6/7/2016 Iron Superconductivity !! o Superconducting Gap in FeAs from PCAR o “Minimal” Model of FeAs planes – Different from CuO 2 !! o Multiband Magnetism and Superconductivity Zlatko Tesanovic, Johns Hopkins University Web: V. Cvetkovic and ZT, arXiv/ T. Y. Chen et al., Nature 453, 1224 (2008) Summer Blockbuster of 2008

Coastline of the Fermi Sea Fermi sea New REOFeAs SC T c  55K ?

What can  tell us about superconducting state ? Standard BCS theory works well in materials like Nb, Sn or Hg. In Pb and more complex systems (Va 3 Sn) one needs “strong coupling” theory (2  /T c  4-6 ) Cooper pair size = coherence length  electrons virtual phonons

Fermi sea What can  tell us about superconducting state ? Our results for FeAs are inconsistent with either of these features

Andreev Spectroscopy 5 Gap value from Andreev peaks 2   13.4 meV 2  /k B T C = 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

 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 !!

At 4.52 K BCS-like gap from BTK analysis 2  /k B T C = 3.68 closer to 3.53 (BCS s-wave) than 4.28 (BCS d-wave) 2  = ± 0.3 meV T C = 42 K

 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/

Emerge systematically ZBA obscures gap ZBA due to SC H field has small effect at 4.5K Emergence of Zero Bias Anomaly (ZBA) Contact size Small contact Large contact

G(V) -1 G(V) P. Xiong, G. Xiao, R. B. Laibowitz, Phys. Rev. Lett. 71, 1907 (1993) ZBA in s-wave Nb

Nb tip on Cu thin film (Chen et al)

V. Cvetkovic and ZT, arXiv/ “Minimal” Model of FeAs layers I “Puckering” of FeAs planes is essential: i)All d-orbitals are near E F ii)Large overlap with As p-orbitals below E F  enhanced itinerancy of d electrons defeats Hund’s rule and large local moment

V. Cvetkovic and ZT, arXiv/ Hund’s Rule Defeated “Puckering” of FeAs planes is essential: i)All d-orbitals are near E F ii)Large overlap with As p-orbitals below E F  enhanced itinerancy of d electrons defeats Hund’s rule and large local moment Hund’s rule rules for Mn 2+ : all five d-electrons line up to minimize Coulomb repulsion  S = 5/2

V. Cvetkovic and ZT, arXiv/ “Minimal” Model of FeAs layers II Important: Near E F e and h bands contain significant admixture of all five Wannier d-orbitals, d xz and d yz of odd parity in FeAs plane) and the remaining three d-orbitals of even parity in FeAs plane 

V. Cvetkovic and ZT, arXiv/ “Minimal” Model of FeAs layers III FeAs are different from CuO 2 Charge carriers are more itinerant and less localized on atomic sites. Multiband description is necessary, unlike an effective single band model of cuprates h1h1 h2h2 e1e1

Interactions in FeAs I V. Cvetkovic and ZT, arXiv/

Interactions in FeAs II Typically, we find V s is dominant  Valley density-wave(s) (VDW) in FeAs V. Cvetkovic and ZT, arXiv/ h1h1 h2h2 e1e1

Valley Density-Wave (VDW) and SC in FeAs Outcome: CDW (structural) and SDW (AF) orders at q = M V. Cvetkovic and ZT, arXiv/ VDW  SC Near VDW transition strong VDW fluctuations enhance interband repulsion.  SC state with ¢ (holes) and - ¢ (electrons).

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