Browsing by Author "Frandsen, Benjamin A."

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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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    Gradual enhancement of stripe-type antiferromagnetism in the spin-ladder material BaFe2S3 under pressure
    (American Physical Society, 2018) Zheng, Liangliang; Frandsen, Benjamin A.; Wu, Changwei; Yi, Ming; Wu, Shan; Huang, Qingzhen; Bourret-Courchesne, Edith; Simutis, G.; Khasanov, R.; Yao, Dao-Xin; Wang, Meng; Birgeneau, Robert J.
    We report pressure-dependent neutron diffraction and muon spin relaxation/rotation measurements combined with first-principles calculations to investigate the structural, magnetic, and electronic properties of BaFe2S3 under pressure. The experimental results reveal a gradual enhancement of the stripe-type ordering temperature with increasing pressure up to 2.6 GPa and no observable change in the size of the ordered moment. The ab initio calculations suggest that the magnetism is highly sensitive to the Fe-S bond lengths and angles, clarifying discrepancies with previously published results. In contrast to our experimental observations, the calculations predict a monotonic reduction of the ordered moment with pressure. We suggest that the robustness of the stripe-type antiferromagnetism is due to strong electron correlations not fully considered in the calculations.
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    Local Orthorhombicity in the Magnetic ${C}_{4}$ Phase of the Hole-Doped Iron-Arsenide Superconductor ${\mathrm{Sr}}_{1\ensuremath{-}x}{\mathrm{Na}}_{x}{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$
    (American Physical Society, 2017) Frandsen, Benjamin A.; Taddei, Keith M.; Yi, Ming; Frano, Alex; Guguchia, Zurab; Yu, Rong; Si, Qimiao; Bugaris, Daniel E.; Stadel, Ryan; Osborn, Raymond; Rosenkranz, Stephan; Chmaissem, Omar; Birgeneau, Robert J.; Rice Center for Quantum Materials
    We report on temperature-dependent pair distribution function measurements of Sr1−xNaxFe2As2, an iron-based superconductor system that contains a magnetic phase with reentrant tetragonal symmetry, known as the magnetic C4 phase. Quantitative refinements indicate that the instantaneous local structure in the C4 phase comprises fluctuating orthorhombic regions with a length scale of ∼2 nm, despite the tetragonal symmetry of the average static structure. Additionally, local orthorhombic fluctuations exist on a similar length scale at temperatures well into the paramagnetic tetragonal phase. These results highlight the exceptionally large nematic susceptibility of iron-based superconductors and have significant implications for the magnetic C4 phase and the neighboring C2 and superconducting phases.
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    Widespread orthorhombic fluctuations in the (Sr,Na)Fe2As2 family of superconductors
    (American Physical Society, 2018) Frandsen, Benjamin A.; Taddei, Keith M.; Bugaris, Daniel E.; Stadel, Ryan; Yi, Ming; Acharya, Arani; Osborn, Raymond; Rosenkranz, Stephan; Chmaissem, Omar; Birgeneau, Robert J.
    We report comprehensive pair distribution function measurements of the hole-doped iron-based superconductor system Sr1−xNaxFe2As2. Structural refinements performed as a function of temperature and length scale reveal orthorhombic distortions of the instantaneous local structure across a large region of the phase diagram possessing average tetragonal symmetry, indicative of fluctuating nematicity. These nematic fluctuations are present up to high doping levels (x≳0.48, near optimal superconductivity) and high temperatures (above room temperature for x=0, decreasing to 150 K for x=0.48), with a typical length scale of 1–3 nm. This work highlights the ubiquity of nematic fluctuations in a representative iron-based superconductor and provides important details about the evolution of these fluctuations across the phase diagram.