Browsing by Author "Brahlek, Matthew"

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    Searching for superconductivity in high entropy oxide Ruddlesden-Popper cuprate films
    Mazza, Alessandro R.; Gao, Xingyao; Rossi, Daniel J.; Musico, Brianna L.; Valentine, Tyler W.; Kennedy, Zachary; Zhang, Jie; Lapano, Jason; Keppens, Veerle; Moore, Robert G.; Brahlek, Matthew; Rost, Christina M.; Ward, Thomas Z. (American Vacuum Society, 2021-11-29)
    In this work, the high entropy oxide A2CuO4 Ruddlesden-Popper (La0.2Pr0.2Nd0.2Sm0.2Eu0.2)2CuO4 is explored by charge doping with Ce+4 and Sr+2 at concentrations known to induce superconductivity in the simple parent compounds, Nd2CuO4 and La2CuO4. Electron doped (La0.185Pr0.185Nd0.185Sm0.185Eu0.185Ce0.075)2CuO4 and hole doped (La0.18Pr0.18Nd0.18Sm0.18Eu0.18Sr0.1)2CuO4 are synthesized and shown to be single crystal, epitaxially strained, and highly uniform. Transport measurements demonstrate that all as-grown films are insulating regardless of doping. Annealing studies show that resistivity can be tuned by modifying oxygen stoichiometry and inducing metallicity but without superconductivity. These results, in turn, are connected to extended x-ray absorption fine structure results, indicating that the lack of superconductivity in the high entropy cuprates likely originates from a large distortion within the Cu-O plane (σ2 > 0.015 Å2) due to A-site cation size variance, which drives localization of charge carriers. These findings describe new opportunities for controlling charge- and orbital-mediated functional responses in Ruddlesden-Popper crystal structures, driven by balancing of cation size and charge variances that may be exploited for functionally important behaviors such as superconductivity, antiferromagnetism, and metal-insulator transitions while opening less understood phase spaces hosting doped Mott insulators, strange metals, quantum criticality, pseudogaps, and ordered charge density waves.
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    What is in a name: Defining "high entropy" oxides
    Brahlek, Matthew; Gazda, Maria; Keppens, Veerle; Mazza, Alessandro R.; McCormack, Scott J.; Mielewczyk-Gryń, Aleksandra; Musico, Brianna; Page, Katharine; Rost, Christina M.; Sinnott, Susan B.; Toher, Cormac; Ward, Thomas Z.; Yamamoto, Ayako (AIP Publishing, 2022-11-04)
    High entropy oxides are emerging as an exciting new avenue to design highly tailored functional behaviors that have no traditional counterparts. Study and application of these materials are bringing together scientists and engineers from physics, chemistry, and materials science. The diversity of each of these disciplines comes with perspectives and jargon that may be confusing to those outside of the individual fields, which can result in miscommunication of important aspects of research. In this Perspective, we provide examples of research and characterization taken from these different fields to provide a framework for classifying the differences between compositionally complex oxides, high entropy oxides, and entropy stabilized oxides, which is intended to bring a common language to this emerging area. We highlight the critical importance of understanding a material's crystallinity, composition, and mixing length scales in determining its true definition.