Iron-based Superconductors

The nature of the magnetic order in superconductors has had a rich and interesting history, and has been a special topic of interest ever since the parent materials of the high T_C cuprates were found to be antiferromagnetic Mott insulators that exhibit huge exchange energies within the Cu-O planes.  The recent discovery of superconductivity in the rare-earth (R) iron-based oxide systems RO_1-xF_xFeAs (R = rare earth) and the (Ca,Ba,Sr)_1-xK_xFe₂As₂ has generated enormous interest because these materials are the first non-copper oxide superconductors with T_c's 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.  We have used neutron scattering at the NCNR to explore the crystal and magnetic structures of a number of these materials, and have found that the parent materials undergo an abrupt structural distortion below ~150-200 K (depending on the system), changing the symmetry from tetragonal (space group P4/nmm) to orthorhombic (space group Cmma, or equivalently monoclinic, space group P112/n) 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 the spin-density-wave type.  The measured spin wave dispersion relations yield a magnon bandwidth of ~0.2 eV, a very large energy scale compared to the ordering temperature, again much like the cuprates.  Doping the system suppresses both the magnetic order and structural distortion in favor of superconductivity.  Therefore, much like high-T_c copper oxides, the superconducting regime in these Fe-based materials occurs in close proximity to a long-range ordered and very energetic antiferromagnetic ground state.  Indeed, a magnetic resonance has now been observed when the system becomes superconducting, with an intensity that follows the form of a superconducting order parameter, just like the cuprates.  It therefore appears likely that magnetic fluctuations are at the heart of the superconducting pairing.

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 huge and very anisotropic change in the crystal structure changed the interpretation of the superconductivity measurements, since the initial measurements were carried out using a solid pressure medium rather than hydrostatic pressure as was done for the diffraction measurements.  It turns out that the collapsed phase is not superconducting under hydrostatic pressure, but is superconducting under the anisotropic pressure where the a-b plane is compressed and the c-axis is relaxed, which should have interesting implications for thin film properties.

Below is a list of publications, including a very brief review of the NCNR work.

Publications

Magnetic Order Close to Superconductivity in the Iron-based Layered La(O_1-xF_x)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. B 78, 024521 (2008).

Neutron scattering study of the oxypnictide superconductor LaO_0.87F_0.13FeAs, 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. B 78, 052508 (2008).

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, 167203 (2008).

Pressure-induced Volume-collapsed Tetragonal Phase of CaFe₂As₂ as seen via Neutron Scattering, 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, Phys. Rev. B 78, 184517 (2008).

The crystalline electric field as a probe for long range antiferromagnetic order and superconductivity in CeFeAsO_1−xF_x, 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, Phys. Rev. Lett. 101, 217002 (2008).

Neutron-Diffraction Measurements of Magnetic Order and a Structural Transition in the Parent BaFe₂As₂ Compound of FeAs-Based High-Temperature Superconductors, Q. Huang, Y. Qiu, W. Bao, M.A. Green, J. W. Lynn, Y. C. Gasparovic, T. Wu, G. Wu, and X. H. Chen, Phys. Rev. Lett. 101, 257003 (2008).

Crystal Structure and Antiferromagnetic Order in NdFeAsO_1-xF_x (x = 0.0 and 0.2):  Superconducting Compounds from Neutron Diffraction Measurements, 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, Phys. Rev. Lett. 101, 257002 (2008).

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 LaOF_xFeAs 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.

NIST press release.

(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.

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