Browsing by Author "Hallas, Alannah M."

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    Disentangling superconducting and magnetic orders in NaFe1−xNixAs using muon spin rotation
    (American Physical Society, 2018) Cheung, Sky C.; Guguchia, Zurab; Frandsen, Benjamin A.; Gong, Zizhou; Yamakawa, Kohtaro; Almeida, Dalson E.; Onuorah, Ifeanyi J.; Bonfá, Pietro; Miranda, Eduardo; Wang, Weiyi; Tam, David W.; Song, Yu; Cao, Chongde; Cai, Yipeng; Hallas, Alannah M.; Wilson, Murray N.; Munsie, Timothy J.S.; Luke, Graeme; Chen, Bijuan; Dai, Guangyang; Jin, Changqing; Guo, Shengli; Ning, Fanlong; Fernandes, Rafael M.; De Renzi, Roberto; Dai, Pengcheng; Uemura, Yasutomo J.
    Muon spin rotation and relaxation studies have been performed on a “111” family of iron-based superconductors, NaFe1−xNixAs, using single crystalline samples with Ni concentrations x=0, 0.4, 0.6, 1.0, 1.3, and 1.5%. Static magnetic order was characterized by obtaining the temperature and doping dependences of the local ordered magnetic moment size and the volume fraction of the magnetically ordered regions. For x=0 and 0.4%, a transition to a nearly-homogeneous long range magnetically ordered state is observed, while for x≳0.4% magnetic order becomes more disordered and is completely suppressed for x=1.5%. The magnetic volume fraction continuously decreases with increasing x. Development of superconductivity in the full volume is inferred from Meissner shielding results for x≳0.4%. The combination of magnetic and superconducting volumes implies that a spatially-overlapping coexistence of magnetism and superconductivity spans a large region of the T−x phase diagram for NaFe1−xNixAs. A strong reduction of both the ordered moment size and the volume fraction is observed below the superconducting TC for x=0.6, 1.0, and 1.3%, in contrast to other iron pnictides in which one of these two parameters exhibits a reduction below TC, but not both. The suppression of magnetic order is further enhanced with increased Ni doping, leading to a reentrant nonmagnetic state below TC for x=1.3%. The reentrant behavior indicates an interplay between antiferromagnetism and superconductivity involving competition for the same electrons. These observations are consistent with the sign-changing s± superconducting state, which is expected to appear on the verge of microscopic coexistence and phase separation with magnetism. We also present a universal linear relationship between the local ordered moment size and the antiferromagnetic ordering temperature TN across a variety of iron-based superconductors. We argue that this linear relationship is consistent with an itinerant-electron approach, in which Fermi surface nesting drives antiferromagnetic ordering. In studies of superconducting properties, we find that the T=0 limit of superfluid density follows the linear trend observed in underdoped cuprates when plotted against TC. This paper also includes a detailed theoretical prediction of the muon stopping sites and provides comparisons with experimental results.
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    Nonsymmorphic symmetry-protected band crossings in a square-net metal PtPb4
    (Springer Nature, 2022) Wu, Han; Hallas, Alannah M.; Cai, Xiaochan; Huang, Jianwei; Oh, Ji Seop; Loganathan, Vaideesh; Weiland, Ashley; McCandless, Gregory T.; Chan, Julia Y.; Mo, Sung-Kwan; Lu, Donghui; Hashimoto, Makoto; Denlinger, Jonathan; Birgeneau, Robert J.; Nevidomskyy, Andriy H.; Li, Gang; Morosan, Emilia; Yi, Ming; Rice Center for Quantum Materials
    Topological semimetals with symmetry-protected band crossings have emerged as a rich landscape to explore intriguing electronic phenomena. Nonsymmorphic symmetries in particular have been shown to play an important role in protecting the crossings along a line (rather than a point) in momentum space. Here we report experimental and theoretical evidence for Dirac nodal line crossings along the Brillouin zone boundaries in PtPb4, arising from the nonsymmorphic symmetry of its crystal structure. Interestingly, while the nodal lines would remain gapless in the absence of spin–orbit coupling (SOC), the SOC, in this case, plays a detrimental role to topology by lifting the band degeneracy everywhere except at a set of isolated points. Nevertheless, the nodal line is observed to have a bandwidth much smaller than that found in density functional theory (DFT). Our findings reveal PtPb4 to be a material system with narrow crossings approximately protected by nonsymmorphic crystalline symmetries.