Browsing by Subject "Cuprate Superconductors"

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    Neutron Scattering, Muon Spin Rotation/Relaxation, and Charge Transport Study of the Electron-Doped Cuprate Superconductors
    (2017-05) LI, Yangmu
    Exploring and understanding the exotic properties, phases, phase transitions exhibited by quantum materials are central research thrusts in contemporary condensed matter physics and materials science. One class of quantum materials that has attracted much attention during the past three decades is the family of copper-oxide (cuprate) high-transition-temperature superconductors. This family not only manifests superconductivity, a technologically useful macroscopic quantum state, but offers great opportunities for us to understand and control quantum phase transitions in the presence of strong electron-electron correlations. Investigating the competing, coexisting, and intertwined orders exhibited by the cuprate materials challenges us to consider new theoretical descriptions. This Thesis focuses on a specific type of cuprate superconductor, namely the electron-doped cuprate materials. The phases exhibited by these materials (antiferromagnetism, superconductivity, and charge order) depend on the degree of chemical substitution and oxygen reduction. Because these chemical manipulations induce simultaneous modifications to multiple parameters (e.g., chemical potential, band structure, local disorder, etc.), the electron-doped cuprate materials exhibit a rich and complex phase diagram that remains to be fully understood. The results presented in this Thesis provide crucial links between the normal and superconducting states and between the electron- and hole-doped parts of the phase diagram. High-quality single crystals of the archetypal electron-doped cuprate superconductor Nd2-xCexCuO4+d (NCCO) are synthesized using the state-of-the-art traveling-solvent floating-zone technique, and characterized by various in-house techniques, including Laue X-ray diffraction, magnetization (using a superconducting quantum interference device), scanning electron microscopy, and energy dispersive X-ray spectroscopy. The magnetic properties (Neel temperature, staggered magnetization, instantaneous spin-spin correlation length, magnetic volume fraction, and spin fluctuation timescales) and the electronic properties (Fermi surface topology, electrical resistivity, Hall constant, magnetoresistivity, upper critical field, superconducting volume fraction, and superfluid density) are studied with neutron scattering, muon spin rotation/relaxation (MuSR), and high-magnetic-field transport measurements at national laboratories in the United States and Canada. Published data for NCCO and other electron-doped cuprates are reanalyzed and compared with the new results obtained for NCCO. Simulations are performed for the instantaneous spin-spin correlation length and magnetoresistivity. A revised phase diagram of the electron-doped cuprates is constructed in the multi-dimensional parameter space of temperature, chemical substitution, and oxygen reduction. Three distinct phases are observed as a function of chemical substitution and oxygen reduction: (1) a long-range antiferromagnetic phase, where the Fermi surface consists of small electron pockets; (2) a bulk superconducting phase, where the Fermi surface consists of small electron and hole pockets; (3) a phase at high doping levels with a large hole Fermi surface. A disorder-smeared, first-order phase transition with microscopic phase separation is identified between the long-range antiferromagnetic and bulk superconducting phases. Specifically, this phase transition is observed to be volume-wise, and distinct spin fluctuation timescales are found for each phase. The magnetoresistivity measurements presented in this Thesis together with previous charge transport studies show two-band (electron and hole) contributions to the normal-state transport properties in electron-doped cuprate samples with bulk superconductivity. In addition, the two-band features are observed in the characteristic properties of the superconductor, including the upper critical field and the superfluid density. Universal scaling is demonstrated between the superconducting transition temperature and the hole superfluid density for both electron- and hole-doped cuprates, which clearly points to hole-related superconductivity in the nominally electron-doped cuprates. This scaling extends the famous Uemura scaling established for the hole-doped cuprates. The analysis of the superfluid density of the electron-doped cuprates follows that of prior theoretical work. Furthermore, new and published data for the ab-plane resistivity, Hall coefficient, cotangent of the Hall angle, and c-axis resistivity for the electron- and hole-doped cuprates are analyzed. The ab-plane resistivity of the electron- and hole-doped cuprates and the c-axis resistivity of the electron-doped cuprates features an upturn at low temperature/doping and a quadratic temperature-dependent contribution. Universal scaling between coefficients that characterize the low-temperature upturn is obtained for both electron- and hole-doped cuprates, indicative of a single underlying origin of the resistivity upturns, regardless of the nominal dopant type. The ab-plane transport scattering rate exhibits a quadratic (Fermi-liquid) temperature dependence, and is nearly independent of doping, compound and carrier type (electrons versus holes).