. Variation in Tc is often found in strongly correlated superconductors, and its presence in WP could be the result of influence from a competing order and/or a non-s-wave gap.

BaNi\\(_2\\)As\\(_2\\), a non-magnetic superconductor counterpart to BaFe\\(_2\\)As\\(_2\\), has been shown to develop nematic order, multiple charge orders, and a dramatic six-fold enhancement of superconductivity via isovalent chemical substitution of Sr for Ba. Here we present high pressure single-crystal and powder x-ray diffraction measurements of BaNi\\(_2\\)As\\(_2\\) to study the effects of tuning lattice density on the evolution of charge order in this system. Single-crystal X-ray experiments track the evolution of the incommensurate (Q=0.28) and commensurate (Q=0.33 and Q=0.5) charge orders, and the tetragonal-triclinic distortion as a function of temperature up to pressures of 10.4 GPa, and powder diffraction experiments at 300 K provide lattice parameters up to 17 GPa. We find that applying pressure to BaNi\\(_2\\)As\\(_2\\) produces a similar evolution of structural and charge-ordered phases as found as a function of chemical pressure in Ba\\(_{1-x}\\)Sr\\(_{x}\\)Ni\\(_2\\)As\\(_2\\) , with coexisting commensurate charge orders appearing on increasing pressure. These phases also exhibit a similar abrupt cutoff at a critical pressure of (9 \\(\\pm\\) 0.5) GPa, where powder diffraction experiments indicate a collapse of the tetragonal structure at higher temperatures. We discuss the relationship between this collapsed tetragonal phase and the discontinuous phase boundary observed at the optimal substitution value for superconductivity in Ba\\(_{1-x}\\)Sr\\(_{x}\\)Ni\\(_2\\)As\\(_2\\)

Recent discoveries of charge order and electronic nematic order in the iron-based superconductors and cuprates have pointed towards the possibility of nematic and charge fluctuations playing a role in the enhancement of superconductivity. The Ba1-xSrxNi2As2 system, closely related in structure to the BaFe2As2 system, has recently been shown to exhibit both types of ordering without the presence of any magnetic order. We report single crystal X-ray diffraction, resistance transport measurements, and magnetization of \\BaSrLate, providing evidence that the previously reported incommensurate charge order with wavevector \\((0,0.28,0)_{tet}\\) in the tetragonal state of \\BaNi~vanishes by this concentration of Sr substitution together with nematic order. Our measurements suggest that the nematic and incommensurate charge orders are closely tied in the tetragonal state, and show that the \\((0,0.33,0)_{tri}\\) charge ordering in the triclinic phase of BaNi2As2 evolves to become \\((0,0.5,0)_{tri}\\) charge ordering at \\(x\\)=0.65 before vanishing at \\(x\\)=0.71.

BaNi\\(_2\\)As\\(_2\\) is a structural analog of the pnictide superconductor BaFe\\(_2\\)As\\(_2\\), which, like the iron-based superconductors, hosts a variety of ordered phases including charge density waves (CDWs), electronic nematicity, and superconductivity. Upon isovalent Sr substitution on the Ba site, the charge and nematic orders are suppressed, followed by a sixfold enhancement of the superconducting transition temperature (\\(T_c\\)). To understand the mechanisms responsible for enhancement of \\(T_c\\), we present high-resolution angle-resolved photoemission spectroscopy (ARPES) measurements of the Ba\\(_{1-x}\\)Sr\\(_{x}\\)Ni\\(_2\\)As\\(_2\\) series, which agree well with our density functional theory (DFT) calculations throughout the substitution range. Analysis of our ARPES-validated DFT results indicates a Lifshitz transition and reasonably nested electron and hole Fermi pockets near optimal substitution where \\(T_c\\) is maximum. These nested pockets host Ni \\(d_{xz}\\)/\\(d_{yz}\\) orbital compositions, which we associate with the enhancement of nematic fluctuations, revealing unexpected connections to the iron-pnictide superconductors. This gives credence to a scenario in which nematic fluctuations drive an enhanced \\(T_c\\).

The electronic nematic phase, wherein electronic degrees of freedom lower the crystal rotational symmetry, is a common motif across a number of high-temperature superconductors. However, understanding the role and influence of nematicity and nematic fluctuations in Cooper pairing is often complicated by the coexistence of other orders, particularly long-range magnetic order. Here we report the enhancement of superconductivity in a model electronic nematic system absent of magnetism, and show that the enhancement is directly born out of strong nematic fluctuations emanating from a tuned quantum phase transition. We use elastoresistance measurements of the Ba\\(_{1-x}\\)Sr\\(_{x}\\)Ni\\(_2\\)As\\(_2\\) substitution series to show that strontium substitution promotes an electronically driven \\(B_{1g}\\) nematic order in this system, and that the complete suppression of that order to absolute zero temperature evokes a dramatic enhancement of the pairing strength, as evidenced by a sixfold increase in the superconducting transition temperature. The direct relation between enhanced pairing and nematic fluctuations in this model system, as well as the interplay with a unidirectional charge density wave order comparable to that found in the cuprates, offers a means to elucidate the role of nematicity in boosting superconductivity.

How superconductivity interacts with charge or nematic order is one of the great unresolved issues at the center of research in quantum materials. Ba\\(_{1-x}\\)Sr\\(_{x}\\)Ni\\(_{2}\\)As\\(_{2}\\) (BSNA) is a charge ordered pnictide superconductor recently shown to exhibit a six-fold enhancement of superconductivity due to nematic fluctuations near a quantum phase transition (at \\(x_c=0.7\\)). The superconductivity is, however, anomalous, with the resistive transition for \\(0.4 < x< x_c\\) occurring at a higher temperature than the specific heat anomaly. Using x-ray scattering, we discovered a new charge density wave (CDW) in BSNA in this composition range. The CDW is commensurate with a period of two lattice parameters, and is distinct from the two CDWs previously reported in this material. We argue that the anomalous transport behavior arises from heterogeneous superconductivity nucleating at antiphase domain walls in this CDW. We also present new data on the incommensurate CDW, previously identified as being unidirectional, showing that is a rotationally symmetric, \"4\\(Q\\)\" state with \\(C_4\\) symmetry. Our study establishes BSNA as a rare material containing three distinct CDWs, and an exciting testbed for studying coupling between CDW, nematic, and SC orders.

We study the temperature dependence of electrical resistivity for currents directed along all crystallographic axes of the spin-triplet superconductor UTe\\(_{2}\\). We focus particularly on an accurate determination of the resistivity along the \\(c\\)-axis (\\(\\rho_c\\)) by using a generalized Montgomery technique that allows extraction of crystallographic resistivity components from a single sample. In contrast to expectations from the observed highly anisotropic band structure, our measurement of the absolute values of resistivities in all current directions reveals a surprisingly nearly isotropic transport behavior at temperatures above Kondo coherence, with \\(\\rho_c \\sim \\rho_b \\sim 2\\rho_a\\), that evolves to reveal qualitatively distinct behaviors on cooling. The temperature dependence of \\(\\rho_c\\) exhibits a peak at a temperature much lower than the onset of Kondo coherence observed in \\(\\rho_a\\) and \\(\\rho_b\\), consistent with features in magnetotransport and magnetization that point to a magnetic origin. A comparison to the temperature-dependent evolution of the scattering rate observed in angle-resolved photoemission spectroscopy experiments provides important insights into the underlying electronic structure necessary for building a microscopic model of superconductivity in UTe\\(_{2}\\).

Kinetics of molecular oxygen / Au (001) surface interaction has been studied at high temperature and near atmospheric pressures of O2 gas with in situ x-ray scattering measurements. We find that the hexagonal reconstruction (hex) of Au (001) surface lifts to (1x1) in the presence of O2 gas, indicating that the (1x1) is more favored when some oxygen atoms present on the surface. The measured lifting rate constant vs. temperature is found to be highest at intermediate temperature exhibiting a 'volcano'-type behavior. At low temperature, the hex-to-(1x1) activation barrier (Eact = 1.3(3) eV) limits the lifting. At high temperature, oxygen adsorption energy (Eads = 1.6(2) eV) limits the lifting. The (1x1)-to-hex activation barrier (Ehex = 0.41(14) eV) is also obtained from hex recovery kinetics. The pressure-temperature (PT) surface phase diagram obtained in this study shows three regions: hex at low P and T, (1x1) at high P and T, and coexistence of the hex and (1x1) at the intermediate P and T.