Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session X11: Fe-based superconductivity - Neutron scattering and magnetismFocus
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Sponsoring Units: DMP GMAG Chair: Songxue Chi, Oak Ridge National Lab Room: LACC 303A |
Friday, March 9, 2018 8:00AM - 8:36AM |
X11.00001: Interplay between Nematicity, Antiferromagnetism and Superconductivity in Iron-Based Superconductors Invited Speaker: Shiliang Li The antiferromagnetism, superconductivity and nematicity are three kinds of orders that have been widely seen in iron-based superconductors. Their relationship is crucial to understand the unconventional superconductivity in these materials. The antiferromagnetic ordered moment varies significantly among different materials but no experimental regular pattern is found. Moreover, whether superconductivity can be enhanced by strong nematic fluctuations is still an open question. In this talk, I will focus on the nematic fluctuations and their interaction with the antiferromagnetic order and superconductivity in iron-based superconductors. I will show that nematic fluctuations can be well studied by measuring resistivity under uniaxial pressure. The variety of the antiferromagnetic ordered moment among various parent compounds and their doped samples may be simply viewed as coming from the different amplitude of nematic fluctuations. Superconductivity will appear when nematic fluctuations are strong enough. However, superconductivity can be suppressed by enhancing nematic fluctuations under uniaxial pressure, which indicates a competition between them. Our results suggest the important role played by nematic fluctuations in iron-based superconductivity. |
Friday, March 9, 2018 8:36AM - 8:48AM |
X11.00002: Neutron spin resonance in the new-type iron-based superconductors Tao Xie, Huiqian Luo, Shiliang Li, Tom Fennell, Bourdarot Frédéric, Louis-Pierre Regnault, Sergey Danilkin, Guochu Deng Most iron-based superconductors show a neutron spin resonance which is a typical evidence for the intimate relationship between magnetism and superconductivity. Here we have grown high quality single crystals of the new-type iron-based superconductors CaLa-112 and CaK-1144 [1], and used inelastic neutron scattering to search the spin resonance. Although both system have similar structure to the 122 system, their spin resonance shows very different behaviors. For the CaLa-112 system, spin resonance is a two-dimensional-like mode and no modulations along L direction, consistent with the transport measurements[2]. Polarized neutron scattering indicates the intensity of resonance is spatially isotropic due to the weak spin-orbit coupling or absence of orbital ordering. For the CaK-1144 system, it is surprised to find out three spin resonance modes at different energies, and all of them are high modulated along L direction, suggesting multi-orbital physics or multiple Fermi surface nestings. Therefore, the spin resonance in iron pnictide superconductors is highly system dependent due to orbital physics. |
Friday, March 9, 2018 8:48AM - 9:00AM |
X11.00003: Anisotropic resonance modes emerging in an antiferromagnetic superconductor Markus Braden, Florian Wasser, Chul-Ho Lee, Kunihiro Kihou, Paul Steffens, Karin Schmalzl, Navid Qureshi The emergence of spin-resonance modes in an antiferromagnetic superconductor was studied on Co underdoped BaFe2As2, for which the local coexistence of large moment antiferromagnetism and superconductivity is well established by local probes. Several unpolarized neutron scattering studies for Co doping of about 4.5% reveal a broad spin-resonance feature in the superconducting state but could not identify the nature of this signal. We have applied the polarization analysis in our neutron scattering experiments, which allows us to separate and identify the anisotropies of the magnetic excitations. In the normal state of Co underdoped BaFe2As2 the antiferromagnetic order results in broad magnetic gaps opening in all three spin directions that are reminiscent of the magnetic response in the parent compound [1], where magnetic anisotropy arising from spin-orbit coupling results in sizeable magnetic gaps. In the superconducting state of the underdoped material two distinct anisotropic resonance excitations can be identified, but in contrast to numerous studies on optimum and over-doped BaFe2As2 there is no isotropic resonance excitation [1], in particular there is no longitudinal contribution to the superconducting resonance. |
Friday, March 9, 2018 9:00AM - 9:12AM |
X11.00004: Spin anisotropy in FeSe Qisi Wang, Hongliang Wo, Yao Shen, Jitae Park, Helen Walker, Mechthild Enderle, Jun Zhao Understanding the magnetism of a high-temperature superconductor is fundamental for elucidating its pairing mechanism. FeSe being structurally the simplest iron-based superconductor has attracted extensive research interest because of its unusual magnetism and intriguing superconducting properties. Here, we use polarized neutron scattering to study the low energy spin excitations in FeSe single crystals. In the superconducting state, two anisotropic resonance modes with different polarizations are observed. The spin anisotropy persists in the normal state, and becomes isotropic above the nematic ordering temperature. The observation of double anisotropic resonance modes in FeSe cannot be readily explained by a simple spin triplet exciton model. Various scenarios for the observed spin anisotropy and its implications for the pairing mechanism will be discussed. |
Friday, March 9, 2018 9:12AM - 9:24AM |
X11.00005: No evidence of the crossover in the spin fluctuations of Ca(Fe1-xCox)2As2 and unusual suppression with temperature for x = 0.030. Aashish Sapkota, Pinaki Das, Anna Boehmer, Benjamin Ueland, Douglas Abernathy, Sergey Budko, Paul Canfield, Andreas Kreyssig, Alan Goldman, Robert McQueeney In the Ca(Fe1-xCox)2As2 family, the superconductivity appears with Co doping after a first-order suppression of an antiferromagnetic order. Here, we present results of our inelastic neutron scattering measurement on Ca(Fe1-xCox)2As2, x=0.026 (antiferromagnetically ordered below TN=64±7 K) and x=0.030 (superconducting below Tc=14.6±1 K). Our results suggest that, unlike Ba(Fe1-xCox)2As2, smooth evolution of the gapped spin waves to diffusive spin fluctuations is absent in Ca(Fe1-xCox)2As2 and the result is consistent with the first order phase transition from magnetically ordered to superconducting state. Additionally, we observed an unusual suppression of the spin fluctuations with decreasing temperature for x=0.030, possibly arising due to compound’s proximity to the collapsed tetragonal phase. |
Friday, March 9, 2018 9:24AM - 9:36AM |
X11.00006: Itinerant antiferromagnetism and its manifestation in the spin dynamics of SrCr2As2 Pinaki Das, N.S. Sangeetha, G.R. Lindemann, Tom Heitmann, Douglas Abernathy, Andreas Kreyssig, David Johnston, Alan Goldman, Robert McQueeney, David Vaknin Neutron diffraction and magnetic susceptibility studies of polycrystalline SrCr2As2 reveal that this compound is an itinerant G-type antiferromagnet below the Neel temperature TN = 590(5) K with the Cr magnetic moments aligned along the tetragonal c axis. The system remains tetragonal to the lowest measured temperature (~12 K). The ordered moment, μ = 1.9(1) μB/Cr, measured at T = 12 K, is significantly reduced compared to its localized value (4 μB/Cr) due to the itinerant character brought about by hybridization between the Cr 3d and As 4p orbitals. Inelastic neutron scattering measurements on powder samples show steep columns of magnetic scattering arising from the magnetic Γ points, with the magnetic spectra persisting up to at least 150 meV. The spectra are modeled using a phenomenological J1-J2-Jc Heisenberg description with very large nearest- (J1) and next-nearest-neighbor (J2) AFM interactions within the square tetragonal basal plane and weak interlayer interactions (Jc). |
Friday, March 9, 2018 9:36AM - 9:48AM |
X11.00007: Coexistence of Ferromagnetic and Antiferromagnetic Fluctuations in Iron-germanide Superconductor Hongliang Wo, Qisi Wang, Yao Shen, Kenji Nakajima, Seiko Kawamura, Paul Steffens, Martin Boehm, Karin Schmalzl, Jitae Park, Thomas Forrest, Yang Zhao, Jeffrey Lynn, Masaaki Matsuda, Jun Zhao The recently discovered iron-germanide superconductor YFe2Ge2 is of great interest because it is a possible new unconventional iron-based superconductor other than iron pnictides/chalcgenides. In contrast to the parent compounds of most iron-based superconductors, YFe2Ge2 does not exhibit long range magnetic order, but instead shows superconductivity below 1.8K. The nature of its magnetism has not been determined. Here, we report inelastic neutron scattering measurements of single crystalline YFe2Ge2. Both stripe and ferromagnetic spin fluctuations are observed over a wide range of energies. These results suggest that the magnetic ground state is highly frustrated and that both stripe spin fluctuations and ferromagnetic fluctuations are candidates for mediating the electron pairing in YFe2Ge2. |
Friday, March 9, 2018 9:48AM - 10:00AM |
X11.00008: Large downward renormalization of spin excitation energies in Ba1-xKxFe2As2 due to electron correlation effects Naoki Murai, Katsuhiro Suzuki, Shin-ichiro Ideta, Masamichi Nakajima, Hiroaki Ikeda, Kiyohisa Tanaka, Ryoichi Kajimoto We report inelastic neutron scattering (INS) study of hole-doped iron-based superconductor Ba1-xKxFe2As2 (x = 0.25). The INS data show a spin-wave-like dispersive feature, with a zone boundary energy of 200 meV. A random phase approximation (RPA) treatment of the dynamical spin susceptibility, incorporating the band renormalization factor of 3 derived from angle-resolved photoemission spectroscopy (ARPES) measurements on the same sample, provides a reasonable description of the observed spin excitations. This analysis shows that the Fe-3d bandwidth narrowing due to electron correlation is directly reflected in the spin excitation energy scale. We will discuss the current state of our analysis relating the magnetic response to the underlying electronic structure of the materials. |
Friday, March 9, 2018 10:00AM - 10:12AM |
X11.00009: Electronic and spin dynamics in the insulating iron pnictide NaFe0.5Cu0.5As Shunhong Zhang, Yanjun He, Jia-Wei Mei, Feng Liu, Zheng Liu NaFe0.5Cu0.5As represents a rare exception in the metallic iron pnictide family, in which a small insulating gap is opened. Based on first-principles study, we provide a comprehensive theoretical characterization of this insulating compound. The Fe3+ spin degree of freedom is quantified as a quasi-1D S=5/2 Heisenberg model. The itinerant As hole state is downfolded to a pxy-orbital hopping model on a square lattice. A unique orbital-dependent Hund's coupling between the spin and the hole is revealed. Several important material properties are analyzed, including (a) factors affecting the small p−d charge-transfer gap; (b) role of the extra interchain Fe; and (c) the quasi-1D spin excitation in the Fe chains. The experimental manifestations of these properties are discussed. |
Friday, March 9, 2018 10:12AM - 10:24AM |
X11.00010: Frustrated magnetism and bicollinear antiferromagnetic order in FeTe Hsin-Hua Lai, Shoushu Gong, Wenjun Hu, Qimiao Si Iron chalcogenides display a variety of electronic orders in their phase diagram. A particularly enigmatic case is FeTe, a metal with co-existing hole and electron Fermi surfaces as in the iron pnictides but has a distinct (pi/2,pi/2) bicollinear (BC) antiferromagnetic order. While local-moment physics has been recognized as essential for the electronic order in FeTe, it has been a long-standing challenge to understand how the bicollinear antiferromagnetic ground state emerges in a proper quantum spin model. We demonstrate that a bilinear-biquadratic spin-1 model on a square lattice harbors the BC order that is quasi-degenerate with the (pi/2, pi/2) plaquette (PL) order over an extended parameter regime [1], but the ring-exchange interactions stabilize the BC order. Recently, it became possible to test our theory in the polarized neutron scattering experiment [2]. This in turn motivated us to calculate the dynamic spin structure factors for the BC and PL orders. We discuss the implications for the overall understanding of the magnetism in the iron-based systems. [1] H.-H. Lai, S.-S. Gong, W.-J. Hu, Q. Si, arXiv:1608.08206. [2] Y. Song et al., preprint (2017). |
Friday, March 9, 2018 10:24AM - 10:36AM |
X11.00011: Polarized Neutron Spectroscopy of Dynamical Magnetism in Parent and Superconducting Fe(Te,Se) Igor Zaliznyak, Andrei Savici, Vasile Garlea, Zhijun Xu, Guangyong Xu, Genda Gu, John Tranquada Inelastic neutron scattering presented perhaps the first experimental indication of the orbital-selective electron localization in the Fe chalcogenides [1]. The follow-up study [2] established the concomitant ferro-orbital order, leading to the "bicollinear" antiferromagnetism, a metallic state, and a loss of magnetic susceptibility[1,2]. Here, we present polarized inelastic neutron surveys of the temperature-dependent magnetic dynamics in both Fe1+yTe, and the optimally doped superconductor, FeTe0.55Se0.45. We observe that the local dynamical correlations in the superconducting sample in the energy range of the spin resonance, at high temperature become similar to those found in the under-doped samples [1], and which are also observed in the optimally doped samples at higher energy. Our results reveal unusual, energy- and temperature-dependent orbital composition of dynamical magnetism in these materials. |
Friday, March 9, 2018 10:36AM - 10:48AM |
X11.00012: Short-Range Nematic Correlations in Iron-Based Superconductors Probed by Pair Distribution Function Analysis Benjamin Frandsen Elucidating the role of nematicity in iron-based superconductors (FeSCs) remains a crucial objective in the field. Nematic fluctuations are thought to be present across wide regions of the FeSC phase diagrams, with possible connections to the superconducting mechanism. However, detecting and characterizing nematic fluctuations poses a significant experimental challenge. Here, we show that atomic pair distribution function (PDF) analysis of neutron and x-ray total scattering data can be a powerful tool to investigate nematic fluctuations. We report on temperature-dependent PDF measurements of representative iron pnictide and chalcogenide systems, which reveal ubiquitous nanometer-scale orthorhombic distortions of the instantaneous local structure even in phases with an average tetragonal structure. These distortions of the local structure reflect the large nematic susceptibility in FeSCs and may have significant implications for superconductivity and neighboring phases in these materials. |
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