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

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

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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

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Brian Sales Oak Ridge National Laboratory

Research Sponsored by DOE BES Division of Materials Sciences and Engineering

A Summary of Recent Experimental Results From Oak Ridge

  • n the new Iron-based superconductors

Conference on Concepts in Electron Correlation HVAR, Croatia, September 24-30 (2008)

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David Mandrus Brian Sales Rongying Jin

Correlated Electron Materials Group

Michael McGuire Wigner Fellow Athena Sefat Wigner Fellow

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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
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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 Tc 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 Ba1-xKxFe2As2 in the ThCr2Si2 structure- Single crystals can be grown from Sn or FeAs flux. No Oxygen

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June 2008 Superconductivity at 18 K reported for LixFeAs 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 Fe1+xSe, 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

  • xygen

Brief History of Quaternary Rare Earth Transition Metal Pnictide Oxides- and other Iron-based superconductors (cont’d)

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

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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

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Structure of LaFeAsO

La Fe As O

  • Tetragonal ZrCuSiAs prototype
  • Space group P4/nmm, Z = 2
  • edge sharing FeAs4 tetrahedra
  • Fe atoms form square nets
  • Fe-Fe bonding likely to be important
  • a = 4.034 A, c = 8.745 A
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Some of Known Fe compounds (Before 1991) with the ThCr2Si2 Structure

EuFe2As2 KFe2As2 BaFe2As2 SrFe2As2 DyFe2B2 HoFe2B2 TmFe2B2 BaFe2P2 CaFe2P2 CeFe2Ge2 ErFe2B2 LuFe2B2 YFe2B2 CeFe2P2 GdFe2B2 TbFe2B2 CeFe2Si2 DyFe2Si2 ErFe2Ge2 EuFe2P2 DyFe2Ge2 ErFe2Si2 EuFe2Si2 LaFe2Ge2 LaFe2P2 SmFe2Ge2 UFe2Ge2 LaFe2Si2 NdFe2Si2 TlFe2Se2 ThFe2Si2 YFe2Si2 UFe2P2 GdFe2Ge2 NdFe2Ge2 TbFe2Ge2 YbFe2Ge2 LuFe2Si2 PrFe2Si2 SmFe2Si2 TmFe2Si2 YbFe2Si2 PrFe2Ge2 ThFe2Ge2 HoFe2Si2 SrFe2P2 TbFe2Si2 TlFe2S2 UFe2Si2 ZrFe2Si2

QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture.

Crystals with ThCr2Si2 Structure (BaFe1.84Co0.16As2)

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S= 2kBT (2eTF) 1,

Basic Properties of Superconducting LaFeAsO0.89F0.11

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Resistance Measurements in High Magnetic Fields Suggest Two Gaps (Like MgB2) and Superconducting Coherence Length 2-3 nm

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F19 NMR Measurements Yield a London Penetration Depth (0) 230 nm

  • K. Ahilan et al., PRB 78 (2008) R100501
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F19 NMR Measurements of Knight Shift (K) and Spin-Lattice Relaxation Rate

  • Small value of F19 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 Tc

  • no clear Hebel-Slichter peak.
  • Temperature dependence suggests

that low frequency spin fluctuations suppressed with decreasing temperature down to Tc

  • K. Ahilan et al., PRB 78 (2008) R100501
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Doping Co Into Fe Planes Results in Robust Superconductivity: LaFe.89Co11AsO, Tc 15 K (Sefat et al PRB in press) BaFe1.84Co0.16As2, Tc= 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

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Knight Shift Data As75 and Co59: BaFe1.84Co0.16As2

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Spin-Lattice Relaxation Rate

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Ba(Fe0.9Co0.1)2As2 Single Crystals

ab(Tc) = 0.46 mcm, c(RT) = 1.27 mcm c(Tc) = 2.1 mcm, c(RT) ~6.5 mcm =c/ab ~4.6

ab c

Aki et al. (NHMFL) Resistivity Anisotropy 4-5, Good For Applications

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Critical Current of Co-doped Single Crystals of BaFe2As2 (from magnetization curves) Already within 1 order of magnitude of best “old” superconductors with similar Tc (Nb3Sn)

BaFe1.84Co0.16As2

Aki et al. NHMFL

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Experimental and Calculated Phonon Density of States for LaFeAsO0.89F0.11 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

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  • FS consists of 5 sheets:
  • 2 high velocity electron cylinders
  • 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

  • Weak hybridization between Fe and As
  • Nearly 2D electronic structure
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Summary of Characteristics of FeAs based Superconductors

  • Rare earth arsenide compounds superconduct with maximum Tc 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 1021 electrons or holes /cm3
  • Measured phonon density of states and theory probably rule out electron-

phonon pairing mechanism

  • Observation of inelastic magnetic resonance in Ba1-xKxFe2As2 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?

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Characteristics of Undoped Parent Compounds: LaFeAsO, BaFe2As2: “Bad Metals” LaFeAsO Resistivity

McGuire et al. PRB in press Sefat et al. PRL in Press

BaFe2As2 Resistivity

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Characteristics of Undoped Parent Compounds: LaFeAsO, BaFe2As2: Magnetic Susceptibility Similar to Chromium Metal

McGuire et al. PRB in press Sefat et al. PRL in Press

LaFeAsO BaFe2As2

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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

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Characteristics of Undoped Parent Compounds: Structural Transition LaFeAsO

Start of Structural Transition Evident at T 200 K

McGuire et al. PRB in press

Structural and Magnetic Transition Evident with HiRes Heat Capacity Elastic constant data suggests second order phase transition

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Characteristics of Undoped Parent Compounds: Transport LaFeAsO

Hall Data Suggests factor of 10 decrease in carrier concentration below Ts. Remaining electrons have higher mobility Sharp increase of thermal conductivity ,which is dominated by lattice, suggests strong electron phonon scattering. Seebeck data complicated, but magnitude is too large for good electron- hole symmetry. Electrons dominate conduction

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McGuire et al. In preparation Systematic Variation of Apparent Carrier Concentration and Magnetoresistance With Rare Earth in Oxy-Arsenide Parent Compounds

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Similarities of Structural/ Magnetic Transition in FeAs Compounds With Itinerant Antiferromagnetism in Cr Metal Magnetism (SDW) due to pairing of electrons and holes of opposite spin by “nesting” electron and hole Fermi Surfaces. Cr moment 0.43 B

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Similarities of Structural/ Magnetic Transition in FeAs Compounds With Itinerant Antiferromagnetism in Cr Metal Pressure drives SDW transition toward T=0, mention possibility

  • f superconductivity arising near QCP if disorder small enough
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Undoped parent compounds are bad metals- not Mott Insulators Structural Transition, Ts, from tetragonal to orthorhombic occurs near 150 K. At or 10-20 K below structural transition, long range antiferromagnetic

  • rder occurs consistent with the formation of a SDW

Fe moment 0.3-0.8 B (From neutron and Mossbauer) In LaFeAsO, structural transition is second order or weakly first order Above Ts, magnetic susceptibility is small and increases with increasing T at least up to 400 K- Susceptibility is very similar to Cr metal Microscopic probes indicate Fe local environment similar to iron metal Transport anomalies associated more with structural transition At high temperatures Fe3s photoemission spectroscopy from CeFeAsO0.89F0.11 suggest fast itinerant iron spin fluctuations with an Fe moment 1 B Summary of Characteristics of Undoped Parent Compounds: LaFeAsO, BaFe2As2

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Layered FeAs materials are exciting new systems with great chemical flexibility Only cuprates have a higher Tc Metallic Fe layer sandwiched between ionic layers, strongly 2D Low carrier density, correlations important Proximity to magnetism Orbital degrees of freedom may be important Pairing mechanism needs to couple electron and hole bands Crystals of BaFe2As2 (and FeSe1-xTex) are good for fundamental physics studies

Summary