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Multiband Effects in Fe- -pnictide Superconductors pnictide Superconductors Multiband Effects in Fe Zlatko Tesanovic Johns Hopkins University E-mail: zbt@pha.jhu.edu Web: http://www.pha.jhu.edu/~zbt V. Cvetkovic and ZT , arXiv/0804.4678 V.

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SLIDE 1
  • Iron Pnictides – Semimetals turned Superconductors
  • Minimal Model of FeAs planes – Different from CuO2 !!
  • Superconducting Gap in FeAs (PCAR, ARPES, mw, STM)
  • Multiband Magnetism and Superconductivity in FeAs

9/26/2008

Multiband Effects in Fe Multiband Effects in Fe-

  • pnictide Superconductors

pnictide Superconductors

Zlatko Tesanovic Johns Hopkins University

E-mail: zbt@pha.jhu.edu Web: http://www.pha.jhu.edu/~zbt

  • V. Cvetkovic and ZT, arXiv/0804.4678
  • T. Y. Chen et al., Nature 453, 1224 (2008)
  • V. Cvetkovic and ZT, arXiv/0808.3742

Summer Blockbuster of 2008 Summer Blockbuster of 2008

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SLIDE 2

Pnictides: Semiconductors Semimetals Superconductors

Pnictides – elements from Group V of Periodic Table: nitrogen, phosphorus, arsenic, antimony and bismuth III-V Semiconductors – formed by elements from Groups III and V: aluminium phosphide, aluminium arsenide, aluminium antimonide, gallium phosphide, gallium arsenide, gallium antimonide, indium phosphide, indium arsenide and indium antimonide plus numerous ternary and quaternary semiconductors.

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SLIDE 3

Fe-pnictides: Semimetals Superconductors

Spring 2008

Iron Pnictides

Greatest web-induced frenzy in history of condensed matter physics: 17 papers on arXiv in a single July day. Comparable to the latest superstring “revolution” (Bagger-Lambert)

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SLIDE 4

Hideo Hosono, TITech

Fe-pnictides: Semimetals Superconductors

May 2006

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SLIDE 5

1111 122 Fe As O RE 122

Cu-oxides versus Fe-pnictides

However, there are also many differences! This may add up to a new and interesting physics

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SLIDE 6

Key Difference: 9 versus 6 d-electrons

  • In CuO2 one d-hole in a half-filled single band
  • In FeAs large and even number of d-holes/several d-like bands

Even (FeAs) vs odd (CuO2) number of d-electrons Nearly half-filled (FeAs) vs nearly-filled (CuO2) d-shell

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SLIDE 7

Fe-pnictides: Semimetals Superconductors

In contrast to CuO2, all d- bands in FeAs are either nearly empty (electrons) or nearly full (holes) and far from being half-filled.

  • FeAs are less

correlated than CuO2

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SLIDE 8
  • C. de la Cruz, et al., Nature 453, 899 (2008)

Phase diagram of Fe-pnictides

Like CuO2, phase diagram FeAs has SDW (AF) in proximity to the SC state. SC coexists with SDW (AF) in 122 compounds

  • H. Chen, et al., arXiv/0807.3950
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SLIDE 9
  • K. Shimizu et al. Nature 412, 316-318 (2001).

Superconductivity in Fe

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SLIDE 10
  • V. Cvetkovic and ZT, arXiv/0804.4678

Minimal Model of FeAs layers I

“Puckering” of FeAs planes is essential: i)All d-orbitals are near EF ii)Large overlap with As p-orbitals below EF enhanced itinerancy of d electrons defeats Hund’s rule and large local moment

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SLIDE 11
  • V. Cvetkovic and ZT, arXiv/0804.4678

Hund’s Rule Defeated

“Puckering” of FeAs planes is essential: i)All d-orbitals are near EF ii)Large overlap with As p-orbitals below EF enhanced itinerancy of d electrons defeats Hund’s rule and large local moment Hund’s rule rules for Mn2+ : all five d-electrons line up to minimize Coulomb repulsion S = 5/2

Haule, Shim and Kotliar, PRL 100, 226402 (2008)

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SLIDE 12
  • V. Cvetkovic and ZT, arXiv/0804.4678

Minimal Model of FeAs layers II

Important: Near EF e and h bands contain significant admixture of all five Wannier d-orbitals, dxz and dyz of odd parity in FeAs plane) and the remaining three d-orbitals

  • f even parity in FeAs plane
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SLIDE 13

Valley Density-Wave (VDW) in Fe-pnictides

Outcome: CDW (structural) and SDW (AF) orders at q = M

  • V. Cvetkovic and ZT, arXiv/0804.4678
  • V. Cvetkovic and ZT, arXiv/0808.3742
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SLIDE 14

Fictitious “Superconductor” VDW in Fe-pnictides

  • V. Cvetkovic and ZT, arXiv/0808.3742
  • V. Cvetkovic and ZT, arXiv/0804.4678

?

What about real superconductivity ?

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SLIDE 15

Coastline of the Fermi Sea

Fermi sea

+

  • Fermi sea

+ +

  • Fermi sea

New REOFeAs SC Tc ∼ 55K

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SLIDE 16

What can Δ tell us about superconducting state ?

Standard BCS theory works well in materials like Nb, Sn or Hg. In Pb and more complex systems (Va3Sn)

  • ne needs “strong coupling” theory (2Δ/Tc ∼ 4-6 )

Cooper pair size = coherence length ξ

electrons virtual phonons

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SLIDE 17

Fermi sea

+ +

  • What can Δ tell us about superconducting state ?

Our results for FeAs appear inconsistent with these features

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SLIDE 18

Andreev Spectroscopy Andreev Spectroscopy

18

Gap value from Andreev peaks 2Δ ≈ 13.4 meV 2Δ/kBTC = 3.68 (BCS) Extra features beyond gap (contact specific) Slanted background [Always G(-V) > G(V)] Experimental setup Gold tip in contact with FeAs SC

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SLIDE 19

Δ in FeAs superconductors I

  • T. Y. Chen et al., Nature 453, 1224 (2008)

Conclusions: Nodeless superconducting gap and no pseudogap behavior. Very different from high temperature cuprate superconductors !!

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SLIDE 20

Δ in FeAs superconductors II

Conclusions: Conventional phonon-mechanism is unlikely but so is Mott limit-induced repulsion of the cuprate d-wave kind. We have something new !! Only a single superconducting gap – sign/phase could be different for holes and electrons.

  • V. Cvetkovic and ZT, arXiv/0804.4678
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SLIDE 21
  • Emerge systematically
  • ZBA obscures gap
  • ZBA due to SC
  • H field has small effect at 4.5K

Emergence of Zero Bias Anomaly (ZBA) Emergence of Zero Bias Anomaly (ZBA)

Contact size

Small contact Large contact

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SLIDE 22

G(V)-1

G(V)

  • P. Xiong, G. Xiao, R. B. Laibowitz, Phys. Rev. Lett. 71, 1907 (1993)

ZBA in s ZBA in s-

  • wave Nb

wave Nb

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SLIDE 23

Nb tip on Cu thin film (Chen Nb tip on Cu thin film (Chen et al et al) )

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SLIDE 24
  • 18 -15 -12 -9
  • 6
  • 3

3 6 9 12 15 18 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0

d4 d4 d5 d5 d4 d4 d3 d3 d3 d3 d3 d2 d2 d2 d2 d2 d2 d2 d2 d1 d1 d1 d1

3.5Ω, 1.19Ω 4.4Ω, 0.62Ω 2.3Ω, 1.27Ω 3.1Ω 4.4Ω 6.6Ω 9.7Ω 10.7Ω

dI/dV V (mV)

Nb tip on Cu thin film (Chen Nb tip on Cu thin film (Chen et al et al) )

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SLIDE 25
  • V. Cvetkovic and ZT, arXiv/0804.4678

Minimal Model of FeAs layers III

FeAs are different from CuO2 Charge carriers are more itinerant and less localized on atomic sites. Multiband description is necessary, unlike an effective single band model of cuprates

h1 h2 e1

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SLIDE 26

Interactions in FeAs I

  • V. Cvetkovic and ZT, arXiv/0804.4678
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SLIDE 27

Interactions in FeAs II

Typically, we find Ws is dominant Valley density-wave(s) (VDW) in FeAs

  • V. Cvetkovic and ZT, arXiv/0804.4678

h1 h2 e1 e-h

  • V. Cvetkovic and ZT, arXiv/0808.3742

These “Josephson” terms do not appear in fictitious superconductor analogy Could they cause real SC ?

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SLIDE 28

Valley Density-Wave (VDW) and SC in FeAs

Outcome: combined SDW (AF) and CDW/structural orders at q = M

  • V. Cvetkovic and ZT, arXiv/0808.3742

VDW SC

Near VDW transition VDW fluctuations enhance interband “Josephson” repulsion.

  • SC state with ∆ (holes) and -∆ (electrons).
  • V. Stanev, J. Kang, ZT, arXiv/0809.0014

“Josephson” terms in k- space (c+c+dd) play key role in SC

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SLIDE 29
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SLIDE 30
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SLIDE 31
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SLIDE 32

THE END THE END

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SLIDE 33
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SLIDE 34
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SLIDE 35

Valley Density-Wave (VDW) in Fe-pnictides

Outcome: CDW (structural) and SDW (AF) orders at q = M

  • V. Cvetkovic and ZT, arXiv/0804.4678
  • V. Cvetkovic and ZT, arXiv/0808.3742
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SLIDE 36

Valley Density-Wave (VDW) in Fe-pnictides

Outcome: CDW (structural) and SDW (AF) orders at q = M

  • V. Cvetkovic and ZT, arXiv/0804.4678
  • V. Cvetkovic and ZT, arXiv/0808.3742
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SLIDE 37

At 4.52 K

BCS-like gap from BTK analysis

2Δ/kBTC = 3.68 closer to 3.53 (BCS s-wave) than 4.28 (BCS d-wave) 2Δ = 13.34 ± 0.3 meV TC = 42 K