Unconventionalsuperconductivitycast iniron...

Unconventional superconductivitycast in ironRayOsbornMaterials Science Division, Argonne National Laboratory,Argonne IL60439, USA

Wednesday, 4 May- 8:45 a.m,

It is now 100 years sincethediscovery ofsuperconductivity byH. Kamerlingh Onnes andover50 years since thepublication byBardeen. Cooper and Schrieffer (BCS) ofthe firstmicroscopic theoryto explainit, butthefield isasvigorous asever. both experimentally and theoretically. In conventionalBCS superconductors, latticevibrations create anattractive potential thatbinds theelectron pairs(Cooper pairs)thatcarry thesupercurrent, butpairformation isdisrupted bymagnetic impurities.

However, there isa growing family ofunconventionalsuperconductors,whose Cooper pairs form in thepresence ofstrong magnetic correlations andsometimes coexist withlong-rangemagnetic order. Theseinclude thecopper-oxide-based high-temperature superconductors, cerium-heavy fermion compounds,andorganic charge transfer salts. In2008,a new group of iron-based pnictideandchalcogenidesuperconductors was added to thislist. Inspiteofsuperconducting critical temperatures (Tc) thatdifferbyover twoorders ofmagnitude, there are striking similarities between these materials. Inall ofthem,superconductivity emerges when magneticorder hasbeen suppressed either bychemicalsubstitutionorbyapplying pressure; withinthesuperconducting state, there isevidence that thesuperconductingenergy gap hasunusual symmetry inconsistent withBCStheory; and theelectronic transportaboveT

cdiffers from thebehaviour ofnormal metals. Allofthese observationshave suggested theroleof

magnetic fluctuations, rather thanlatticevibrations, inpairformation.

Inthis talk, Iwill discuss recent results onthenew iron-based superconductors andhow similarities tothecopper-oxide andothersuperconductors mask significant differences in theirunderlying electronicstructure andthestrength oftheirelectron correlations, making a universal theory ofunconventionalsuperconductivity anelusive prospect.

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Unconventional SuperconductivityCast in Iron

Leiden – April 8, 1911Heike Kamerlingh Onnes, Gilles Holst, Gerrit Flim

Dirk van Delft and Peter Kes, Physics Today (Sept. 2010)

“Kwik nagenoeg nul”

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Superconductivity and the Holy Grail

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Physics World 24 (April, 2011).

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Dawning of the Iron Age

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Atomic Structure of Iron-Based Superconductors

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Outline

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(e) (f) (g) (h)

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Superconductivity The Limits of Conventional Behaviour

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Fermi Surfaces in Iron Arsenides

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Fermi Surface Nesting

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Electron Correlations and Mott-Hubbard Insulators

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Conventional Superconductivity

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Energy Gap Bose-Einstein Condensation

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Spallation Neutron Source, Oak Ridge, Tennessee

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Inelastic Neutron Scattering&('"!(

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Energy Conservation

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Magnetic Cross Section

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Dynamic Magnetic Susceptibility

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MERLIN

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Detector coverage " steradians

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Superconductivity in MgB2: A Case Study

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Electron-Phonon Coupling in the Iron Arsenides

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Theoretical:Actual: Tc = 39K

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Quantum Criticality

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Ba1-xKxFe2As2A Typically Unconventional Superconductor

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EPL Publications on Ba1-xKxFe2As2

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Antiferromagnetic Order in BaFe2As2

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Phase Diagram of Ba1-xKxFe2As2

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30

60

90

120

150

T (K

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K content (x)

TC

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Superconducting Gaps in Ba0.6K0.4Fe2As2

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Resonant Spin ExcitationsA New Probe of Fermi Surface Nesting

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S(Q,!) in Ba1-xKxFe2As2

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Resonant Spin Excitation in Ba0.6K0.4Fe2As2 (Tc = 38K)

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Observations of Resonant Spin Excitations

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Resonance in the Copper Oxide Superconductors

H. F. Fong et al, Nature 398, 588 (1999)

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Resonant Spin Excitations in Unconventional Superconductors

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Itinerant Theories of the Resonance

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Spin Resonance

Energy

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Resonance Condition

χ(Q,ω) =χ0(Q, ω)

1− J(Q)χ0(Q, ω)

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Unconventional Gap Symmetries

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coskx cosky

Extended s±-wave

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Origin of the Spin Resonance

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Extended s±-wave

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Origin of the Spin Resonance

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Extended s±-wave

s-wave

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Fermi Surface Nesting vs Hole Doping

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Doping Dependence of the Resonance

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x=0.3 x=0.5 x=0.7 x=0.9

10

15

20

25

0.5 1.0 1.5 2.02

4

6

Q (Å-1)

0.5 1.0 1.5 2.0 0.5 1.0 1.5 2.0 0.5 1.0 1.5 2.0

(a) (b) (c) x 2 (d) x 2.667

(e) (f) (g) (h)

0.0 0.2 0.4 0.6 0.8 1.00

30

60

90

120

150

T (K

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K content (x)

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Hole-Doping

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Itinerant Model of the Spin Resonance

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0

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1

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Splitting of the Magnetic Response (and Tc)

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x=0.3 x=0.5 x=0.7 x=0.9

10

15

20

25

0.5 1.0 1.5 2.02

4

6

Q (Å-1)

0.5 1.0 1.5 2.0 0.5 1.0 1.5 2.0 0.5 1.0 1.5 2.0

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(e) (f) (g) (h)

0.4 0.6 0.80.8

1.0

1.2

1.4

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x x

0.4 0.6 0.8

10

15

0

10

20

30

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Disappearing Resonance

69

0 5 100

20

40

2!-! (meV)

M. Eschrig, Adv. Phys. 55, 47 (2006)

2"

Spin Resonance

Energy

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4

6

8

10

8

10

12

14

6

8

10

4

5

6

4 8 12 16 20

4

5

6

Energy Transfer (meV)

x = 0.3

x = 0.4

x = 0.5

x = 0.7

x = 0.9

4

6

8

10

8

10

12

14

6

8

10

4

5

6

4 8 12 16 20

4

5

6

Energy Transfer (meV)

x = 0.3

x = 0.4

x = 0.5

x = 0.7

x = 0.9

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oping

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SDW vs Superconductivity

M. Neupane et al, Phys. Rev. B 83, 094522 (2011)

Hole-Doping

D. Singh

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Copper vs Iron

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Copper Oxides Iron Arsenides

“The iron-based superconductors liberate us from the belief that the Mott physics is essential for high-temperature superconductivity, which is what has kept us stuck for such a long time.”Dung-Hai Lee

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“To take [the iron superconductors] as giving you information about the cuprates is to try to reconstruct the human figure from a cubist painting.”Philip Anderson

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Is there a glue?

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“This mythology is popular among science journalists, who dramatize both the element of competition

and the search for The Secret.”

P. W. Anderson, Science 316, 1705 (2007)

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66

Collaborators