Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session W39: Focus Session: Iron Based Superconductors: Neutron Scattering and Magnetism |
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Sponsoring Units: DCMP DMP Chair: Pengcheng Dai, University of Tennessee-Knoxville Room: F150 |
Thursday, March 18, 2010 11:15AM - 11:51AM |
W39.00001: Recent Neutron Studies of the Iron-based Magnetic Superconductors Invited Speaker: We present results of recent neutron scattering investigations at the NCNR of the crystal structures, magnetic structures, and spin dynamics of the iron-based ROFe(As,P) (R=La, Ce, Pr, Nd), (Ba,Sr,Ca)Fe$_{2}$As$_{2}$, and Fe$_{1-x}$(Se-Te) superconductors [1]. All the undoped materials exhibit universal behavior, where a tetragonal-to-orthorhombic structural transition occurs between $\sim $140-220 K, at or below which the systems order antiferromagnetically. The magnetic structure within the a-b plane consists of chains of parallel Fe spins that are coupled antiferromagnetically in the orthogonal direction, with an ordered moment typically less than 1 $\mu$$_{B}$. Hence these are itinerant electron magnets, with a spin structure that is consistent with Fermi-surface nesting. The exchange interactions are strong, with spin-wave bandwidths $\sim $200 meV. The rare-earth moments order antiferromagnetically at low T like ``conventional'' magnetic-superconductors, while the crystal field excitations can be employed to study the properties of the superconducting state. With doping in CeFeAsO$_{1-x}$F$_{x}$, LaFeAsO$_{1-x}$F$_{x}$, and SrFe$_{2-y}$Ni$_{y}$As$_{2}$ [2] the structural and magnetic transitions are suppressed in favor of superconductivity. The application of pressure in CaFe$_{2}$As$_{2}$ transforms the system from a magnetically ordered orthorhombic material to a ``collapsed'' non-magnetic tetragonal system. In the superconducting doping regime, well defined spin correlations and a clear magnetic resonance in the magnetic excitation spectrum that tracks the superconducting order parameter are observed, reminiscent of the cuprate superconductors [3]. The overall results clearly indicate that the magnetic properties are a key element in these iron-based superconductors. Further information and references can be found at http://www.ncnr.nist.gov/staff/jeff \\[4pt] [1] For a recent neutron review see J. W. Lynn and P. Dai, Physica C \textbf{469}, 469 (2009). \\[0pt] [2] N. Kumar, et al., Phys. Rev. B \textbf{80}, 144524 (2009). \\[0pt] [3] S. Li, et al., Phys. Rev. B \textbf{79}, 174527 (2009). \\[4pt] It is a pleasure to acknowledge my collaborators at the NCNR and the many collaborators with the following groups: P. Dai (U. Tennessee/ORNL), N. L. Wang (Beijing), R. J. Cava (Princeton U.), A. Goldman (Ames Lab), W. Bao (LANL), S. Dhar (TIFR), J. P. Paglione (U. Maryland). Please see [1] for a complete list of co-authors. [Preview Abstract] |
Thursday, March 18, 2010 11:51AM - 12:03PM |
W39.00002: ABSTRACT WITHDRAWN |
Thursday, March 18, 2010 12:03PM - 12:15PM |
W39.00003: Interplay between Fe and Nd magnetism in NdFeAsO single crystals W. Tian, W. Ratcliff II, J.W. Lynn, J.-Q. Yan, M.G. Kim, B. Jensen, K. Dennis, R.W. McCallum, T.A. Lograsso, R.J. McQueeney, A.I. Goldman, A. Kreyssig Millimeter-sized NdFeAsO single crystals were grown out of NaAs flux and characterized by ac-susceptibility, magnetization, electrical resistivity, and specific heat measurements. The microscopic nature of the observed phase transitions has been determined by neutron scattering experiments at BT9, NIST, Gathersburg, and high-resolution x-ray diffraction studies. The tetragonal-to-orthorhombic lattice distortion is followed by the reported antiferromagnetic ordering of the Fe moments. At low-temperatures, the onset of the antiferromagnetic order of the Nd moments is accompanied with a rearrangement of the Fe moments. The observed phase transitions underline a complex coupling between the Fe and Nd magnetism. -- The work at the Ames Laboratory was supported by the US DOE, Office of Science, under contract No. DE-AC02-07CH11358. [Preview Abstract] |
Thursday, March 18, 2010 12:15PM - 12:27PM |
W39.00004: Lattice distortion and magnetic quantum phase transition in CeFeAs1-xPxO C.R. dela Cruz, W.Z. Hu, S. Li, Q. Huang, M. Green, J. Lynn, G.F. Chen, N.L. Wang, H. Mook, Q. Si, P. Dai A feature of the parent compounds of the Fe-based superconductors is the structural distortion that occurs in the vicinity of the onset of long range magnetic order of the Fe-spins. In the RFeAsO(R=rare earth) family, the magneto-structural transition is suppressed in favor of superconductivity upon doping charge carriers into the system, which alters the system electronically and crystallographically as well. To understand the lattice effect on the suppression of the AFM ground state itself, it is important to isoelectronically tune the crystal lattice structure without the influence on charge carrier doping and superconductivity. Here we use neutron powder diffraction to show that replacing the larger arsenic with smaller phosphorus in CeFeAs1-xPxO simultaneously suppresses the AFM order and orthorhombic distortion near x = 0.4, providing evidence for a magnetic quantum phase transition. Furthermore, we find that the pnictogen height in these iron arsenides is an important controlling parameter for their electronic and magnetic properties, and may play an important role in electron pairing and superconductivity. [Preview Abstract] |
Thursday, March 18, 2010 12:27PM - 1:03PM |
W39.00005: Systematic evolution of magnetism with doping in AFe$_{2}$As$_{2}$ superconductors Invited Speaker: The AFe$_{2}$As$_{2}$ (A=Ba,Sr,Ca) based superconductors (SC) are antiferromagnetic (AFM) metals with a layered crystal structure. Electron or hole doping suppresses the AFM transition and leads to the appearance of a SC phase in the presence of AFM spin fluctuations. We have studied the evolution of static magnetic order and spin excitations as a function of doping in Ba(Fe$_{1-x}$Co$_{x})_{2}$As$_{2}$ using neutron and x-ray scattering. The spin wave spectra in the AFM parent compounds (A=Ca) reveal large magnetic exchange within the Fe layers and weaker interlayer exchange. Spin fluctuations in the optimally doped SC compositions (x $>$ 7{\%}), with no long-range AFM order, are more two-dimensional (2D) in character and highlighted by a 2D magnetic resonance feature that develops below T$_{C}$. Within a narrow compositional range (3 $<$ x $<$ 6{\%}) at the onset of SC, AFM and SC can actually coexist and compete with each other. This competition is revealed by a strong suppression of the AFM order parameter below T$_{C}$. The spin excitations in the underdoped compositions are notably more 3D than optimally doped compositions, including a magnetic resonance that has strong c-axis dispersion. Overall, the results suggest that the approach to SC in Ba(Fe$_{1-x}$Co$_{x})_{2}$As$_{2}$ coincides with competing weak magnetic order and a crossover in the dimensionality of the system. [Preview Abstract] |
Thursday, March 18, 2010 1:03PM - 1:15PM |
W39.00006: Neutron scattering studies of competing magnetic and superconducting order parameters in Ba(Fe$_{1-x}$Co$_{x})_{2}$As$_{2}$ Daniel Pratt, Rafael Fernandez, Wei Tian, Jerel Zaretsky, Andreas Kreyssig, Shibabrata Nandi, Min Gyu Kim, Alex Thaler, Ni Ni, Sergey Bud'ko, Paul Canfield, Robert McQueeney, Jorg Schmalian, Alan Goldman Neutron and x-ray diffraction studies show that the simultaneous first-order transition to an orthorhombic and antiferromagnetic (AFM) ordered state in BaFe$_{2}$As$_{2}$ splits into two transitions, and T$_{N }$as well as the ordered moment decrease as x ({\%}Co) increases.~ Recent experiments have demonstrated that for a range of doping, superconductivity and magnetism co-exist homogeneously and compete for electrons.~ The magnetic order parameters have been measured for a series of Co doping from x = 0 to 6.3{\%} and a clear drop in sublattice magnetization is observed at T$_{C}$ for each compound.~ Comparison of these measurements to mean field calculations has provided insight into these competing order parameters.~ The good agreement between the measurement and calculations show that neutron scattering is a useful probe for understanding the symmetry of the superconducting gap in the iron-arsenide superconductors.$^{ }$ The work at the Ames Laboratory was supported by the US DOE, office of science, under contract No. DE-AC02-07CH11358. [Preview Abstract] |
Thursday, March 18, 2010 1:15PM - 1:27PM |
W39.00007: Magnetic order and lattice distortion in Rh- and Cu-doped BaFe$_{2}$As$_{2}$ single crystals A. Kreyssig, M.G. Kim, S. Nandi, W. Tian, J. Zarestky, A. Thaler, N. Ni, S.L. Bud'ko, P.C. Canfield, R.J. McQueeney, A.I. Goldman Recent investigations of superconducting Co-doped BaFe$_{2}$As$_{2}$ have highlighted the interplay between superconductivity, magnetism and structure. Here we report on the antiferromagnetic order, lattice distortion and their response to superconductivity in Rh-doped BaFe$_{2}$As$_{2}$ and compare the behavior with non-superconducting Cu-doped BaFe$_{2}$As$_{2}$ single crystals. Results of the neutron scattering experiment performed at HB1A, HFIR, Oak Ridge, are correlated with high-resolution x-ray diffraction, resistance and magnetization measurements. The magnetic and structural phase transitions are similarly suppressed by the different dopings and the temperature dependencies of the order parameters are comparable, whereas only the Rh-doped sample shows a reduction of the antiferromagnetically ordered Fe moment in the superconducting state as reported for the Co-doped series. -- The work at the Ames Laboratory was supported by the US DOE, office of science, under contract No. DE-AC02-07CH11358. [Preview Abstract] |
Thursday, March 18, 2010 1:27PM - 1:39PM |
W39.00008: Vortex Imaging in Ba(Fe$_{0.93}$Co$_{0.07}$)$_{2}$As$_{2}$ using Small Angle Neutron Scattering P. Das, T.R. O'Brien, M.R. Eskildsen, M. Laver, C.D. Dewhurst, N. Ni, A. Kreyssig, S.L. Bud'ko, P.C. Canfield, A.I. Goldman We report extended studies of the field dependence of the vortex configuration in superconducting Ba(Fe$_{0.93}$Co$_{0.07})_{2}$As$_{2}$ at $T \sim $ 0.1$T_{c}$ ($T_{c}$ = 21 K) using Small Angle Neutron Scattering (SANS). SANS measurements show a ring of scattering indicating a highly disordered vortex lattice mainly due to strong pinning in this material and a very broad rocking curve width. At lower applied fields ($<$ 1 T) the measured scattering vectors ($q)$ are close to the value expected for a hexagonal vortex lattice (VL) [M.R. Eskildsen \textit{et al.}, Phys. Rev. B 79, 100501(R) (2009)] but with increasing fields it falls between $q_{hex}$ and $q_{square}$. A deviation from the hexagonal VL can be related to a Fermi-surface or gap anisotropy in the basal plane. Moreover an exponential fit to the scattered intensity as a function of field for H $>$ 1 T yields a significantly smaller slope than what was found for smaller fields. This work was supported by NSF grants DMR-0804887, PHY-0552843 and DMR-0454672, and DOE BES contract No. DE-AC02-07CH11358. [Preview Abstract] |
Thursday, March 18, 2010 1:39PM - 1:51PM |
W39.00009: Neutron Scattering Study of the Magnetic Structure of NdFeAsO Yiming Qiu, Wei Bao, Qingzhen Huang, Mark Green, T. Wu, G. Wu, Xianhui Chen We report the neutron scattering studies of the non-superconducting NdFeAsO. In NdFeAsO, there is a tetragonal to orthorhombic structural transition at Ts$\approx$150 K, where an anomaly in resistivity also occurs. A long range magnetic order with the wave-vector (1/2, 1/2, 0)$_T$ forms below T$_N$=1.96 K. This long range order is dominated by the rare earth Nd ions, however, both the Nd and smaller Fe moments contribute to the antiferromagnetic structure. No LaFeAsO-like SDW ordering below Ts was observed in the initial study[1]. We will address the difference in magnetic structures. [Preview Abstract] |
Thursday, March 18, 2010 1:51PM - 2:03PM |
W39.00010: Crystal growth and neutron scattering studies of single crystal NaFeAs and its Co-doped superconductors Meng Wang, Chenglin Zhang, Jun Zhao, Miaoyin Wang, Huiqian Luo, Pengcheng Dai There have been much recent interests in studying FeAs based superconductors. Although there are many families of FeAs-based materials, most work have focused on the BaFe2As2 (122) family of materials because the availability of single crystals of these materials. To expand the single crystal growth capability and compare similarities and differences between different classes of FeAs-based superconductors, we report the growth and neutron scattering studies of single crystals of 111 phase NaFeAs by the self-flux method. By using FeAs as flux, we grow NaFeAs single crystals with diameters of about 2-5mm. We discuss the detailed crystal growth method and present neutron scattering results on these single crystals. [Preview Abstract] |
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