Browsing by Author "Wang, Weiyi"
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Item c-axis pressure-induced antiferromagnetic order in optimally P-doped BaFe2(As0.70P0.30)2 superconductor(Springer Nature, 2018) Hu, Ding; Wang, Weiyi; Zhang, Wenliang; Wei, Yuan; Gong, Dongliang; Tam, David W.; Zhou, Panpan; Li, Yu; Tan, Guotai; Song, Yu; Georgii, Robert; Pedersen, Björn; Cao, Huibo; Tian, Wei; Roessli, Bertrand; Yin, Zhiping; Dai, PengchengSuperconductivity in BaFe2(As1−xPx)2 iron pnictides emerges when its in-plane two-dimensional (2D) orthorhombic lattice distortion associated with nematic phase at Ts and three-dimensional (3D) collinear antiferromagnetic order at TN (Ts = TN) are gradually suppressed with increasing x, reaching optimal superconductivity around x = 0.30 with Tc ≈ 30 K. Here we show that a moderate uniaxial pressure along the c-axis in BaFe2(As0.70P0.30)2 spontaneously induces a 3D collinear antiferromagnetic order with TN = Ts > 30 K, while only slightly suppresses Tc. Although a ~ 400 MPa pressure compresses the c-axis lattice while expanding the in-plane lattice and increasing the nearest-neighbor Fe–Fe distance, it barely changes the average iron-pnictogen height in BaFe2(As0.70P0.30)2. Therefore, the pressure-induced antiferromagnetic order must arise from a strong in-plane magnetoelastic coupling, suggesting that the 2D nematic phase is a competing state with superconductivity.Item Coexistence of Ferromagnetic and Stripe Antiferromagnetic Spin Fluctuations in SrCo2As2(American Physical Society, 2019) Li, Yu; Yin, Zhiping; Liu, Zhonghao; Wang, Weiyi; Xu, Zhuang; Song, Yu; Tian, Long; Huang, Yaobo; Shen, Dawei; Abernathy, D.L.; Niedziela, J.L.; Ewings, R.A.; Perring, T.G.; Pajerowski, Daniel M.; Matsuda, Masaaki; Bourges, Philippe; Mechthild, Enderle; Su, Yixi; Dai, PengchengWe use inelastic neutron scattering to study energy and wave vector dependence of spin fluctuations in SrCo2As2, derived from SrFe2−xCoxAs2 iron pnictide superconductors. Our data reveal the coexistence of antiferromagnetic (AF) and ferromagnetic (FM) spin fluctuations at wave vectors QAF=(1,0) and QFM=(0,0)/(2,0), respectively. By comparing neutron scattering results with those of dynamic mean field theory calculation and angle-resolved photoemission spectroscopy experiments, we conclude that both AF and FM spin fluctuations in SrCo2As2 are closely associated with a flatband of the eg orbitals near the Fermi level, different from the t2g orbitals in superconducting SrFe2−xCoxAs2. Therefore, Co substitution in SrFe2−xCoxAs2 induces a t2g to eg orbital switching, and is responsible for FM spin fluctuations detrimental to the singlet pairing superconductivity.Item 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.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 High-energy magnetic excitations from heavy quasiparticles in CeCu2Si2(Springer Nature, 2021) Song, Yu; Wang, Weiyi; Cao, Chongde; Yamani, Zahra; Xu, Yuanji; Sheng, Yutao; Löser, Wolfgang; Qiu, Yiming; Yang, Yi-feng; Birgeneau, Robert J.; Dai, PengchengMagnetic fluctuations is the leading candidate for pairing in cuprate, iron-based, and heavy fermion superconductors. This view is challenged by the recent discovery of nodeless superconductivity in CeCu2Si2, and calls for a detailed understanding of the corresponding magnetic fluctuations. Here, we mapped out the magnetic excitations in superconducting (S-type) CeCu2Si2 using inelastic neutron scattering, finding a strongly asymmetric dispersion for E ≲ 1.5 meV, which at higher energies evolves into broad columnar magnetic excitations that extend to E ≳ 5 meV. While low-energy magnetic excitations exhibit marked three-dimensional characteristics, the high-energy magnetic excitations in CeCu2Si2 are almost two-dimensional, reminiscent of paramagnons found in cuprate and iron-based superconductors. By comparing our experimental findings with calculations in the random-phase approximation,we find that the magnetic excitations in CeCu2Si2 arise from quasiparticles associated with its heavy electron band, which are also responsible for superconductivity. Our results provide a basis for understanding magnetism and superconductivity in CeCu2Si2, and demonstrate the utility of neutron scattering in probing band renormalization in heavy fermion metals.Item Local breaking of fourfold rotational symmetry by short-range magnetic order in heavily overdoped Ba(Fe1−xCux)2As2(American Physical Society, 2017) Wang, Weiyi; Song, Yu; Hu, Ding; Li, Yu; Zhang, Rui; Harriger, L.W.; Tian, Wei; Cao, Huibo; Dai, PengchengWe investigate Cu-doped Ba(Fe1−xCux)2As2 with transport, magnetic susceptibility, and elastic neutron scattering measurements. In the heavily Cu-doped regime where long-range stripe-type antiferromagnetic order in BaFe2As2 is suppressed, Ba(Fe1−xCux)2As2 (0.145≤x≤0.553) samples exhibit spin-glass-like behavior in magnetic susceptibility and insulating-like temperature dependence in electrical transport. Using elastic neutron scattering, we find stripe-type short-range magnetic order in the spin-glass region identified by susceptibility measurements. The persistence of short-range magnetic order over a large doping range in Ba(Fe1−xCux)2As2 likely arises from local arrangements of Fe and Cu that favor magnetic order, with Cu acting as vacancies relieving magnetic frustration and degeneracy. These results indicate locally broken fourfold rotational symmetry, suggesting that stripe-type magnetism is ubiquitous in iron pnictides.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 Nature of the spin resonance mode in CeCoIn5(Springer Nature, 2020) Song, Yu; Wang, Weiyi; Van Dyke, John S.; Pouse, Naveen; Ran, Sheng; Yazici, Duygu; Schneidewind, A.; Čermák, Petr; Qiu, Y.; Maple, M.B.; Morr, Dirk K.; Dai, PengchengSpin-fluctuation-mediated unconventional superconductivity can emerge at the border of magnetism, featuring a superconducting order parameter that changes sign in momentum space. Detection of such a sign-change is experimentally challenging, since most probes are not phase-sensitive. The observation of a spin resonance mode (SRM) from inelastic neutron scattering is often seen as strong phase-sensitive evidence for a sign-changing superconducting order parameter, by assuming the SRM is a spin-excitonic bound state. Here we show that for the heavy fermion superconductor CeCoIn5, its SRM defies expectations for a spin-excitonic bound state, and is not a manifestation of sign-changing superconductivity. Instead, the SRM in CeCoIn5 likely arises from a reduction of damping to a magnon-like mode in the superconducting state, due to its proximity to magnetic quantum criticality. Our findings emphasize the need for more stringent tests of whether SRMs are spin-excitonic, when using their presence to evidence sign-changing superconductivity.Item Neutron scattering studies of doped iron pnictides(2019-02-28) Wang, Weiyi; Dai, PengchengUnraveling the mechanism of unconventional superconductivity is one of the long-sought crown jewels in condensed matter physics. Outstanding among many of the challenges is the critical importance of electron-electron interactions, which gives rise to new emergent collective electronic orders, including magnetism, electronic nematic order, and superconductivity. These degrees of freedom are closely intertwined in iron-based superconductors. The interplay between these orders, can be unraveled by studying their co-evolution across phase diagrams, which nature conveniently grants us access through chemical substitution. Such an understanding provides the foundation for a description that unifies these different orders as different flavors of the same electrons, and sets the stage for future applications of these fascination materials. Within this thesis I present several neutron scattering works on prototypical iron-based superconductors, derived from BaFe$_{2}$As$_{2}$ and NaFeAs. Neutron scattering directly probes the magnetic correlations of atoms, and it also allows for the measurement of the structural properties that directly couple to the electronic nematic order. Complemented by physical property measurements that directly probe superconductivity, this enables us to carry out a comprehensive survey of how these entangled orders behave across the temperature and doping phase diagram. In Ni doped NaFeAs, we found both nematic and magnetic orders compete with superconductivity, demonstrating an intricate balance between the different orders that results from the fact they are different manifestations of the same electrons. At high temperature or high doping-levels, where these orders were assumed to be absent, using high-resolution neutron diffraction and neutron Larmor diffraction techniques we uncovered local orthorhombic distortions, indicating electronic nematic order persists on a local length scale, over a much large region of the phase diagram than previous thought. Our observation of these unexpected local orthorhombic distortions demonstrate fluctuations of the electronic nematic order that are maximized near a putative quantum critical point, when coupled with random strain fields, result in locally pinned electronic nematic states, from which global superconductivity precipitates. In a similar vein, locally-broken fourfold rotational symmetry is also observed in heavily Cu-doped Ba(Fe$_{1-x}$Cu$_{x}$)$_2$As$_2$. Combining transport, magnetic susceptibility, elastic and inelastic neutron scattering measurements, we identified for the first time the existence of short-range magnetic order over a large doping range beyond the putative quantum critical point. This discovery reveals while the system on average is tetragonal and exhibit four-fold rotational symmetry, small patches can exhibit locally broken fourfold symmetry, giving rise to magnetic order. Our findings point to an inherent tendency towards the lowering of crystallographic symmetry, on a local scale if not globally, providing the backdrop from which superconductivity emerges. Our neutron scattering results on partially detwinned NaFe$_{0.985}$Co$_0.015$As show that neutron spin resonance in this system only appears at the antiferromagnetic wave vector. Combined with electronic band analysis, our results indicate that the neutron spin resonances in NaFe$_{0.985}$Co$_0.015$As arise mostly from quasiparticle excitations between the hole and electron Fermi surfaces with the $d_{\rm yz}$ orbital character. Our analysis suggests that intraorbital quasiparticle scattering of the $d_{\rm yz}$-$d_{\rm yz}$ orbitals are important for superconductivity. The combination of these works provides a significant advance in the understanding of the interplay between electronic nematic order, magnetism and unconventional superconductivity in iron-based superconductors, underscores the physics at the local scale which is critical for potential applications, and motivates future studies to examine the intricate interplay of complex orders at the local scale.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 Possible Mott transition in layered Sr2Mn3As2O2ᅠsingle crystals(American Physical Society, 2019) Chen, Chih-Wei; Wang, Weiyi; Loganathan, Vaideesh; Carr, Scott V.; Harriger, Leland W.; Georgen, C.; Nevidomskyy, Andriy H.; Dai, Pengcheng; Huang, C.-L.; Morosan, E.Single crystals of Sr2Mn3As2O2 have been grown for the first time, for which we show a possible layer-selective Mott insulator behavior. This compound stands out as a hybrid structure of MnO2 and MnAs layers, analogously to the active CuO2 and FeAs layers, respectively, in the cuprate and iron-based high-temperature superconductors. Electrical transport, neutron diffraction measurements, together with density functional theory calculations on Sr2Mn3As2O2 single crystals converge toward a picture of independent magnetic order at T1∼79 K and T2∼360 K for the two Mn sublattices, with insulating behavior at odds with the metallic behavior predicted by calculations. Furthermore, our inelastic neutron-scattering studies of spin-wave dispersions for the Mn(1) sublattice reveal an effective magnetic exchange coupling of SJ∼3.7 meV. This is much smaller than those for the Mn(2) sublattice.Item Spin excitation anisotropy in the paramagnetic tetragonal phase of BaFe2As2(American Physical Society, 2017) Li, Yu; Wang, Weiyi; Song, Yu; Man, Haoran; Lu, Xingye; Bourdarot, Frédéric; Dai, PengchengWe use neutron polarization analysis to study temperature dependence of the spin excitation anisotropy in BaFe 2 As 2 , which has a tetragonal-to-orthorhombic structural distortion at T s and antiferromagnetic (AF) phase transition at T N with ordered moments along the orthorhombic a axis below T s ≈ T N ≈ 136 K. In the paramagnetic tetragonal state at 160 K, spin excitations are isotropic in spin space with M a = M b = M c , where M a , M b , and M c are spin excitations polarized along the a -, b -, and c -axis directions of the orthorhombic lattice, respectively. On cooling towards T N , significant spin excitation anisotropy with M a > M b ≈ M c develops below 3 meV with a diverging M a at T N . The in-plane spin excitation anisotropy in the tetragonal phase of BaFe 2 As 2 is similar to those seen in the tetragonal phase of its electron and hole-doped superconductors, suggesting that spin excitation anisotropy is a direct probe of doping dependence of spin-orbit coupling and its connection to superconductivity in iron pnictides.Item Temperature and polarization dependence of low-energy magnetic fluctuations in nearly optimally doped NaFe0.9785Co0.0215As(American Physical Society, 2017) Song, Yu; Wang, Weiyi; Zhang, Chenglin; Gu, Yanhong; Lu, Xingye; Tan, Guotai; Su, Yixi; Bourdarot, Frédéric; Christianson, A.D.; Li, Shiliang; Dai, PengchengWe use unpolarized and polarized neutron scattering to study the temperature and polarization dependence of low-energy magnetic fluctuations in nearly optimally doped NaFe0.9785Co0.0215As, with coexisting superconductivity (Tc≈19 K) and weak antiferromagnetic order (TN≈30 K, ordered moment ≈0.02μB/Fe). A single spin resonance mode with intensity tracking the superconducting order parameter is observed, although energy of the mode only softens slightly upon approaching Tc. Polarized neutron scattering reveals that the single resonance is mostly isotropic in spin space, similar to overdoped NaFe0.935Co0.045As but different from optimal electron-, hole-, and isovalently doped BaFe2As2 compounds, all featuring an additional prominent anisotropic component. Spin anisotropy in NaFe0.9785Co0.0215As is instead present at energies below the resonance, which becomes partially gapped below Tc, similar to the situation in optimally doped YBa2Cu3O6.9. Our results indicate that anisotropic spin fluctuations in NaFe1−xCoxAs appear in the form of a resonance in the underdoped regime, become partially gapped below Tc near optimal doping, and disappear in overdoped compounds.Item Weaker nematic phase connected to the first order antiferromagnetic phase transition inᅠSrFe2As2ᅠcompared toᅠBaFe2As2(American Physical Society, 2019) Tam, David W.; Wang, Weiyi; Zhang, Li; Song, Yu; Zhang, Rui; Carr, Scott V.; Walker, H.C.; Perring, Toby G.; Adroja, D.T.; Dai, PengchengUnderstanding the nature of the electronic nematic phase in iron pnictide superconductors is important for elucidating its impact on high-temperature superconductivity. Here we use transport and inelastic neutron scattering to study spin excitations and in-plane resistivity anisotropy in uniaxial pressure detwinned BaFe2As2 and SrFe2As2, the parent compounds of iron pnictide superconductors. While BaFe2As2 exhibits weakly first-order tetragonal-to-orthorhombic structural and antiferromagnetic (AF) phase transitions below Ts>TN≈138K, SrFe2As2 has strongly coupled first-order structural and AF transitions below Ts=TN≈210K. We find that the direct signatures of the nematic phase persist to lower temperatures above the phase transition in the case of SrFe2As2 compared to BaFe2As2. Our findings support the conclusion that the strongly first-order nature of the magnetic transition in SrFe2As2 weakens the nematic phase and resistivity anisotropy in the system.