Browsing by Author "Chi, Songxue"
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Item Dynamic Spin-Lattice Coupling and Nematic Fluctuations in NaFeAs(American Physical Society, 2018) Li, Yu; Yamani, Zahra; Song, Yu; Wang, Weiyi; Zhang, Chenglin; Tam, David W.; Chen, Tong; Hu, Ding; Xu, Zhuang; Chi, Songxue; Xia, Ke; Zhang, Li; Cui, Shifeng; Guo, Wenan; Fang, Ziming; Liu, Yi; Dai, PengchengItem Electron doping evolution of the neutron spin resonance in NaFe1−xCoxAs(American Physical Society, 2016) Zhang, Chenglin; Lv, Weicheng; Tan, Guotai; Song, Yu; Carr, Scott V.; Chi, Songxue; Matsuda, M.; Christianson, A.D.; Fernandez-Baca, J.A.; Harriger, L.W.; Dai, PengchengNeutron spin resonance, a collective magnetic excitation coupled to superconductivity, is one of the most prominent features shared by a broad family of unconventional superconductors including copper oxides, iron pnictides, and heavy fermions. In this paper, we study the doping evolution of the resonances in NaFe1−xCoxAs covering the entire superconducting dome. For the underdoped compositions, two resonance modes coexist. As doping increases, the low-energy resonance gradually loses its spectral weight to the high-energy one but remains at the same energy. By contrast, in the overdoped regime we only find one single resonance, which acquires a broader width in both energy and momentum but retains approximately the same peak position even when Tc drops by nearly a half compared to optimal doping. These results suggest that the energy of the resonance in electron overdoped NaFe1−xCoxAs is neither simply proportional to Tc nor the superconducting gap but is controlled by the multiorbital character of the system and doped impurity scattering effect.Item Experimental elucidation of the origin of the ‘double spin resonances’ in Ba(Fe1−xCox)2As2(American Physical Society, 2016) Wang, Meng; Yi, M.; Sun, H. L.; Valdivia, P.; Kim, M.G.; Xu, Z. J.; Berlijn, T.; Christianson, A.D.; Chi, Songxue; Hashimoto, M.; Lu, D.H.; Li, X.D.; Bourret-Courchesne, E.; Dai, Pengcheng; Lee, D.H.; Maier, T.A.; Birgeneau, R.J.We report a combined study of the spin resonances and superconducting gaps for underdoped (Tc=19 K), optimally doped (Tc=25 K), and overdoped (Tc=19 K) Ba(Fe1−xCox)2As2 single crystals with inelastic neutron scattering and angle resolved photoemission spectroscopy. We find a quasi-two-dimensional spin resonance whose energy scales with the superconducting gap in all three compounds. In addition, anisotropic low energy spin excitation enhancements in the superconducting state have been deduced and characterized for the under and optimally doped compounds. Our data suggest that the quasi-two-dimensional spin resonance is a spin exciton that corresponds to the spin singlet-triplet excitations of the itinerant electrons. However, the intensity enhancements of the anisotropic spin excitations are dominated by the out-of-plane spin excitations of the ordered moments due to the suppression of damping in the superconducting state. Hence we offer an interpretation of the double energy scales differing from previous interpretations based on anisotropic superconducting energy gaps and systematically explain the doping-dependent trend across the phase diagram.Item Local orthorhombic lattice distortions in the paramagnetic tetragonal phase of superconducting NaFe1−xNixAs(Springer Nature, 2018) Wang, Weiyi; Song, Yu; Cao, Chongde; Tseng, Kuo-Feng; Keller, Thomas; Li, Yu; Harriger, L.W.; Tian, Wei; Chi, Songxue; Yu, Rong; Nevidomskyy, Andriy H.; Dai, PengchengUnderstanding the interplay between nematicity, magnetism and superconductivity is pivotal for elucidating the physics of iron-based superconductors. Here we use neutron scattering to probe magnetic and nematic orders throughout the phase diagram of NaFe1-xNixAs, finding that while both static antiferromagnetic and nematic orders compete with superconductivity, the onset temperatures for these two orders remain well separated approaching the putative quantum critical points. We uncover local orthorhombic distortions that persist well above the tetragonal-to-orthorhombic structural transition temperature Ts in underdoped samples and extend well into the overdoped regime that exhibits neither magnetic nor structural phase transitions. These unexpected local orthorhombic distortions display Curie-Weiss temperature dependence and become suppressed below the superconducting transition temperature Tc, suggesting that they result from the large nematic susceptibility near optimal superconductivity. Our results account for observations of rotational symmetry breaking above Ts, and attest to the presence of significant nematic fluctuations near optimal superconductivity.Item A Mott insulator continuously connected to iron pnictide superconductors(Springer Nature, 2016) Song, Yu; Yamani, Zahra; Cao, Chongde; Li, Yu; Zhang, Chenglin; Chen, Justin S.; Huang, Qingzhen; Wu, Hui; Tao, Jing; Zhu, Yimei; Tian, Wei; Chi, Songxue; Cao, Huibo; Huang, Yao-Bo; Dantz, Marcus; Schmitt, Thorsten; Yu, Rong; Nevidomskyy, Andriy H.; Morosan, Emilia; Si, Qimiao; Dai, Pengcheng; Rice Center for Quantum MaterialsIron-based superconductivity develops near an antiferromagnetic order and out of a bad-metal normal state, which has been interpreted as originating from a proximate Mott transition. Whether an actual Mott insulator can be realized in the phase diagram of the iron pnictides remains an open question. Here we use transport, transmission electron microscopy, X-ray absorption spectroscopy, resonant inelastic X-ray scattering and neutron scattering to demonstrate that NaFe1−xCuxAs near x≈0.5 exhibits real space Fe and Cu ordering, and are antiferromagnetic insulators with the insulating behaviour persisting above the Néel temperature, indicative of a Mott insulator. On decreasing x from 0.5, the antiferromagnetic-ordered moment continuously decreases, yielding to superconductivity ∼x=0.05. Our discovery of a Mott-insulating state in NaFe1−xCuxAs thus makes it the only known Fe-based material, in which superconductivity can be smoothly connected to the Mott-insulating state, highlighting the important role of electron correlations in the high-Tc superconductivity.Item Neutron spin resonance as a probe of Fermi surface nesting and superconducting gap symmetry in Ba0.67K0.33(Fe1−xCox)2As2(American Physical Society, 2018) Zhang, Rui; Wang, Weiyi; Maier, Thomas A.; Wang, Meng; Stone, Matthew B.; Chi, Songxue; Winn, Barry; Dai, PengchengWe use inelastic neutron scattering to study the energy and wave-vector dependence of the superconductivity-induced resonance in hole-doped Ba0.67K0.33(Fe1−xCox)2As2 (x=0 and 0.08 with Tc≈37 and 28 K, respectively). In previous work on electron-doped Ba(Fe0.963Ni0.037)2As2 (TN=26K and Tc=17 K), the resonance is found to peak sharply at the antiferromagnetic (AF) ordering wave vector QAF along the longitudinal direction, but disperses upwards away from QAF along the transverse direction [Kim et al., Phys. Rev. Lett. 110, 177002 (2013)]. For hole-doped x=0 and 0.08 without AF order, we find that the resonance displays a ringlike upward dispersion away from QAFalong both the longitudinal and transverse directions. By comparing these results with calculations using the random phase approximation, we conclude that the dispersive resonance is a direct signature of isotropic superconducting gaps arising from nested hole-electron Fermi surfaces.Item Phase diagram and neutron spin resonance of superconducting NaFe1−xCuxAs(American Physical Society, 2017) Tan, Guotai; Song, Yu; Zhang, Rui; Lin, Lifang; Xu, Zhuang; Tian, Long; Chi, Songxue; Graves-Brook, M.K.; Li, Shiliang; Dai, PengchengWe use transport and neutron scattering to study the electronic phase diagram and spin excitations of NaFe1−xCuxAs single crystals. Similar to Co- and Ni-doped NaFeAs, a bulk superconducting phase appears near x≈2% with the suppression of stripe-type magnetic order in NaFeAs. Upon further increasing Cu concentration the system becomes insulating, culminating in an antiferromagnetically ordered insulating phase near x≈50%. Using transport measurements, we demonstrate that the resistivity in NaFe1−xCuxAs exhibits non-Fermi-liquid behavior near x≈1.8%. Our inelastic neutron scattering experiments reveal a single neutron spin resonance mode exhibiting weak dispersion along c axis in NaFe0.98Cu0.02As. The resonance is high in energy relative to the superconducting transition temperature Tc but weak in intensity, likely resulting from impurity effects. These results are similar to other iron pnictides superconductors despite that the superconducting phase in NaFe1−xCuxAs is continuously connected to an antiferromagnetically ordered insulating phase near x≈50% with significant electronic correlations. Therefore, electron correlations is an important ingredient of superconductivity in NaFe1−xCuxAs and other iron pnictides.Item Topology stabilized fluctuations in a magnetic nodal semimetal(Springer Nature, 2023) Drucker, Nathan C.; Nguyen, Thanh; Han, Fei; Siriviboon, Phum; Luo, Xi; Andrejevic, Nina; Zhu, Ziming; Bednik, Grigory; Nguyen, Quynh T.; Chen, Zhantao; Nguyen, Linh K.; Liu, Tongtong; Williams, Travis J.; Stone, Matthew B.; Kolesnikov, Alexander I.; Chi, Songxue; Fernandez-Baca, Jaime; Nelson, Christie S.; Alatas, Ahmet; Hogan, Tom; Puretzky, Alexander A.; Huang, Shengxi; Yu, Yue; Li, MingdaThe interplay between magnetism and electronic band topology enriches topological phases and has promising applications. However, the role of topology in magnetic fluctuations has been elusive. Here, we report evidence for topology stabilized magnetism above the magnetic transition temperature in magnetic Weyl semimetal candidate CeAlGe. Electrical transport, thermal transport, resonant elastic X-ray scattering, and dilatometry consistently indicate the presence of locally correlated magnetism within a narrow temperature window well above the thermodynamic magnetic transition temperature. The wavevector of this short-range order is consistent with the nesting condition of topological Weyl nodes, suggesting that it arises from the interaction between magnetic fluctuations and the emergent Weyl fermions. Effective field theory shows that this topology stabilized order is wavevector dependent and can be stabilized when the interband Weyl fermion scattering is dominant. Our work highlights the role of electronic band topology in stabilizing magnetic order even in the classically disordered regime.Item Two spatially separated phases in semiconducting Rb0.8Fe1.5S2(American Physical Society, 2014) Wang, Meng; Tian, Wei; Valdivia, P.; Chi, Songxue; Bourret-Courchesne, E.; Dai, Pengcheng; Birgeneau, R.J.We report neutron scattering and transport measurements on semiconducting Rb0.8Fe1.5S2, a compound isostructural and isoelectronic to the well-studied A0.8FeySe2(A=K,Rb,Cs,Tl/K) superconducting systems. Both resistivity and dc susceptibility measurements reveal a magnetic phase transition at T=275K. Neutron diffraction studies show that the 275 K transition originates from a phase with rhombic iron vacancy order which exhibits an in-plane stripe antiferromagnetic ordering below 275 K. In addition, the stripe antiferromagnetic phase interdigitates mesoscopically with an ubiquitous phase with 5√×5√ iron vacancy order. This phase has a magnetic transition at TN=425K and an iron vacancy order-disorder transition at TS=600K. These two different structural phases are closely similar to those observed in the isomorphous Se materials. Based on the close similarities of the in-plane antiferromagnetic structures, moments sizes, and ordering temperatures in semiconducting Rb0.8Fe1.5S2 and K0.81Fe1.58Se2, we argue that the in-plane antiferromagnetic order arises from strong coupling between local moments. Superconductivity, previously observed in the A0.8FeySe2−zSz system, is absent in Rb0.8Fe1.5S2, which has a semiconducting ground state. The implied relationship between stripe and block antiferromagnetism and superconductivity in these materials as well as a strategy for further investigation is discussed in this paper.