Iron-based Superconductors
The recent discovery of superconductivity in the rare-earth (R) iron-based oxide systems RO1-xFxFeAs (R = rare earth) and the (Ca,Ba,Sr)1-xKxFe2As2 has generated enormous interest because these materials are the first non-copper oxide superconductors with Tc exceeding 50 K. The parent (non-superconducting) materials like LaOFeAs and SrFe2As2 are metallic but show anomalies near 150-200 K in both resistivity and dc magnetic susceptibility. Optical conductivity and theoretical calculations suggested that LaOFeAs exhibits a spin-density-wave (SDW) instability that is suppressed with doping electrons to form superconductivity, but there had been no direct evidence of the SDW order. We have used neutron scattering at the NCNR to explore the crystal and magnetic structures of a number of these materials. We have found that they undergoe an abrupt structural distortion below ~150 K, changing the symmetry from tetragonal (space group P4/nmm) to monoclinic (space group P112/n, or equivalently orthorhombic Cmma) at low temperatures. This structural transition is the primary cause of transport and thermodynamic anomalies, but it is closely followed by the development of long range SDW-type antiferromagnetic order, at or just below the structural transition. The magnetic structures are all simple, commensurate configurations, but with a small ordered moment ranging from 0.25-1 Bohr magnetons. The small moment indicates that these are itinerant electron systems, and that the ordering can be described as of the spin-density-wave type. Doping the system suppresses both the magnetic order and structural distortion in favor of superconductivity. Therefore, much like high-Tc copper oxides, the superconducting regime in these Fe-based materials occurs in close proximity to a long-range ordered antiferromagnetic ground state.
For the undoped (Ca,Ba,Sr)Fe2As2 systems, pressure has recently been found to drive them superconducting. Our pressure and temperature-dependent neutron diffraction measurements for CaFe2As2 show that a transition from the ambient-pressure magnetically ordered orthorhombic phase to a non-magnetic "collapsed" tetragonal phase precedes the onset of superconductivity at lower temperature. By "collapsed" we mean that the c-axis decreases by 10%, while band structure calculations indicate that the Fe moment has collapsed as well.
The crystal structure measurements were carried on the BT-1 high resolution powder diffractometer, while the magnetic structures and magnetic order parameters weret observed using the BT-7 thermal triple axis instrument operated in powder diffraction mode with the position sensitive detector.
Publications
Magnetic Order Close to Superconductivity in the Iron-based Layered La(O1-xFx)FeAs systems, C. de la Cruz, Q. Huang, J. W. Lynn, J. Li, W. Ratcliff II, J. L. Zarestky, H. A. Mook, G. F. Chen, J. L. Luo, N. L. Wang, and P. Dai, P. Dai, Nature 453, 899 (2008). Supplementary Material.
Intrinsic Properties of Stoichiometric LaOFeP, T. M. McQueen, M. Regulacio, A. J. Williams, Q. Huang, J. W. Lynn, Y. S. Hor, D.V. West, and R. J. Cava, Phys. Rev. B78, 024521 (2008).
Neutron scattering study of the oxypnictide superconductor LaO0.87F0.13FeAs, Y. Qiu, M. Kofu, Wei Bao, S.-H. Lee, Q. Huang, T. Yildirim, J. R. D. Copley, J. W. Lynn, T. Wu, G. Wu, and X. H. Chen, Phys. Rev. B78, 052508 (2008).
Magnetic Order of the Iron Spins in NdOFeAs, Y. Chen, J. W. Lynn, G. F. Chen, G. Li, Z. C. Li, J. L. Luo, N. L. Wang, P. Dai, C. dela Cruz, and H. A. Mook, Phys. Rev. B78 , 064515 (2008).
Doping Evolution of Antiferromagnetic Order and Structural Distortion in LaFeAsO1-xFx, Q. Huang, J. Zhao, J. W. Lynn, G. F. Chen, J. L. Lou, N. L. Wang, and P. Dai, Phys. Rev. B 78, 054529 (2008).
Structural and Magnetic Phase Diagram of CeFeAsO1-xFx and its Relationship to High-Temperature Superconductivity, J. Zhao, Q. Huang, C. de al Cruz, S. Li, J. W. Lynn, Y. Chen, M. A. Green, G. F. Chen, G. Li, Z. C. Li, J. L. Luo, N. L. Wang, and P. Dai (submitted).
Magnetic order in BaFe2As2, the parent compound of the FeAs based superconductors in a new structural family, Q. Huang, Y. Qiu, W. Bao, J.W. Lynn, M.A. Green, Y.C. Gasparovic, T. Wu, G. Wu, and X. H. Chen, (submitted).
Spin and Lattice Structure of Single Crystal SrFe2As2, Jun Zhao, W. Ratcliff-II, J. W. Lynn, G. F. Chen, J. L. Luo, N. L. Wang, Jiangping Hu, and Pengcheng Dai, Phys. Rev. B 78, 140504(R) (2008).
Structure and Magnetic Order in the NdFeAsO1−xFx Superconductor System, Y. Qiu, W. Bao, Q. Huang, T. Yildirim, J. M. Simmons, M. A. Green, J.W. Lynn, Y.C. Gasparovic, J. Li, T. Wu, G. Wu, and X.H. Chen, preprint.(preprint)
Squeezing the Magnetism out of Superconducting CaFe2As2--a Volume and Moment "Collapsed" Superconducting Phase, A. Kreyssig, M. A. Green, Y. B. Lee, G. D. Samolyuk, P. Zajdel, J. W. Lynn, S. L. Bud’ko, M. S. Torikachvili, N. Ni, S. Nandi, J. Leão, S. J. Poulton, D. N. Argyriou, B. N. Harmon, P. C. Canfield, R. J. McQueeney, and A. I. Goldman, (submitted).
Lattice and Magnetic structures of PrFeAsO, PrFeAsO0.85F0.15 and PrFeAsO0.85, Jun Zhao, Q. Huang, Clarina de la Cruz, J. W. Lynn, M. D. Lumsden, Z. A. Ren, Jie Yang, Xiaolin Shen, Xiaoli Dong, Zhongxian Zhao, and Pengcheng Dai, Phys. Rev. B 78, (in press)
The crystalline electric field as a probe for long range antiferromagnetic order and superconductivity in CeFeAsO1−xFx, S. Chi, D. T. Adroja, T. Guidi, R. Bewley, Shliang Li, Jun Zhao, J. W. Lynn, C. M. Brown, Y. Qiu, G. F. Chen, J. L. Lou, N. L. Wang, and Pengcheng Dai, [http://arxiv.org/abs/0807.4986] (preprint).
Low energy spin waves and magnetic interactions in SrFe2As2, J. Zhao, D.-X. Yao, S. Li, T. Hong, Y. Chen, S. Chang, W. Ratcliff II, J. W. Lynn, H. A. Mook, G. F. Chen, J. L. Luo, N. L. Wang, E. W. Carlson, J. Hu, and P. Dai, Phys. Rev. Lett. 101, [http://arxiv.org/abs/0808.2455](in press).
Anatomy of a Nature article:
The excitement generated by these new superconductors means that the field moves ahead very quickly, and our initial work on the LaOFxFeAs is a paradigm for the frantic pace of research. The sample arrived in Knoxville from the Beijing Institute of Physics on Friday morning, March 28, and Clarina dela Cruz flew to Washington with the sample, arriving at the NCNR that evening. The sample was sealed in a holder and cooled to base temperature on the BT-1 instrument. The first full diffraction pattern revealed the (surprising) structural distortion, and we spent until Sunday mapping out the structural phase transition. Sunday afternoon we moved the sample to BT-7 to search for magnetic order. Early Monday morning we had the magnetic diffraction pattern and began to measure the order parameter. The reactor was scheduled to shutdown for refueling at 8 AM, but we requested and were granted an additional hour of operation to finish the order parameter, which was successful. The measurement team consisted of Qing Huang, Jiying Li, William Ratcliff, II and myself from the NCNR, and of course Clarina dela Cruz from the University of Tennessee--Knoxville.
The manuscript was submitted for publication on Tuesday, April 1. Nature 453, 899 (2008). Supplementary Material.
(Left) Graphical representation of the splitting of the {220} peak as the structural phase transition proceeds. The horizontal axis is diffraction angle, and the vertical axis is temperature. (Right) quantitative data. If you follow the peaks from low temperature where they are well separated, the peaks move towards each other with increasing temperature, suggesting a second order type transition, but then the peaks rapidly collapse into a single peak, which could indicate that the transition is actually weakly first order. This is an aspect that needs further detailed measurements in the vicinity of the transition.
Two versions of the coarse resolution/high intensity diffraction data obtained on BT-7. The first one shows full scale, where the magnetic peaks are barely discernible. The expanded intensity scale reveals three magnetic peaks observed at low temperature (blue points). Two additional magnetic peaks occur under the strong fundamental structural peaks, and are evident when a subtraction is performed. Red points were taken at 170 K.
The structural transition was a surprise as it was thought that the strong anomalies in the bulk properties were due to the formation of a spin-density-wave magnetic state. But even more of a surprise was the observation that the magnetic transition occurred at a temperature that was substantially below the structural transition. To make absolutely certain that these were two separate transitions, the magnetic order parameter was re-measured at the High Flux Isotope Reactor at Oak Ridge National Lab, with identical results.
Magnetic structure and order parameter (solid blue points) obtained at the NCNR. Green squares--repeat of the order parameter measurement at HFIR.
