Browsing by Author "Dai, Pengcheng"
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Item 3D optical lattice system for ultra-cold Lithium 6(2014-04-25) Yang, Tsung-Lin; Hulet, Randall G.; Killian, Thomas C.; Dai, PengchengOptical lattice experiments have proved to be a versatile tool for studying strongly correlated quantum systems. The motivation behind these experiments is to use the cold atoms to emulate a solid state system for which the Hamiltonian is analytically or numerically non-solvable. In our experiment, we construct a 3D simple cubic lattice system which realizes the Fermi-Hubbard model and provides us the opportunity to study the antiferromagnetic (AFM) insulator phase. In order to establish evaporative cooling in the optical lattice and to enlarge the size of the AFM phase, we implement a compensation potential additional to the standard lattice beam. In this thesis, I will present detailed steps to construct and calibrate the compensated 3D optical lattice system.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 Absence of ${E}_{2g}$ Nematic Instability and Dominant ${A}_{1g}$ Response in the Kagome Metal ${\mathrm{CsV}}_{3}{\mathrm{Sb}}_{5}$(American Physical Society, 2024) Liu, Zhaoyu; Shi, Yue; Jiang, Qianni; Rosenberg, Elliott W.; DeStefano, Jonathan M.; Liu, Jinjin; Hu, Chaowei; Zhao, Yuzhou; Wang, Zhiwei; Yao, Yugui; Graf, David; Dai, Pengcheng; Yang, Jihui; Xu, Xiaodong; Chu, Jiun-HawEver since the discovery of the charge density wave (CDW) transition in the kagome metal CsV3Sb5, the nature of its symmetry breaking has been under intense debate. While evidence suggests that the rotational symmetry is already broken at the CDW transition temperature (𝑇CDW), an additional electronic nematic instability well below 𝑇CDW has been reported based on the diverging elastoresistivity coefficient in the anisotropic channel (𝑚𝐸2𝑔). Verifying the existence of a nematic transition below 𝑇CDW is not only critical for establishing the correct description of the CDW order parameter, but also important for understanding low-temperature superconductivity. Here, we report elastoresistivity measurements of CsV3Sb5 using three different techniques probing both isotropic and anisotropic symmetry channels. Contrary to previous reports, we find the anisotropic elastoresistivity coefficient 𝑚𝐸2𝑔 is temperature independent, except for a step jump at 𝑇CDW. The absence of nematic fluctuations is further substantiated by measurements of the elastocaloric effect, which show no enhancement associated with nematic susceptibility. On the other hand, the symmetric elastoresistivity coefficient 𝑚𝐴1𝑔 increases below 𝑇CDW, reaching a peak value of 90 at 𝑇*=20 K. Our results strongly indicate that the phase transition at 𝑇* is not nematic in nature and the previously reported diverging elastoresistivity is due to the contamination from the 𝐴1𝑔 channel.Item Anisotropic magnon damping by zero-temperature quantum fluctuations in ferromagnetic CrGeTe3(Springer Nature, 2022) Chen, Lebing; Mao, Chengjie; Chung, Jae-Ho; Stone, Matthew B.; Kolesnikov, Alexander I.; Wang, Xiaoping; Murai, Naoki; Gao, Bin; Delaire, Olivier; Dai, PengchengSpin and lattice are two fundamental degrees of freedom in a solid, and their fluctuations about the equilibrium values in a magnetic ordered crystalline lattice form quasiparticles termed magnons (spin waves) and phonons (lattice waves), respectively. In most materials with strong spin-lattice coupling (SLC), the interaction of spin and lattice induces energy gaps in the spin wave dispersion at the nominal intersections of magnon and phonon modes. Here we use neutron scattering to show that in the two-dimensional (2D) van der Waals honeycomb lattice ferromagnetic CrGeTe3, spin waves propagating within the 2D plane exhibit an anomalous dispersion, damping, and breakdown of quasiparticle conservation, while magnons along the c axis behave as expected for a local moment ferromagnet. These results indicate the presence of dynamical SLC arising from the zero-temperature quantum fluctuations in CrGeTe3, suggesting that the observed in-plane spin waves are mixed spin and lattice quasiparticles fundamentally different from pure magnons and phonons.Item Anisotropic neutron spin resonance in underdoped superconducting NaFe1−xCoxAs(American Physical Society, 2014) Zhang, Chenglin; Song, Yu; Regnault, L.-P.; Su, Yixi; Enderle, M.; Kulda, J.; Tan, Guotai; Sims, Zachary C.; Egami, Takeshi; Si, Qimiao; Dai, PengchengWe use polarized inelastic neutron scattering (INS) to study spin excitations in superconducting NaFe0.985Co0.015As (C15) with static antiferromagnetic (AF) order along the a axis of the orthorhombic structure and NaFe0.935Co0.045As (C45) without AF order. In previous unpolarized INS work, spin excitations in C15 were found to have a dispersive sharp resonance near Er1=3.25 meV and a broad dispersionless mode at Er2=6 meV. Our neutron polarization analysis reveals that the dispersive resonance in C15 is highly anisotropic and polarized along the a and c axes, while the dispersionless mode is isotropic similar to that of C45. Since the a-axis polarized spin excitations of the anisotropic resonance appear below Tc, our data suggests that the itinerant electrons contributing to the magnetism are also coupled to the superconductivity.Item Anomalous Metamagnetism in the Low Carrier Density Kondo Lattice YbRh3Si7(American Physical Society, 2018) Rai, Binod K.; Chikara, S.; Ding, Xiaxin; Oswald, Iain W.H.; Schönemann, R.; Loganathan, V.; Hallas, A.M.; Cao, H.B.; Stavinoha, Macy; Chen, T.; Man, Haoran; Carr, Scott; Singleton, John; Zapf, Vivien; Benavides, Katherine A.; Chan, Julia Y.; Zhang, Q.R.; Rhodes, D.; Chiu, Y.C.; Balicas, Luis; Aczel, A.A.; Huang, Q.; Lynn, Jeffrey W.; Gaudet, J.; Sokolov, D.A.; Walker, H.C.; Adroja, D.T.; Dai, Pengcheng; Nevidomskyy, Andriy H.; Huang, C.-L.; Morosan, E.We report complex metamagnetic transitions in single crystals of the new low carrier Kondo antiferromagnet YbRh3Si7. Electrical transport, magnetization, and specific heat measurements reveal antiferromagnetic order at TN=7.5 K. Neutron diffraction measurements show that the magnetic ground state of YbRh3Si7 is a collinear antiferromagnet, where the moments are aligned in the ab plane. With such an ordered state, no metamagnetic transitions are expected when a magnetic field is applied along the c axis. It is therefore surprising that high-field magnetization, torque, and resistivity measurements with H∥c reveal two metamagnetic transitions at μ0H1=6.7 T and μ0H2=21 T. When the field is tilted away from the c axis, towards the ab plane, both metamagnetic transitions are shifted to higher fields. The first metamagnetic transition leads to an abrupt increase in the electrical resistivity, while the second transition is accompanied by a dramatic reduction in the electrical resistivity. Thus, the magnetic and electronic degrees of freedom in YbRh3Si7 are strongly coupled. We discuss the origin of the anomalous metamagnetism and conclude that it is related to competition between crystal electric-field anisotropy and anisotropic exchange interactions.Item Antiferromagnetic order and spin dynamics in iron-based superconductors(American Physical Society, 2015) Dai, PengchengHigh-transition temperature (high-Tc) superconductivity in the iron pnictides or chalcogenides emerges from the suppression of the static antiferromagnetic order in their parent compounds, similar to copper oxide superconductors. This raises a fundamental question concerning the role of magnetism in the superconductivity of these materials. Neutron scattering, a powerful probe to study the magnetic order and spin dynamics, plays an essential role in determining the relationship between magnetism and superconductivity in high-Tc superconductors. The rapid development of modern neutron time-of-flight spectrometers allows a direct determination of the spin dynamical properties of iron-based superconductors throughout the entire Brillouin zone. In this paper, an overview is presented of the neutron scattering results on iron-based superconductors, focusing on the evolution of spin-excitation spectra as a function of electron and hole doping and isoelectronic substitution. Spin dynamical properties of iron-based superconductors are compared with those of copper oxide and heavy fermion superconductors and the common features of spin excitations in these three families of unconventional superconductors and their relationship with superconductivity are discussed.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 Chemical tuning of electrical transport in Ti1−xPtxSe2−y(American Physical Society, 2015) Chen, Justin S.; Wang, Jiakui K.; Carr, Scott V.; Vogel, Sven C.; Gourdon, Olivier; Dai, Pengcheng; Morosan, E.The structural and transport properties of polycrystalline Ti1−xPtxSe2−y(x≤0.13,y≤0.2) are studied, revealing highly tunable electrical properties, spanning nearly ten orders of magnitude in scaled resistivity. Using x-ray and neutron diffraction, Pt is found to dope on the Ti site. In the absence of Pt doping (for x=0), Se deficiency (y>0) increases the metallic character of TiSe2, while a large increase of the low-temperature resistivity is favored by a lack of Se deficiency (y=0) and increasing amounts of doped Pt (x>0). The chemical tuning of the resistivity in Ti1−xPtxSe2−y with Se deficiency and Pt doping results in a metal-to-insulator transition. Simultaneous Pt doping and Se deficiency (x,y>0) confirms the competition between the two opposing trends in electrical transport, with the main outcome being the suppression of the charge density wave transition below 2 K for y=2x=0.18. Band structure calculations on a subset of Ti1−xPtxSe2−y compositions are in line with the experimental observations.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 Conventional superconductivity in the doped kagome superconductor Cs(V0.86Ta0.14)3Sb5 from vortex lattice studies(Springer Nature, 2024) Xie, Yaofeng; Chalus, Nathan; Wang, Zhiwei; Yao, Weiliang; Liu, Jinjin; Yao, Yugui; White, Jonathan S.; DeBeer-Schmitt, Lisa M.; Yin, Jia-Xin; Dai, Pengcheng; Eskildsen, Morten RingA hallmark of unconventional superconductors is a complex electronic phase diagram where intertwined orders of charge-spin-lattice degrees of freedom compete and coexist. While the kagome metals such as CsV3Sb5 also exhibit complex behavior, involving coexisting charge density wave order and superconductivity, much is unclear about the microscopic origin of the superconducting pairing. We study the vortex lattice in the superconducting state of Cs(V0.86Ta0.14)3Sb5, where the Ta-doping suppresses charge order and enhances superconductivity. Using small-angle neutron scattering, a strictly bulk probe, we show that the vortex lattice exhibits a strikingly conventional behavior. This includes a triangular symmetry with a period consistent with 2e-pairing, a field dependent scattering intensity that follows a London model, and a temperature dependence consistent with a uniform superconducting gap. Our results suggest that optimal bulk superconductivity in Cs(V1−xTax)3Sb5 arises from a conventional Bardeen-Cooper-Schrieffer electron-lattice coupling, different from spin fluctuation mediated unconventional copper- and iron-based superconductors.Item Correlation-driven electronic reconstruction in FeTe1−xSex(Springer Nature, 2022) Huang, Jianwei; Yu, Rong; Xu, Zhijun; Zhu, Jian-Xin; Oh, Ji Seop; Jiang, Qianni; Wang, Meng; Wu, Han; Chen, Tong; Denlinger, Jonathan D.; Mo, Sung-Kwan; Hashimoto, Makoto; Michiardi, Matteo; Pedersen, Tor M.; Gorovikov, Sergey; Zhdanovich, Sergey; Damascelli, Andrea; Gu, Genda; Dai, Pengcheng; Chu, Jiun-Haw; Lu, Donghui; Si, Qimiao; Birgeneau, Robert J.; Yi, Ming; Rice Center for Quantum MaterialsElectronic correlation is of fundamental importance to high temperature superconductivity. While the low energy electronic states in cuprates are dominantly affected by correlation effects across the phase diagram, observation of correlation-driven changes in fermiology amongst the iron-based superconductors remains rare. Here we present experimental evidence for a correlation-driven reconstruction of the Fermi surface tuned independently by two orthogonal axes of temperature and Se/Te ratio in the iron chalcogenide family FeTe1−xSex. We demonstrate that this reconstruction is driven by the de-hybridization of a strongly renormalized dxy orbital with the remaining itinerant iron 3d orbitals in the emergence of an orbital-selective Mott phase. Our observations are further supported by our theoretical calculations to be salient spectroscopic signatures of such a non-thermal evolution from a strongly correlated metallic phase into an orbital-selective Mott phase in dxy as Se concentration is reduced.Item Correlation-driven metal-insulator transition in proximity to an iron-based superconductor(American Physical Society, 2017) Charnukha, A.; Yin, Z.P.; Song, Y.; Cao, C.D.; Dai, Pengcheng; Haule, K.; Kotliar, G.; Basov, D.N.We report the direct spectroscopic observation of a metal to correlated-insulator transition in the family of iron-based superconducting materials. By means of optical spectroscopy we demonstrate that the excitation spectrum of NaFe1−xCuxAs develops a large gap with increasing copper substitution. Dynamical mean-field theory calculations show a good agreement with the experimental data and suggest that the formation of the charge gap requires an intimate interplay of strong on-site electronic correlations and spin-exchange coupling, revealing the correlated Slater-insulator nature of the antiferromagnetic ground state. Our calculations further predict the high-temperature paramagnetic state of the same compound to be a highly incoherent correlated metal. We verify this prediction experimentally by showing that the doping-induced weakening of antiferromagnetic correlations enables a thermal crossover from an insulating to an incoherent metallic state. Redistribution of the optical spectral weight in this crossover uncovers the characteristic energy of Hund's-coupling and Mott-Hubbard electronic correlations essential for the electronic localization. Our results demonstrate that NaFe1−xCuxAs continuously transitions from the typical itinerant phases of iron pnictides to a highly incoherent metal and ultimately a correlated insulator. Such an electronic state is expected to favor high-temperature superconductivity.Item Critical quadrupole fluctuations and collective modes in iron pnictide superconductors(American Physical Society, 2016) Thorsmølle, V.K.; Khodas, M.; Yin, Z.P.; Zhang, Chenglin; Carr, S.V.; Dai, Pengcheng; Blumberg, G.The multiband nature of iron pnictides gives rise to a rich temperature-doping phase diagram of competing orders and a plethora of collective phenomena. At low dopings, the tetragonal-to-orthorhombic structural transition is closely followed by a spin-density-wave transition both being in close proximity to the superconducting phase. A key question is the nature of high-Tc superconductivity and its relation to orbital ordering and magnetism. Here we study the NaFe1−xCoxAs superconductor using polarization-resolved Raman spectroscopy. The Raman susceptibility displays critical enhancement of nonsymmetric charge fluctuations across the entire phase diagram, which are precursors to a d-wave Pomeranchuk instability at temperature θ(x). The charge fluctuations are interpreted in terms of quadrupole interorbital excitations in which the electron and hole Fermi surfaces breathe in-phase. Below Tc, the critical fluctuations acquire coherence and undergo a metamorphosis into a coherent in-gap mode of extraordinary strength.Item Diffusive excitonic bands from frustrated triangular sublattice in a singlet-ground-state system(Springer Nature, 2023) Gao, Bin; Chen, Tong; Wu, Xiao-Chuan; Flynn, Michael; Duan, Chunruo; Chen, Lebing; Huang, Chien-Lung; Liebman, Jesse; Li, Shuyi; Ye, Feng; Stone, Matthew B.; Podlesnyak, Andrey; Abernathy, Douglas L.; Adroja, Devashibhai T.; Duc Le, Manh; Huang, Qingzhen; Nevidomskyy, Andriy H.; Morosan, Emilia; Balents, Leon; Dai, PengchengMagnetic order in most materials occurs when magnetic ions with finite moments arrange in a particular pattern below the ordering temperature. Intriguingly, if the crystal electric field (CEF) effect results in a spin-singlet ground state, a magnetic order can still occur due to the exchange interactions between neighboring ions admixing the excited CEF levels. The magnetic excitations in such a state are spin excitons generally dispersionless in reciprocal space. Here we use neutron scattering to study stoichiometric Ni2Mo3O8, where Ni2+ ions form a bipartite honeycomb lattice comprised of two triangular lattices, with ions subject to the tetrahedral and octahedral crystalline environment, respectively. We find that in both types of ions, the CEF excitations have nonmagnetic singlet ground states, yet the material has magnetic order. Furthermore, CEF spin excitons from the tetrahedral sites form a dispersive diffusive pattern around the Brillouin zone boundary, likely due to spin entanglement and geometric frustrations.Item Direct observation of magnon-phonon coupling in yttrium iron garnet(American Physical Society, 2017) Man, Haoran; Shi, Zhong; Xu, Guangyong; Xu, Yadong; Chen, Xi; Sullivan, Sean; Zhou, Jianshi; Xia, Ke; Shi, Jing; Dai, PengchengThe magnetic insulator yttrium iron garnet (YIG) with a ferrimagnetic transition temperature of ∼ 560 K has been widely used in microwave and spintronic devices. Anomalous features in spin Seeback effect (SSE) voltages have been observed in Pt/YIG and attributed to magnon-phonon coupling. Here, we use inelastic neutron scattering to map out low-energy spin waves and acoustic phonons of YIG at 100 K as a function of increasing magnetic field. By comparing the zero and 9.1 T data, we find that instead of splitting and opening up gaps at the spin wave and acoustic phonon dispersion intersecting points, magnon-phonon coupling in YIG enhances the hybridized scattering intensity. These results are different from expectations of conventional spin-lattice coupling, calling for different paradigms to understand the scattering process of magnon-phonon interactions and the resulting magnon polarons.Item Direct observation of spin excitation anisotropy in the paramagnetic orthorhombic state of BaFe2−xNixAs2(American Physical Society, 2018) Man, Haoran; Zhang, Rui; Park, J.T.; Lu, Xingye; Kulda, J.; Ivanov, A.; Dai, PengchengWe use transport and inelastic neutron-scattering measurements to investigate single crystals of iron pnictide BaFe2−xNixAs2(x=0,0.03), which exhibit a tetragonal-to-orthorhombic structural transition at Ts and stripe antiferromagnetic order at TN(Ts≥TN). Using a tunable uniaxial pressure device, we detwin the crystals and study their transport and spin excitation properties at antiferromagnetic wave-vector S1(1,0) and its 90∘ rotated wave-vector S2(0,1) under different pressure conditions. We find that uniaxial pressure necessary to detwin and maintain the single domain orthorhombic antiferromagnetic phase of BaFe2−xNixAs2 induces resistivity and spin excitation anisotropy at temperatures above zero pressure Ts. In the uniaxial pressure-free detwinned sample, spin excitation anisotropy between S1(1,0) and S2(0,1) first appears in the paramagnetic orthorhombic phase below Ts. These results are consistent with predictions of spin nematic theory, suggesting the absence of structural or nematic phase transition above Ts in iron pnictides.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 Diverse Neutron Scattering Measurements in Unconventional Superconductors(2017-01-18) Carr, Scott V; Dai, PengchengSuperconductivity is an amazing phenomena. Beyond the eye-popping demos and promises of floating trains, it is a macroscopic manifestation of pure quantum physics. Where most quantum effects are smothered by thermal energy even at the lowest of temperatures, superconductivity brings the strangeness of quantum mechanics to reasonably accessible temperatures. An while it does have applications in terms of effecient charge transfer and the production of powerful magnetic fields, this is not why we study it. The mystery of its origin, oddness of its effects, and beauty of its consequences drives us to delve into the richness of this amazing phenomena. One key player in the physics of these exotic superconductors appears to be magnetism. Magnetism is a constant presence in the phase diagrams of exotic superconductors and appears to couple to the superconducting transition in some interesting ways. The details of the connection remain unknown, but it is clear that magnetism, either through static order, fluctuations, or both, is an important aspect of the crystalline environments which foster superconductivity. Through the tool of neutron scattering I investigate this connection across an array or different compounds using a variety of techniques. In this thesis, I conclude on the role of phase separation in superconducting KyFe1.6+xSe2 using neutron diffraction, perform a survey of magnetic excitations across the superconducting phase diagram in NaFe1−xCoxAs with neutron time-of-flight spectroscopy, and reveal new magnetic properties of the old heavy fermion superconductor UPt3 via more traditional neutron measurements.Item Doping effects of Cr on the physical properties of BaFe1.9−xNi0.1CrxAs2(American Physical Society, 2018) Gong, Dongliang; Xie, Tao; Zhang, Rui; Birk, Jonas; Niedermayer, Christof; Han, Fei; Lapidus, S.H.; Dai, Pengcheng; Li, Shiliang; Luo, HuiqianWe present a systematic study on the heavily Cr doped iron pnictides BaFe1.9−xNi0.1CrxAs2 by using elastic neutron scattering, high-resolution synchrotron x-ray diffraction (XRD), resistivity, and Hall transport measurements. When the Cr concentration increases from x=0 to 0.8, neutron diffraction experiments suggest that the collinear antiferromagnetism persists in the whole doping range, where the Néel temperature TN coincides with the tetragonal-to-orthorhombic structural transition temperature Ts, and both of them keeps around 35 K. The magnetic ordered moment, on the other hand, increases within increasing x until x=0.5, and then decreases with further increasing x. Detailed refinement of the powder XRD patterns reveals that the Cr substitutions actually stretch the FeAs4 tetrahedron along the c axis and lift the arsenic height away from the Fe-Fe plane. Transport results indicate that the charge carriers become more localized upon Cr doping, then changes from electron type to hole type around x=0.5. Our results suggest that the ordered moment and the ordered temperature of static magnetism in iron pnictides can be decoupled and tuned separately by chemical doping.