Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session V11: Fe-based superconductors - Material synthesis and discoveryFocus
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Sponsoring Units: DMP Chair: Brian Sales, Oak Ridge National Lab Room: LACC 303A |
Thursday, March 8, 2018 2:30PM - 3:06PM |
V11.00001: Progress and perspectives in materials chemistry of iron-based superconductors Invited Speaker: Dirk Johrendt My talk reviews discoveries and recent developments of superconducting iron based compounds from the view of materials chemistry. Initiated by the first observations of superconductivity in the pnictide-oxides LaFePO (2006) and LaFeAsO1-xFx (2008), tremendous activities have meanwhile spawned an impressive family of iron-based superconducting materials, which is not inferior to the cuprates. Superconductivity in iron-based compounds emerges in Fe2X2 layers (X = As, P, Se, S) as the common structural motif. It is intriguing how combinations of these active layers with a variety of separating blocking layers yielded many new superconductors with partly novel crystal structures, for example (CaFeAs)10Pt4As8 with Tc up to 40 K. Remarkable combinations of separating layers occur in the recently discovered self-doped compounds RbFe2As2[GdOFeAs]2 and KFe2As2[CaFFeAs]2. Subtle manipulations of the Fe2X2 or the blocking layers by doping control the superconducting properties. Especially the 122-type compounds based on BaFe2As2 proved to be extremely adaptive to doping. Superconductivity in iron arsenides emerges during destabilization of antiferromagnetism by doping or pressure, while the intertwining with magnetism is less clear in iron selenides where nematic magnetism plays a role. The exciting discovery of superconductivity up to Tc = 100 K in FeSe single layers on STO raised the question if wide separation of FeSe layers by intercalation may also produce such high Tc. For example [(Li1-xFex)OH]FeSe exhibits Tc = 42 K and shows the rare coexistence of ferromagnetism and superconductivity. In spite of this rich crystal chemistry, the highest Tc in iron based bulk materials is currently 55 K. However, since the true factors for high Tc are not yet understood, only explorative synthesis of further iron based or related systems can pave the way to higher Tc above the important 77 K threshold. |
Thursday, March 8, 2018 3:06PM - 3:18PM |
V11.00002: Substitution and doping in iron pnictides S. Schuppler, P. Nagel, M.-J. Huang, R. Eder, Th. Wolf, M. Merz For insight into the composition-dependent electronic structure of iron pnictides, we performed a systematic study of spatial structure and electronic states by x-ray diffraction and x-ray absorption. A large number of compositions in the (Ba,Sr,AM)(Fe,TM)2(As,P)2 family of compounds was investigated, covering the substitution of Ba by Sr or alkali metals (AM); of Fe by transition metals (TM); and of As by P. Our observations on doping effects upon such substitutions include “reluctant” doping (charge carriers are only partially transferred away from the substituent) or “site-decoupled” doping (transferred charge carriers affect either Fe sites or As sites but not both). Here, we focus on isovalent substitutions. The energy-level schemes derived from electronic and spatial structure differ from the “standard” ones in ways that may help explain the systematics in these substitution systems. Indirect, structural effects of substitution appear to be more important for magnetism and superconductivity in iron pnictides than the direct, charge-carrier doping effects. |
Thursday, March 8, 2018 3:18PM - 3:30PM |
V11.00003: Prediction of Antiferromagnetism in the Chromium-Pnictide BaCr2P2 Confirmed by Synthesis Radi Jishi, Jose Rodriguez, Timothy Haugan, Michael Susner We have carried out density-functional theory (DFT) calculations for the chromium-pnictide BaCr2P2, which is analogous to BaFe2As2, a parent compound for iron-pnictide superconductors. Evolutionary methods combined with DFT predict that the chromium analog has the same crystal structure as the latter. DFT also predicts Néel antiferromagentic order on the chromium sites. We have confirmed the DFT predictions directly after the successful synthesis of polycrystalline samples of BaCr2P2. X-ray diffraction recovers the predicted crystal structure to high accuracy, while magnetic susceptibility and specific-heat measurements are consistent with a transition to an antiferromagnetically ordered state below TN ~ 330 K. |
Thursday, March 8, 2018 3:30PM - 3:42PM |
V11.00004: Non-Bulk Superconductivity in Fluorine-doped BaFe2As2 at Tc~22 K Gregory Stewart, G. Tam, J. S. Kim, B. Demaske, S. Phillpot, H. Maruyama, J. Nino Hiramatsu et al.1 first found non-bulk superconductivity at ~22 K in SrFe2As2 exposed to water vapor. Our own work with exposure to water vapor gave varying results, as well as up-to-severe degradation of the crystal surface after exposure. The present work explores exposure of single crystal BaFe2As2 to other reactive gases, with consistent results from exposure to fluorine (5% in Argon) gas for 20 minutes at room temperature. The depth of the fluorine is ≤ 10 µ, and XPS data indicates that fluorine replaces As in the lattice. The superconductivity is observed in the resistivity but not in the magnetic susceptibility. A tentative model to explain what is happening microscopically to cause the superconductivity is proposed. |
Thursday, March 8, 2018 3:42PM - 3:54PM |
V11.00005: Identification of Excess Fe in Superconducting Single-layer FeSe/SrTiO3 Films Yong Hu, Yu Xu, Yingyan Wang, Lin Zhao, Xingjiang Zhou
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Thursday, March 8, 2018 3:54PM - 4:06PM |
V11.00006: Magnetic and pairing tendencies in quasi 1D multi-orbital BaFe2S3 ladders and chains Nirav Patel, Alberto Nocera, Gonzalo Alvarez, Ryotaro Arita, Adriana Moreo, Elbio Dagotto The discovery of superconductivity in the two-leg ladder compound BaFe2S3 opened a novel avenue of research because it represents the first report of pairing in a quasi-one-dimensional iron-based superconductor. Ladders and chains can be far more accurately studied using many-body techniques and model Hamiltonians than their layered counterparts. As a first step, we studied a two-orbital Hubbard model, derived from first principles, that describes individual BaFe2S3 ladders using the density matrix renormalization technique. We found indications of pairing between two holes in a region with orbital selective Mott phase characteristics [1]. By studying a simplified model on large systems, the two-orbital chain, we uncovered an instability toward pairing that is enhanced with increasing Hund and antiferromagnetic Heisenberg couplings. The analysis of the chain pair-pair correlations indicate that pairs form inter-orbital singlets on neighboring sites and different orbitals [2]. These results suggest that magnetic fluctuations may explain superconductivity in the iron-based ladder superconductors. Future directions of research in ladders and chains will be discussed. |
Thursday, March 8, 2018 4:06PM - 4:18PM |
V11.00007: Observation of a G-type short range antiferromagnetic order in expanded FeS Meng Wang, Ming Yi, Zhijun Xu, Huibo Cao, E. Bourret-Courchesne, Jeffrey Lynn, Robert Birgeneau We report structural studies on FeS single crystals obtained from RbxFe2-yS2 using a hydrothermal method. Neutron diffraction measurements show the iron vacancy order and block antiferromagnetic order in RbxFe2-yS2 are absent in the samples. Interestingly, a G-type short range antiferromagnetic order with moments aligned along the c-axis is observed. Its Neel temperature is determined at 165 K. The data reveal that the short range antiferromagnetic order corresponds to expended inter-layer space of FeS. Our results indicate that FeS is in the vicinity of a magnetically ordered ground state and provide insights on understanding a variety of phenomena observed on FeS and ongoing research on the iron chalcogenide FeSe1-xSx system. |
Thursday, March 8, 2018 4:18PM - 4:30PM |
V11.00008: Superconducting FeSe film made by electrochemical deposition Yoshihiko Takano FeSe films were electrochemically deposited on rolling-assisted biaxially textured substrate (RABiTS) tape and, we observed zero resistivity in the as-electrodeposited film without annealing. We investigated the influence of dipping time in a reaction solution before applying voltages. When the RABiTS tape was dipped in the solution, a thin Se film formed on the substrate before voltage was applied, and then the FeSe film was deposited on the tape by applying voltage. The compositional ratio of the FeSe film deviated from the stoichiometric ratio as the dipping time increased before applying voltage. We observed zero resistivity in the as-electrodeposited FeSe film prepared on the substrate when it was dipped into the solution while applying voltage.[1,2] |
Thursday, March 8, 2018 4:30PM - 4:42PM |
V11.00009: Composition and annealing effects on superconductivity in sintered and arc-melted Fe1+εTe0.50Se0.50 Gregorio Ponti, Madison Foreman, Shirin Mozaffari, John Markert We present the results of x-ray diffraction (XRD), resistivity, and ac magnetic susceptibility measurements to study the effects of the iron concentration on the “11”- structure superconductor Fe1+εTe0.50Se0.50 (0 ≤ ε ≤ 0.15). Samples were initially sintered in sealed quartz tubes or melted in an arc furnace and later annealed at a variety of temperatures: 425°C, 600°C, 675°C for sintered samples and 650°C for arc-melted specimens. The XRD data show a predominant tetragonal phase with a secondary hexagonal phase. The resistivity measurements show that the superconducting transition temperature, TC ≈ 14 K, is independent of iron concentration 1+ε, suggesting a single superconducting line compound. The magnetic screening fraction varies with Fe concentration and annealing temperature, with a maximum value at ε = 0.07. Larger superconducting fractions are associated with larger a-axis lattice constants, hinting at a possible chalcogen-vacancy electron doping. Thus we suggest that such behavior originates from an electron-doped, chalcogen deficient stoichiometry. |
Thursday, March 8, 2018 4:42PM - 4:54PM |
V11.00010: Abstract Withdrawn
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Thursday, March 8, 2018 4:54PM - 5:06PM |
V11.00011: Direct Determination of the Crystal Structure of Superconducting Ca10Pt4As8((Fe1-xPtx)2As2)5 Zhen Wang, Amar B. Karki, Jiayun Pan, Lijun Wu, Jing Tao, Yimei Zhu, Jiandi Zhang, Rongying Jin, E Plummer Layered Ca10Pt4As8(Fe2As2)5 is unique because it is the only-known system that contains an metallic spacer between adjacent Fe2As2 layers and becomes optimally superconducting without chemical doping at ambient pressure. However, the unsolved crystal structure limits our understanding of structure-property relationship. Using electron diffraction and high-resolution scanning transmission electron microscopy (STEM), we are able to directly determine the structure of this superconductor: it forms a monoclinic structure (P 2 1/n) with the unit cell along the c direction doubled compared that found previously, i.e., c = 20.18 Å. Stacking faults along c-axis are commonly observed in the crystal. With the establishment of the crystal structure, the unique transport properties of our superconducting Ca10Pt4As8((Fe1-xPtx)2As2)5 (x ~ 0.05) single crystals with Tc = 34 K can be explained. |
Thursday, March 8, 2018 5:06PM - 5:18PM |
V11.00012: First-principles study of magnetism, lattice dynamics, and superconductivity in LaFeSiHx Linda Hung, Taner Yildirim The structural, electronic, magnetic, and vibrational properties of LaFeSiHx for x between 0 and 1 are investigated using density functional calculations. We find that the electronic and magnetic properties are strongly controlled by the hydrogen concentration x in LaFeSiHx. While fully hydrogenated LaFeSiH has a striped antiferromagnetic ground state, the underdoped LaFeSiHx for x ≦ 0.75 , is not magnetic within the virtual crystal approximation or with explicit doping of supercells. The antiferromagnetic configuration breaks the symmetry of Fe d-orbitals and increases electron-phonon coupling up to 50%, especially for modes in the 20-50 meV range that are associated with Fe atomic movement. We find competing nearest and next-nearest neighbor exchange interactions and significant spin-phonon coupling, qualitatively similar but smaller in magnitude compared those found in LaOFeAs superconductors. The superconducting Tc for antiferromagnetic LaFeSiHx, assuming conventional superconductivity via McMillan's equation, therefore is computed to be 2-10 K, in contrast to Tc≈0 for the nonmagnetic material. We also predict that the LaFeSiHx could be a good proton conductor due to phase stability with a wide range of hydrogen concentration x < 1. |
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