Browsing by Author "Li, Yu"
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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 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 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 magnetic excitations in NaFe1−xCoxAs(American Physical Society, 2016) Carr, Scott V.; Zhang, Chenglin; Song, Yu; Tan, Guotai; Li, Yu; Abernathy, D.L.; Stone, M.B.; Granroth, G.E.; Perring, T.G.; Dai, PengchengWe use time-of-flight (TOF) inelastic-neutron-scattering (INS) spectroscopy to investigate the doping dependence of magnetic excitations across the phase diagram of NaFe 1 − x Co x As with x = 0 , 0.0175, 0.0215, 0.05, and 0.11 . The effect of electron doping by partially substituting Fe by Co is to form resonances that couple with superconductivity, broaden, and suppress low-energy ( E ≤ 80 meV) spin excitations compared with spin waves in undoped NaFeAs. However, high-energy ( E > 80 meV) spin excitations are weakly Co-doping-dependent. Integration of the local spin dynamic susceptibility χ ' ' ( ω ) of NaFe 1 − x Co x As reveals a total fluctuating moment of 3.6 μ 2 B /Fe and a small but systematic reduction with electron doping. The presence of a large spin gap in Co-overdoped nonsuperconducting NaFe 0.89 Co 0.11 As suggests that Fermi surface nesting is responsible for low-energy spin excitations. These results parallel the Ni-doping evolution of spin excitations in BaFe 2 − x Ni x As 2 in spite of the differences in crystal structures and Fermi surface evolution in these two families of iron pnictides, thus confirming the notion that low-energy spin excitations coupling with itinerant electrons are important for superconductivity, while weakly doping-dependent high-energy spin excitations result from localized moments.Item Highly sensitive acetylene detection based on multi-pass retro-reflection-cavity-enhanced photoacoustic spectroscopy and a fiber amplified diode laser(Optical Society of America, 2019) Ma, Yufei; Qiao, Shunda; He, Ying; Li, Yu; Zhang, Zhonghua; Yu, Xin; Tittel, Frank K.In this paper, a multi-pass retro-reflection-cavity-enhanced photoacoustic spectroscopy (PAS) based gas sensor is reported for the first time. The multi-pass retro-reflection-cavity consisted of two right-angle prisms and was designed to reflect the laser beam to pass through the photoacoustic (PA) cell four times, which improved the acetylene (C2H2)-PAS sensor signal level significantly. The optical power of a near-infrared distributed feedback (DFB) diode laser emitting a continuous wave (CW) was amplified to 1000 mW with an erbium-doped fiber amplifier. The background noise was reduced with wavelength modulation spectroscopy (WMS) and 2nd harmonic demodulation techniques. The linear optical power and concentration response of such a PAS sensor were investigated, and the experimental results showed excellent characteristics. When the integration the time of the sensor system was set to 1 s, the minimum detection limit (MDL) for C2H2 detection was 8.17 ppb, which corresponds to a normalized noise equivalent absorption coefficient (NNEA) of 1.84 × 10−8 cm−1W/√Hz. The long-term stability of such a multi-pass retro-reflection-cavity-enhanced PAS based C2H2 sensor was evaluated by an Allan deviation analysis. It was demonstrated that the multi-pass retro-reflection-cavity-enhanced PAS sensor had an excellent stability. An MDL of 600 ppt was achieved when the integration time was set to ~1000 s. It was verified that the method of multi-pass retro-reflection-cavity-enhanced PAS with an amplified laser source improved the sensor performance significantly. If an appropriate cavity design with increasing reflection times is used, the MDL of such a PAS-based sensor can be further improved.Item In-plane uniaxial pressure-induced out-of-plane antiferromagnetic moment and critical fluctuations in BaFe2As2(Springer Nature, 2020) Liu, Panpan; Klemm, Mason L.; Tian, Long; Lu, Xingye; Song, Yu; Tam, David W.; Schmalzl, Karin; Park, J. T.; Li, Yu; Tan, Guotai; Su, Yixi; Bourdarot, Frédéric; Zhao, Yang; Lynn, Jeffery W.; Birgeneau, Robert J.; Dai, PengchengA small in-plane external uniaxial pressure has been widely used as an effective method to acquire single domain iron pnictide BaFe2As2, which exhibits twin-domains without uniaxial strain below the tetragonal-to-orthorhombic structural (nematic) transition temperature Ts. Although it is generally assumed that such a pressure will not affect the intrinsic electronic/magnetic properties of the system, it is known to enhance the antiferromagnetic (AF) ordering temperature TN ( < Ts) and create in-plane resistivity anisotropy above Ts. Here we use neutron polarization analysis to show that such a strain on BaFe2As2 also induces a static or quasi-static out-of-plane (c-axis) AF order and its associated critical spin fluctuations near TN/Ts. Therefore, uniaxial pressure necessary to detwin single crystals of BaFe2As2 actually rotates the easy axis of the collinear AF order near TN/Ts, and such effects due to spin-orbit coupling must be taken into account to unveil the intrinsic electronic/magnetic properties of the system.Item Interplay of multiple degrees of freedom and various emergent phenomenon in iron based material(2018-04-20) Li, Yu; Dai, PengchengThe term ’condensed matter’ is p opularized by Anderson based on his famous philosophic idea ”more is different”. It refers to the hierarchical structure of science. As the system grows from a few particles to a large and complex aggregates and involves a massive increase of degrees of freedom, new physics emerges which is not a simple extrap olation from the elementary building blo cks. This brings us the various exotic quantum phenomenon observed in correlated electron system, such as high T c cuprate and iron pnictide superconductors. In the cuprate and newly discovered iron based superconductors, there are multiple interaction competing or coexisting with each other, and it is usually believed that the high Tc superconductivity arises from the suppression of the long range antiferromagnetic (AF) order, meanwhile the ’normal states’ in the parent compounds are not normal and exhibit exotic properties such as the pseudo gap phase in cuprate and nematic phase in iron pnictides. In these phases, multiple degrees of freedom such as spin, charge, orbital, and lattice are entangled and it is often difficult to distinguish the driven force of the various emergent behavior. Cuprate superconductors are usually considered as doped Mott insulator where the kinetic energy and on-site Coulomb repulsion U are two dominant energy scales comp eting with each other. In iron pnictides, however, the strongly enhanced mass of electrons is not dominated by Hubbard U but the Hund’s coupling J. The extra orbital degree of freedom pushes the system leads to bad metals and the emergence of orbital selective b ehavior. In LiFeAs which is b elieved to b e in proximity to the orbital selective Mott transition, large orbital differentiation have b een calculated by DFMT and observed by ARPES exp eriments. In my inelastic neutron scattering exp eriment, I discovered that the electron doping dep endence of the low energy spin fluctuations is opp osite to that observed in Ba122 system. Comparing with the Fermi surfaces results from ARPES, I concluded it is the natural results of orbital dep endent band renormalization and consistent with the nesting picture successfully applied in 122 system. More significantly, based on the whole spin excitation sp ectrum, I discovered the two-branch feature of the spin excitations suggesting orbital selective spin excitations in LiFeAs system. With spin-orbit coupling, the spins in solid can feel the lo cal surrounding charge environment and prefer certain orientation in spin space. This gives the magnetic anisotropy and lead to gap op ening in the spin excitation sp ectrum. With p olarized neutron inelastic scattering exp eriment, one can determine the dynamic magnetic resp onse along different crystal axes. In the previous neutron scattering exp eriments, it was found that there are evolutions of spin anisotropy from undop ed to overdop ed comp ounds or from low to high temp eratures in 122 families. In Ni-dop ed BaFe 2As2, spin excitation anisotropy can be used as a probe of orbital ordering in the paramagnetic tetragonal phase. In my polarized neutron study on BaFe2As2, I successfully observed the critical scattering of Ma component while the other two not. This suggests that the low energy effective model near the magnetic transition in BaFe2As2 is a Heisenberg model with Ising spin anisotropy. The nematic phase in iron pnictide is a typical emergent phenomenon involving multiple degrees of freedom coupled with each other. It is difficult to distinguish the driving force of the nematicity which is still an open question up to now. In NaFeAs parent comp ound, I studied the coupling b etween the spin and lattice degree of freedom and their effect on the phonon and magnon sp ectra. I demonstrated that the IPTA phonon which is asso ciated with the orthorhombic distortion in the nematic phase, has a softening b ehavior p ersisting far ab ove Ts resulting in a nonlinear dispersion in the long wave length regime (small q). Such a phonon softening and accompanied spectral weight redistribution of spin waves are the natural consequence of the spin-phonon coupling and one-phonon-two-magnon scattering channel has to be involved. Our results suggests that the spin-phonon coupling might play important role in determine the physical properties associated with the nematic phase. In conclusion, I studied the complex interplay among several degrees of freedom in iron pnictides including 122 and 111 systems and the effect of their coupling on the physical properties. My results provide a significant advances in understanding the normal state of iron pnictides and unveiling the underlying physics which might be responsible for high Tc superconductivity.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 Neutron spin resonance as a probe of superconducting gap anisotropy in partially detwinned electron underdoped ${\mathrm{NaFe}}_{0.985}{\mathrm{Co}}_{0.015}\mathrm{As}$(American Physical Society, 2015) Zhang, Chenglin; Park, J.T.; Lu, Xingye; Yu, Rong; Li, Yu; Zhang, Wenliang; Zhao, Yang; Lynn, J.W.; Si, Qimiao; Dai, PengchengWe use inelastic neutron scattering (INS) to study the spin excitations in partially detwinned NaFe0.985Co0.015As which has coexisting static antiferromagnetic (AF) order and superconductivity (Tc=15 K, TN=30 K). In previous INS work on a twinned sample, spin excitations formed a dispersive sharp resonance near Er1=3.25 meV and a broad dispersionless mode at Er1=6 meV at the AF ordering wave vector QAF=Q1=(1,0) and its twinned domain Q2=(0,1). For partially detwinned NaFe0.985Co0.015As with the static AF order mostly occurring at QAF=(1,0), we still find a double resonance at both wave vectors with similar intensity. Since Q1=(1,0) characterizes the explicit breaking of the spin rotational symmetry associated with the AF order, these results indicate that the double resonance cannot be due to the static and fluctuating AF orders but originate from the superconducting gap anisotropy.Item Neutron spin resonance as a probe of superconducting gap anisotropy in partially detwinned electron underdoped NaFe0.985Co0.015As(American Physical Society, 2015) Zhang, Chenglin; Park, J.T.; Lu, Xingye; Yu, Rong; Li, Yu; Zhang, Wenliang; Zhao, Yang; Lynn, J.W.; Si, Qimiao; Dai, PengchengWe use inelastic neutron scattering (INS) to study the spin excitations in partially detwinned NaFe0.985Co0.015As which has coexisting static antiferromagnetic (AF) order and superconductivity (Tc=15 K, TN=30 K). In previous INS work on a twinned sample, spin excitations formed a dispersive sharp resonance near Er1=3.25ﾠmeV and a broad dispersionless mode at Er1=6 meV at the AF ordering wave vector QAF=Q1=(1,0) and its twinned domain Q2=(0,1). For partially detwinned NaFe0.985Co0.015As with the static AF order mostly occurring at QAF=(1,0), we still find a double resonance at both wave vectors with similar intensity. Since Q1=(1,0) characterizes the explicit breaking of the spin rotational symmetry associated with the AF order, these results indicate that the double resonance cannot be due to the static and fluctuating AF orders but originate from the superconducting gap anisotropy.Item Orbital Selective Spin Excitations and their Impact on Superconductivity of LiFe1−xCoxAs(American Physical Society, 2016) Li, Yu; Yin, Zhiping; Wang, Xiancheng; Tam, David W.; Abernathy, D.L.; Podlesnyak, A.; Zhang, Chenglin; Wang, Meng; Xing, Lingyi; Jin, Changqing; Haule, Kristjan; Kotliar, Gabriel; Maier, Thomas A.; Dai, PengchengWe use neutron scattering to study spin excitations in single crystals of LiFe0.88Co0.12As, which is located near the boundary of the superconducting phase of LiFe1−xCoxAs and exhibits non-Fermi-liquid behavior indicative of a quantum critical point. By comparing spin excitations of LiFe0.88Co0.12As with a combined density functional theory and dynamical mean field theory calculation, we conclude that wave-vector correlated low energy spin excitations are mostly from the dxy orbitals, while high-energy spin excitations arise from the dyz and dxz orbitals. Unlike most iron pnictides, the strong orbital selective spin excitations in the LiFeAs family cannot be described by an anisotropic Heisenberg Hamiltonian. While the evolution of low-energy spin excitations of LiFe1−xCoxAs is consistent with the electron-hole Fermi surface nesting conditions for the dxy orbital, the reduced superconductivity in LiFe0.88Co0.12As suggests that Fermi surface nesting conditions for the dyz and dxz orbitals are also important for superconductivity in iron pnictides.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.