BaFe 2 As 2 : A Model Platform for Unconventional Superconductivity. David Mandrus, Oak Ridge National Lab.

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1 BaFe 2 As 2 : A Model Platform for Unconventional Superconductivity David Mandrus, Oak Ridge National Lab.

2 Correlated Electron Materials Group David Mandrus Brian Sales Rongying Jin (now at LSU) Michael McGuire Wigner Fellow Athena Sefat Wigner Fellow

3 Collaborators ORNL: A. Christianson, M. Lumsden, S. E. Nagler (neutrons); J. Howe (electron microscopy); A. Payzant (X-rays); D. Christen (flux dynamics); M. Pan (tunneling); D. J. Singh (theory) NHML/FSU: Larbalestier group, Tozer group UCSD: Maple group (pressure), Basov group (IR) McMaster: Imai group (NMR), Y. Mozharivskyj (crystallography) Julich/Liege: R. Hermann (Mossbauer) UTK: Keppens group (elastic properties), Mannella group (X-ray spectroscopy, ARPES), Egami group (PDF), Plummer group (STM) LANL: F. Ronning, R. Movshovich, E. D. Bauer, J. D. Thompson Houston: S. Pan group LSU: Plummer Group, Zhang group, Jin group UIUC: L. Greene group Presentation_name

4 Pnictides are the Missing Link Between Cuprates and HFs Heavy Fermion Superconductors HF physics + SC Iron-based Superconductors Cuprates Oxide physics + SC Chemically very different, but evidence points to similar SC mechanism

5 Great Similarity of Phase Diagrams S. Sanna, et al. PRL 93, G. Knebel, et al. PRB 74, (2006) Canfield group, arxiv: J. Zhao, et al. Nature Materials 7, 953 (2008) H. Luetkens, et al. Nature Materials 8, 305 (2009)

6 Universal Behavior of Spin Resonance Y. J. Uemura, Cond-mat: Greven group, Cond-mat: E r ~ 4k B T c E r /2Δ ~ 0.64

7 Motivation and Perspective Evidence points to a common mechanism underlying HF, Pnictide, and Cuprate SC Focus on materials-specific properties Work toward a predictive understanding Doping studies are important in this regard 122 Materials show great chemical flexibility

8 Some of Known Fe compounds (Before 1991) with the ThCr 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 DyFe 2 ErFe 2 Ge 2 EuFe 2 P 2 DyFe 2 Ge 2 ErFe 2 EuFe 2 LaFe 2 Ge 2 LaFe 2 P 2 SmFe 2 Ge 2 UFe 2 Ge 2 LaFe 2 NdFe 2 TlFe 2 Se 2 ThFe 2 YFe 2 UFe 2 P 2 GdFe 2 Ge 2 NdFe 2 Ge 2 TbFe 2 Ge 2 YbFe 2 Ge 2 LuFe 2 PrFe 2 SmFe 2 TmFe 2 YbFe 2 PrFe 2 Ge 2 ThFe 2 Ge 2 HoFe 2 SrFe 2 P 2 TbFe 2 TlFe 2 S 2 UFe 2 ZrFe 2 Crystals with ThCr 2 Structure (BaFe 1.84 Co 0.16 As 2 )

9 BaFe 2 As 2 Basic Properties ThCr 2 structure I4/mmm Layers of edge-sharing FeAs 4 tetrahedra Coupled SPT-AFM transition ~140 K M. Rotter, et al. PRL 101, (2008) D. Johrendt, R. Poettgen

10 Magnetic Order BaFe 2 As Managed by UT-Battelle for the Department of Energy 1 D Chains of parallel spin Fe atoms. Y. Su, et al. PRB 79, (2009)

11 BaFe 2 As 2 Properties

12 BaFe 2 As 2 is not very anisotropic ρ c /ρ ab 5 M. A. Tanatar, et al. PRB 79, (2009) (Canfield group)

13 Surface of BaFe 2 As 2 T = 4.3 K Cleaved surface of BaFe2As2 is As terminated Ba layer destroyed, some random Ba atoms observed on surface No surface reconstruction Some evidence of orbital order since only one DOS of one type of As was detected In collaboration with E. W. Plummer group & S.H. Pan group Bulk As-Fe and As-Ba interlayer distances are A and A respectively

14 PRL 101, (2008)

15 Phase Diagram Ba 1-x K x Fe 2 As 2 M. Rotter, et al. Angew. Chem. I nt. Ed. 47, 7949 (2008) J. T. Park, et al. PRL 102, (2009) Stuttgart group Keimer, Hinkov Electronically phase separated on a scale of ~65 nm

16 Electron rather than hole doping Superconductivity robust to in-plane disorder Evidence for s symmetry superconducting state Very clean system experimentally crystals quite homogeneous Presentation_name

17 Ba(Fe 1-x Co x ) 2 As 2 Phase Diagram C. Lester, et al. PRB 79, (2009) I.R. Fisher, S. M. Hayden groups D. K. Pratt, et al. Cond-mat P. C. Canfield, R. J. McQueeney groups

18 No superconducting phase separation --S.-H. Pan

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20 Resonance in Single Crystal BaCo Fe 1.84 As As 2

21 Neutron Scattering on Underdoped BaFe 1.92 Co 0.08 As 2 Tc = 11 K Drop in intensity of Bragg Peak below Tc Resonance obeseved at 4.5 mev = 4.7 k B Tc Spectral weight transferred from Bragg peaks to resonance? A. D. Christianson, et al. Cond-mat Scattering is 3D vs. 2D in optimally doped

22 advantages of RUS: all elastic constants can be obtained in one measurement small samples (mm 3 ) Veerle Keppens Amplitude (V) x x x x x10 6 Frequency (Hz) The University of Tennessee

23 0 T Modulus 0 T Strain-Order Parameter Coupling at Phase Transitions Bi-Linear coupling: F c = βqε Quadratic coupling: F c = βq 2 ε Modulus θ = T a c c 0 c c T T h c c T T c c < = > = β

24 BaFe 2 As 2 f(mhz) T(K)

25 Squared Frequency (MHz 2 ) Squared Frequency (MHz ) BaFe 1.8 Co 0.2 As 2 BaFe 2 As Temperature (K) Temperature (K)

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28 Susceptibility not Curie-Weiss -- looks like Cr

29 E F is only mev above the bottom of the electron bands Usual assumption that k B T<< E F is not valid

30 Sharp Features in the DOS and a variable carrier concentration can give you a susceptibility that increases with T D. Mandrus, et al. PRB 51, 4763 (1995)

31 This simple model, with only somewhat unreasonable parameters, can describe Hall, Seebeck coefficient, and susceptibility as a function of T and doping. N 0 = 1.4 x /cm 3 E H = 250 K m e * = 17 m 0 m h * = 30 m 0 B. C. Sales, et al. Cond-mat:

32 Concluding Speculation Peak in T.C. Correlates With Resonance J. G. Checkelsky, et al (Ong group) Lumsden, et al. PRL (2009)

33 Discussion Questions 1) What causes the structural and magnetic transitions to separate in Ba(Fe 1-x Co x ) 2 As 2 2) How do we explain the temperature dependence of the susceptibility? 3) Are spin fluctuations causing the elastic constants to soften?