Soft x-ray Spectroscopy Adventures in Pnictide Land.

Transcription

1 Soft x-ray Spectroscopy Adventures in Pnictide Land. Norman Mannella University of Tennessee - Knoxville Michael A. McGuire, Athena S. Sefat, Brian C. Sales, Rongying Jin, D. Mandrus Samples F. Bondino, E. Magnano, M. Malvestuto, F. Offi, G. Panaccione, F. Parmigiani PES, XAS, XES 1111 system P. Vilmercati, A. Fedorov, I. Vobornik, G. Panaccione ARPES 122 system C. Parks Cheney, F. Bondino, P. Vilmercati, T. Callcott, J. Freeland XAS 122 system D. J. Singh Theory E. W. Plummer, M. Stocks Discussions

2 Complex Electron Systems High Temperature Superconductivity (Cu Oxides) Colossal Magnetoresistance (Mn Oxides) Dilute Magnetic Semiconductors (Mn:GaAs) Large Thermoelectric Effect (Co Oxides)

3 Complex Electron Systems Complex electron systems exhibit spectacular and unexpected phenomena arising from the interactions between the electrons or other degrees of freedom (such as the lattice or spin). Interactions are strong Beyond the usual approximation schemes based on single particle approximations MANY BODY PROBLEM Lattice Charge Orbital Spin Research Interest: interplay and competition of several degrees of freedom

4 Spallation Neutron Source

5 Cray XT3 supercomputer ~250 Teraflops use by proposal Center for Nanophase Materials Sciences

6 Complex Electron Systems Exciting opportunities What Fe-based SC Manganites La 2-2x 2xSr 1+2x Mn 2 O 7 Cobaltates Na x CoO 2 GdBaCo 2 O 5+ 5+d Delafossites CuRh 1-x Mg x O 2 Binary Oxides VO 2, V 2 O 3, Fe 3 O 4, EuO How PE ARPES XAS XES EXAFS ARPES LASER-ARPESARPES MBE STM Where Synchrotron radiation Serf UTK nmannell@utk.edu, ,

7 Experiments performed at UTK and Synchrotron Radiation Facilities ALS, Elettra, CAMD, Aladin, SSRL

8 The Soft X-Ray Spectroscopies Energy Energy Energy Energy Core Level and Angle Resolved Photoemission XPS ARPES X-Ray Absorption X-Ray Emission XAS XES Resonant Inelastic X-Ray Scattering RIXS h s VB Vacuum level Fermi level, E F s VB VB s s VB h inelastic Core levels h h h h elastic Stoichiometry - DOS Chemical Environment Band Structure Quasi-particles Surface Unoccupied states Near-Surface Bulk Core levels Occupied states Bulk Element-specific Methods Elementary Excitations d-d Transitions

9 Energy Crisis Saving energy loss from transmission lines is one of the most important applications of High T c superconductors

10 Breaking the supremacy of Cuprates

11 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) Feb 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) 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

12 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 Square planar nets of Fe atoms in a tetrahedral environment is the common feature of all four superconducting structure types

13 How high is the T c now? Cuprate Iron-based SC

14 Why Fe-based superconductors are so exciting Excellent model systems for unconventional SC --a lot can be done with DFT --structurally simple & homogeneous --nice crystals can be grown Huge materials space to look for new SCs --good test of predictive ability of modern theory

15 LaFeAsO Structure Iron in tetrahedral coordination, Fe-Fe distance 2.85 Å Fe-Fe distance 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

16 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 )

17 Lattice and Magnetic Structure Adrian Cho, Science 320, 870(2008)

18 Square planar nets of Fe atoms in a tetrahedral environment is the common feature of all four superconducting structure types

19 Cuprates Rev. Mod. Phys. 79, 353 (2007) J. Zhao et al., Nature Materials 7, 953 (2008). Similarities Spin, charge both involved 2D crystal structure Similarity of phase diagrams Differences Lower Tc CuO 2 Planes (Octahedral) vs. Tetrahedral coordination Mott insulator AFM vs. Metallic SDW groundstate Single Band vs. Multi-band

20 Some Key Questions Are the new FeAs SCs similar or dissimilar to the cuprates HTSC? Phase diagram shows competition between a magnetically ordered, Spin Density Waves state and Superconductivity? = signatures of fluctuations associated with the proximity to AF state? Clarify interplay between magnetism and superconductivity, i.e. competition between SC and SDW

21 Are the new FeAs SCs similar or dissimilar to the cuprates HTSC? Electronic structure of the normal state A hallmark of cuprates HTSC: The non-fermi liquid normal state Strong electron correlations (i.e. on-site Coulomb repulsions) in the narrow Cu 3d bands

22 Cu 2+ O 2- Cu 2+ + U Cu 1+ O 2- Cu 3+ Cu 2+ O 2- Cu 2+ + D Cu 1+ O 1- Cu 2+ 2E(d n ) + U E(d n+1 ) + E(d n-1 ) E(d n ) + E(L) + D E(d n+1 ) + E(L -1 ) U < D Mott Hubbard U > D Charge Transfer

23 Energy Kin. Energy Energy Photoemission X-ray absorption (XAS) h h s Vacuum level s VB h h Vacuum level Fermi level, E F VB Fermi level, E F Core levels h Upon creation of a core hole, different screening channels leave definite signatures in photoemission and absorption spectra

24 Energy Kin. Energy h h Core PE - stoichiometry, BE shifts, satellites s VB h h Vacuum level Fermi level, E F E B = h - KE Core levels Binding Energy

25 Cu 2p - XPS Nd 1.85 Ce 0.15 CuO 4-d Nd 2 CuO 4 Steeneken et al., Phys. Rev. Lett. 90, (2003)

26 CeFeAsO 0.89 F 0.11 J. Zhao et al., Nature Materials 7, (2008).

27 Fe 2p - XPS Nd 2 CuO4 Similar to Fe 2p in metallic iron or iron intermetallics

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29 Energy X-ray absorption Spectroscopy (XAS) s VB Vacuum level Fermi level, E F unoccupied DOS (Matrix elements ) h Photon Energy

30 Appears more similar to that of metallic iron or iron intermetallics Fe 2p - XAS

31

32 CeFeAsO 0.89 F 0.11 HIGH DOS at the Fermi Level D. H. Lu et al., Nature 455, 81 (2008) W. Malaeb et al., J. Phys. Soc. Jpn. 77, (2008)

33 CeFeAsO 0.89 F 0.11 Ce 4f LaFeAsO 0.89 F 0.11

34 Resonant Photoemission VL CB VL CB Fe 3d (unoccupied) VB Fe 3d (occupied) E F E F VB Fe 3d (occupied) Coulomb interaction Fe 2p, Fe 3p Fe 2p, Fe 3p

35

36 Behavior similar to that of metallic Ni and Fe M. Weinelt et al., PRL. 78, 967 (1997) S. Hufner et al., PRB 61, (2000)

37 Energy Vacuum level Fermi level, E F s XES Band structure, PDOS Photon Energy

38

39 Are the new FeAs SCs similar or dissimilar to the cuprates HTSC? FeAs-SC do NOT show apparent signatures of Mott physics Itinerant Fe electrons Phase diagram shows competition between a magnetically ordered, Spin Density Waves state and Superconductivity? = signatures of fluctuations associated with the proximity to AF state?

40 Multiplet splitting in core levels of transition metal oxides BE = h - E kin = E*(N-1) - E 0 (N) Final state Initial state Open shell, net spin S V h Upon photoemission, two possible final states 3s J eff 3s,VB(3d) 3s 3s higher energy DE lower energy The splitting between the two peaks is given by DE (2S V + 1) J eff 3s,VB(3d) (Van Vleck Theorem) Binding Energy energy (ev)

41 Intensity (Arb. Units) Detection of Multiplet Splittings in Fe 3s Photoemission Spectra h = 481 ev h = 650 ev h = 7596 ev Binding Energy (ev)

42 Multiplet splittings are usually interpreted as local moments How to handle itinerant systems?

43 F. R. McFeely et al., Solid State. Com. 15, 1051 (1974) The Van Vleck model holds for itinerant systems.. D. G. van Campen & L. E. Klebanoff, Phys. Rev. B 49, 2040 (1994)

44 Fe

45 According to Mossbauer, NMR, magnetic susceptibility.. Fe is NON magnetic magnetic susceptibility accounted by a free Ce ion (3 +, i.e.f 1 )

46 Ce 3d - XAS Ce is 3 +, i.e. f 1

47 Fast time scale in PES ( s) + absence of local moment Fe 3s multiplet splitting indicates Fe spin fluctuations ( mb) Same scenario in FeAl, nonmagnetic with fluctuating magnetism

48 Are the new FeAs SCs similar or dissimilar to the cuprates HTSC? FeAs-SC do NOT show apparent signatures of Mott physics Itinerant Fe electrons Phase diagram shows competition between a magnetically ordered, Spin Density Waves state and Superconductivity Phys. Rev. Lett. 101 (2008), ? = signatures of fluctuations associated with the proximity to AF state? Fe 3s multiplet splitting indicates Fe spin fluctuations ( mb) in the normal state

49 H. Q. Yuan et al., Nature 457, 565 (2009) Despite their layered structure, (Ba,K)Fe 2 As 2 exhibit superconducting properties that are quite isotropic, a behavior drastically different compared to that of other layered superconductors. It has been proposed that the nearly isotropic critical field of (Ba,K)Fe 2 As 2 might be linked to its distinctive three-dimensional electronic structure and Fermi surface (FS) topology, the reduced dimensionality is not a prerequisite for high temperature superconductivity, contrary to what has been suggested for cuprate HTSC

50 h (ev) 30 k z (Ǻ -1 ) Binding Energy (ev) Binding Energy (ev) 4.8

51 Angle Resolved PhotoEmission Spectroscopy (ARPES) E h E B kin p k 2mE sin kin Direct measurement of the electronic structure in 2D materials

52 Energy Angle Resolved PhotoEmission Spectroscopy (ARPES) Angle (Momentum)

53 Binding Energy (ev) M Ba(Fe 1-x Co x ) 2 As Parent (x=0) (a) 0.00 Doped (x=0.1) (b) G k x (Ǻ deg -1 ) k x deg (Ǻ -1 ) ev 60 ev (c) 0.00 (d) Doped Parent ev 52 ev k x (Ǻ deg ) k x deg (Ǻ -1 )

54 Binding Energy (ev) BaFe 1.8 Co 0.2 As ev 58 ev 56 ev 54 ev 52 ev 50 ev 48 ev ev 44 ev 42 ev 40 ev 38 ev 36 ev 34 ev k x (Ǻ -1 ) Band at 200 mev captured by LDA using z LDA but not z EXP

55 Intensity (Arb. Units) Binding Energy (ev) BaFe 2 As ev 71 ev 66 ev 56 ev 46 ev 41 ev 36 ev k // (Ǻ -1 ) Binding Energy (ev)

56 Weak hybridization between Fe and As Nearly 2D electronic structure 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

57 Hole pocket Electron pocket 60 h (ev) hv(ev) k x (Ǻ -1 ) k // (Ǻ -1 )

58 k z (Ǻ -1 ) hv(ev) k x (Ǻ -1 )

59 k z (Ǻ -1 ) Phys. k x Rev. (Ǻ -1 ) B 79, (R) (2009))

60 Polarized XAS on BaFe 2 As 2 X-ray spectroscopy seems to suggest low U values Full calculations including matrix elements and also effect of the core W.L. Yang, T. Devereaux et al., PRB 80, (2009)

61 XAS Intensity (Arb. Units) (a) Pristine H V q 25 H V q y x Cleaved H V Photon Energy (ev)

62 rb. Units) Density XAS of Intensity States (Arb. Units) XAS Intensity (Ar H Calculated XAS BaFe 2 As 2 q = 25 V Photon Energy (ev) 706 H y V H (x 4) q = 90 V H IN PLANE OUT OF PLANE As p (x+y) Calculated XAS Fe d TOTAL V q H x q = 25 V H Fe d (xz+yz) Fe d (x 2 -y 2 ) IN PLANE V H (x 4) OUT Fe OF s (x PLANE 10) As p (x+y) -2 0=E F Energy relative to E F (ev) Fe d (z 2 ) Fe d (xy) Fe d TOTAL H V q y x

63 Density XAS of States Intensity (Arb. (Arb. Units) Units) XAS Intensity (Arb H Calc. XAS q = 25 V H q = 90 V - H V (x 4) H Photon Energy (ev) As s IN PLANE OUT OF PLANE V q H y x As d TOTAL (Arb. Units) Calc. XAS q = 25 V H V - H (x 4) 0=E F As d (xy) As d (xz+yz) As d (x 2 -y 2 ) As d (z 2 ) As s Energy relative to E F (ev) IN PLANE OUT OF PLANE As d TOTAL H V q y x

64 Density of States (Arb. Units) XAS Intensity (Arb. Units) Photon Energy (ev) y q = 90 V H V q H Calculated XAS y x 710 and 712 ev Hybridization with As q = 30 V H H V q x (V H)x4 IN PLANE OUT OF PLANE As p (x+y) Fe d TOTAL Fe d (xz+yz) Fe d (x 2 -y 2 ) Fe d (z 2 ) Fe d (xy) Fe s (x 10) -2 0=E F Energy relative to E F (ev)

65 Density of States (Arb. Units) XAS Intensity (Arb. Units) 1320 Photon Energy (ev) y q = 90 V H V q H x y q = 25 V H H V q x (V H)X4 As s As d TOTAL As d (xy) As d (xz+yz) As d (x 2 -y 2 ) As d (z 2 ) 0=E F Energy relative to E F (ev) 30

66 FeSC are quite unique high temperature superconductors Itinerant Fe d-band character, High density of states at E F, No signatures of strong local Mott-Hubbard type correlations that characterize cuprate HTSC. (F. Bondino et al., Phys. Rev. Lett. 101, (2008)). FeSC show three-dimensional FS topology ( reduced dimensionality is not a necessary condition for high temperature superconductivity?) (P. Vilmercati et al., Phys. Rev. B 79, (R) (2009)) High degree of Fe-As hybridization. The energies and directions of Fe and As d unoccupied orbitals in agreement with the predictions of DFT (C. Parks Cheney et al., Phys. Rev. B, in press) These results assist in establishing a unique character of the FeSC materials, a new class of high temperature superconductors, quite unlike the cuprates.

67 Itinerant Spin Fluctuations in the Normal State of CeFeAsO 0.89 F 0.11 F. Bondino et al., Phys. Rev. Lett. 101, (2008) 3D Fermi Surface Topology in Ba(Fe 1-x Co x ) 2 As 2 P. Vilmercati et al., Phys. Rev. B 79, (R) (2009) Orbital Symmetry and Bonding Topology in Ba(Fe 1-x Co x ) 2 As 2 C. Parks Cheney et al., Phys. Rev. B, in press.