Brian Sales Oak Ridge National Laboratory Research Sponsored by DOE - PowerPoint PPT Presentation

A Summary of Recent Experimental Results From Oak Ridge on the new Iron-based superconductors Conference on Concepts in Electron Correlation HVAR, Croatia, September 24-30 (2008) Brian Sales Oak Ridge National Laboratory Research Sponsored by DOE BES Division of Materials Sciences and Engineering 1 Managed by UT-Battelle for the Department of Energy

Correlated Electron Materials Group Brian Sales David Mandrus Rongying Jin Athena Sefat Michael McGuire Wigner Fellow Wigner Fellow 2 Managed by UT-Battelle for the Department of Energy

Collaborators ORNL: A. Christianson, M. Lumsden, H. Mook (neutrons); J. Howe, M. Chisholm (electron microscopy); A. Payzant (X-rays); D. Christen (flux dynamics); M. Pan (tunneling) NHML/FSU: Larbalestier group UCSD: Maple group (pressure), Basov group (IR) McMaster: Imai group (NMR) Julich/Liege: R. Hermann (Mossbauer) UTK: Keppens group (elastic properties) UTK: Manella group (X-ray spectroscopy) SDSU: Oseroff group (ESR) Wayne State: Z. Zhou group (nanotransport) Argonne: J. Freeland (X-ray spectroscopy) LBNL: C. Booth (EXAFS) UTK: Egami group (PDF) UTK: Plummer group (STM) Fribourg: Bernhard group (optics) Imperial College: Cohen group (proton irradiation) Stanford: Kapitulnik group (Sagnac); Mohler group (scanning SQUID) Kentucky: Brill group (ac heat capacity) Discussions: D. J. Singh, I. Mazin, S. E. Nagler, E. Dagotto, T. Schulthess, D. Scalapino, T. Maier, A. Castro-Neto, Z. Hiroi 3 Managed by UT-Battelle for the Department of Energy Presentation_name

Brief History of Quaternary Rare Earth Transition Metal Pnictide Oxides- and other Iron-based superconductors � Jeitschko group reports first RETPnO compounds (1994) � Jeitschko group reports 18 quaternary arsenides, eg. LaFeAsO (2000)- J.Alloys and Compounds 302 (2000) 70 Superconductivity at 4 K reported for LaFePO, raised to 7 K with F doping- � Kamihara et al. JACS 128 (2006) 10012 Feb. 2008 Superconductivity at 26 K reported for F doped LaFeAsO- � Kamihara et al. JACS 130 (2008) 3296 Pressure increases T c to 41 K Takahashi et al. Nature Letters. � March-April 2008 Groups in Beijing IOP push Tc up to about 50 K by replacing La by other light rare earths (Ce, Pr, Nd,Sm Gd). First materials prepared and studied in US at ORNL- Sefat et al. Phys. Rev. B. 77 (2008) 174503. May 2008 Rotter et al. (Chemistry group in Munich) report superconductivity � at 38 K for Ba 1-x K x Fe 2 As 2 in the ThCr 2 Si 2 structure- Single crystals can be grown from Sn or FeAs flux. No Oxygen 4 Managed by UT-Battelle for the Department of Energy

Brief History of Quaternary Rare Earth Transition Metal Pnictide Oxides- and other Iron-based superconductors (cont ’ d) � June 2008 Superconductivity at 18 K reported for Li x FeAs by IOP group- 3rd structure type (we also observed sc in our lab 2 days before preprint appeared on server). � July 2008 Superconductivity at 8K in Fe 1+x Se, reported by Hsu et al.(group from Taiwan), increases to 27 K under pressure (Mizuguchi et al Japan) Single crystals can be grown 4th structure type- NO arsenic or oxygen QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. Square planar nets of Fe atoms in a tetrahedral environment is the common feature of all four superconducting structure types 5 Managed by UT-Battelle for the Department of Energy

LaFeAsO Structure Iron in tetrahedral coordination, Fe-Fe distance 2.85 Å Fe-Fe distance 2.477 Å in iron metal Large Number of Compounds! La O La As Fe As La O La Table from Quebe et al. J. Alloys and Compounds 302 (2000) 74 6 Managed by UT-Battelle for the Department of Energy

Structure of LaFeAsO La Fe As O • Tetragonal ZrCuSiAs prototype • Space group P4/nmm , Z = 2 • edge sharing FeAs 4 tetrahedra • Fe atoms form square nets • Fe-Fe bonding likely to be important • a = 4.034 A, c = 8.745 A 7 Managed by UT-Battelle for the Department of Energy

Some of Known Fe compounds (Before 1991) with the ThCr 2 Si 2 Structure EuFe 2 As 2 KFe 2 As 2 BaFe 2 As 2 SrFe 2 As 2 DyFe 2 B 2 HoFe 2 B 2 TmFe 2 B 2 BaFe 2 P 2 CaFe 2 P 2 CeFe 2 Ge 2 ErFe 2 B 2 LuFe 2 B 2 YFe 2 B 2 CeFe 2 P 2 GdFe 2 B 2 TbFe 2 B 2 CeFe 2 Si 2 DyFe 2 Si 2 ErFe 2 Ge 2 EuFe 2 P 2 DyFe 2 Ge 2 ErFe 2 Si 2 EuFe 2 Si 2 LaFe 2 Ge 2 LaFe 2 P 2 SmFe 2 Ge 2 UFe 2 Ge 2 LaFe 2 Si 2 NdFe 2 Si 2 TlFe 2 Se 2 ThFe 2 Si 2 YFe 2 Si 2 UFe 2 P 2 GdFe 2 Ge 2 NdFe 2 Ge 2 TbFe 2 Ge 2 YbFe 2 Ge 2 LuFe 2 Si 2 PrFe 2 Si 2 SmFe 2 Si 2 TmFe 2 Si 2 YbFe 2 Si 2 PrFe 2 Ge 2 ThFe 2 Ge 2 HoFe 2 Si 2 SrFe 2 P 2 TbFe 2 Si 2 TlFe 2 S 2 UFe 2 Si 2 ZrFe 2 Si 2 Crystals with ThCr 2 Si 2 Structure (BaFe 1.84 Co 0.16 As 2 ) QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. 8 Managed by UT-Battelle for the Department of Energy

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Basic Properties of Superconducting LaFeAsO 0.89 F 0.11 S= � 2 k B T (2eT F ) �1 , 11 Managed by UT-Battelle for the Department of Energy

Resistance Measurements in High Magnetic Fields Suggest Two Gaps (Like MgB 2 ) and Superconducting Coherence Length � 2-3 nm 12 Managed by UT-Battelle for the Department of Energy

F 19 NMR Measurements Yield a London Penetration Depth � (0) � 230 nm K. Ahilan et al., PRB 78 (2008) R100501 13 Managed by UT-Battelle for the Department of Energy

F 19 NMR Measurements of Knight Shift (K) and Spin-Lattice Relaxation Rate Small value of F 19 Knight shift � indicate spin transfer from FeAs layers is small- consistent with 2-D nature of compound � Spin-Lattice relaxation rate- decrease monotonically with temperature to T c - no clear Hebel-Slichter peak. Temperature dependence suggests � that low frequency spin fluctuations suppressed with decreasing temperature down to T c K. Ahilan et al., PRB 78 (2008) R100501 14 Managed by UT-Battelle for the Department of Energy

Doping Co Into Fe Planes Results in Robust Superconductivity: LaFe .89 Co 11 AsO, T c � 15 K (Sefat et al PRB in press) BaFe 1.84 Co 0.16 As 2 , T c = 22 K (Sefat et al. PRL, in press) Different from Cuprates- This result plus NMR (Imai JPSJ in press) and ARPES (Manella in preparation) results suggests symmetry of superconducting order parameter is type of “s-wave” probably 2 gaps 15 Managed by UT-Battelle for the Department of Energy

Knight Shift Data As 75 and Co 59 : BaFe 1.84 Co 0.16 As 2 16 Managed by UT-Battelle for the Department of Energy

Spin-Lattice Relaxation Rate 17 Managed by UT-Battelle for the Department of Energy

Resistivity Anisotropy � 4-5, Good For Applications Ba(Fe 0.9 Co 0.1 ) 2 As 2 Single Crystals � ab � c � ab (Tc) = 0.46 m � cm, � c (RT) = 1.27 m � cm � c (Tc) = 2.1 m � cm, � c (RT) ~6.5 m � cm Aki et al. (NHMFL) � = � c / � ab ~4.6 18 Managed by UT-Battelle for the Department of Energy

Critical Current of Co-doped Single Crystals of BaFe 2 As 2 (from magnetization curves) Already within 1 order of magnitude of best “old” superconductors with similar T c (Nb 3 Sn) BaFe 1.84 Co 0.16 As 2 Aki et al. NHMFL 19 Managed by UT-Battelle for the Department of Energy

Experimental and Calculated Phonon Density of States for LaFeAsO 0.89 F 0.11 With either DOS, an electron-phonon pairing mechanism is unlikely (This is some of the first neutron data taken at the SNS) Christianson et al. PRL, in press 20 Managed by UT-Battelle for the Department of Energy

• FS consists of 5 sheets: • Weak hybridization between Fe and As • 2 high velocity electron cylinders • Nearly 2D electronic structure • 2 lower velocity hole cylinders • 1 3D hole pocket • 3D hole pocket shrinks with doping • Theories focus on pairing interaction between elect. and hole bands 21 Managed by UT-Battelle for the Department of Energy

Summary of Characteristics of FeAs based Superconductors •Rare earth arsenide compounds superconduct with maximum T c �55 K •Average coherence length is 2-3 nm, with maximum critical field � 100 T Zero temperature penetration depth from NMR � 220 nm •Resistivity anisotropy � 4-6 •Low Carrier Concentration � 10 21 electrons or holes /cm 3 •Measured phonon density of states and theory probably rule out electron- phonon pairing mechanism •Observation of inelastic magnetic resonance in Ba 1-x K x Fe 2 As 2 powder (Christianson et al. Nature in press) suggests magnetic pairing mechanism •NMR and ARPES strongly suggest “s-wave” or “extended s-wave” superconducting order parameter •Correlations not a strong as for cuprates, What about undoped parent compounds? 22 Managed by UT-Battelle for the Department of Energy

Characteristics of Undoped Parent Compounds: LaFeAsO, BaFe 2 As 2 : “Bad Metals” LaFeAsO Resistivity BaFe 2 As 2 Resistivity Sefat et al. PRL in Press McGuire et al. PRB in press 23 Managed by UT-Battelle for the Department of Energy

Characteristics of Undoped Parent Compounds: LaFeAsO, BaFe 2 As 2 : Magnetic Susceptibility Similar to Chromium Metal BaFe 2 As 2 LaFeAsO McGuire et al. PRB in press Sefat et al. PRL in Press 24 Managed by UT-Battelle for the Department of Energy

Characteristics of Undoped Parent Compounds: Magnetic Order in LaFeAsO from Neutron Scattering Simple Striped AF Order with Fe Moment � 0.5 � B Ordering wavevector: 0.5, 0.5, 0.5 McGuire et al. PRB in press 25 Managed by UT-Battelle for the Department of Energy