Iron-Pnictide Superconductors as Multiband Correlated (Semi)Metals

1
Iron-Pnictide Superconductors as Multiband
Correlated (Semi)Metals
Zlatko Tesanovic, Johns Hopkins
University E-mail zbt_at_pha.jhu.edu
Web http//www.pha.jhu.edu/zbt
T. Y. Chen et al., Nature 453, 1224 (2008) V.
Cvetkovic and ZT, EPL 85, 37002 (2009)
arXiv/0804.4678 V. Cvetkovic and ZT,
arXiv/0808.3742

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

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Pnictides (Greek for chocking)Semiconductors ?
Semimetals ? Superconductors
3
Fe-pnictides Semimetals ? Superconductors
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Fe-pnictides Semimetals ? Superconductors
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Tc (K)
2008
courtesy of J. Hoffman
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Superconductivity in Fe
K. Shimizu et al. Nature 412, 316-318 (2001).
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Cu-oxides versus Fe-pnictides
However, there are also many differences! This
may add up to new and interesting physics
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Key Difference 9 versus 6 d-electrons

• In CuO2 a single hole in a filled 3d orbital
  shell
• A suitable single band model should work

• In FeAs large and even number of d-holes
• A multiband model is likely necessary

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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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Phase diagram of Fe-pnictides
Like CuO2, phase diagram of FeAs has SDW (AF) in
proximity to the SC state.
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ARPES
ARPES and dHvA see coherent (metallic) bands in
reasonable agreement with LDA.
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Minimal Model of FeAs layers I
V. Cvetkovic and ZT, EPL 85, 37002 (2009)
arXiv/0804.4678

• Puckering of FeAs planes is essential
• All d-orbitals are near EF
• Large overlap with As p-orbitals below EF ?
  enhanced itinerancy of d electrons
• defeats Hunds rule and large local moment

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Minimal Model of FeAs layers II
V. Cvetkovic and ZT, EPL 85, 37002 (2009)
arXiv/0804.4678
We consider an effective 2D model with 5 Fe 3
As orbitals
Two orbital model (S. Raghu, et al, PRB 77,
220503R (2008)) reconstructs the FS shape but not
its orbital content
dxz
dyz
dxy
dxx-yy
d2zz-xx-yy
odd parity
even parity
The importance of Fe 3d As 4p
hybridization Without pnictide atoms many
hopping processes would vanish by symmetry.
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Hunds Rule Defeated
V. Cvetkovic and ZT, EPL 85, 37002 (2009)
arXiv/0804.4678
Hunds rule rules for Mn2 all five
d-electrons line up to minimize Coulomb
repulsion ? S 5/2

• Puckering of FeAs planes is essential
• All d-orbitals are near EF
• Large overlap with As p-orbitals below EF ?
  enhanced itinerancy of d electrons
• defeats Hunds rule and large local moment

Haule, Shim and Kotliar, PRL 100, 226402 (2008)
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Minimal Model of FeAs layers III
V. Cvetkovic and ZT, EPL 85, 37002 (2009)
arXiv/0804.4678
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 of even
parity in FeAs plane ?
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Nesting and Valley Density-Wave (VDW) in
Fe-pnictides I
V. Cvetkovic and ZT, EPL 85, 37002 (2009)
arXiv/0804.4678
V. Cvetkovic and ZT, arXiv/0808.3742
Turning on moderate interactions ?
VDW itinerant multiband CDW (structural), SDW
(AF) and orbital orders at q M
ed
d
m
c
ec
SDW, CDW, ODW or combinations thereof ? VDW
Semiconductor
Semimetal
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Nesting and Valley Density-Wave (VDW) in
Fe-pnictides II
V. Cvetkovic and ZT, EPL 85, 37002 (2009)
arXiv/0804.4678
V. Cvetkovic and ZT, arXiv/0808.3742
d
c
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Fictitious Superconductor ? VDW in
Fe-pnictides
V. Cvetkovic and ZT, EPL 85, 37002 (2009)
arXiv/0804.4678
V. Cvetkovic and ZT, arXiv/0808.3742
(. . .)
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The Coastline of the Fermi Sea
?
New REOFeAs SC Tc ? 55K
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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
(V3Sn) one needs strong coupling theory (2?/Tc
? 4-6 )
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What can ? tell us about superconducting state ?
Our results for FeAs appear inconsistent with
these features
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? in FeAs superconductors I
T. Y. Chen et al., Nature 453, 1224 (2008)
BTK analysis ? 2D 13.34 0.3 meV TC 42 K
Conclusions Point-contact Andreev reflection
spectroscopy indicates a nodeless superconducting
gap and no SC pseudogap behavior. Very different
from high Tc cuprate superconductors !!
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? in FeAs superconductors II
V. Cvetkovic and ZT, EPL 85, 37002 (2009)
arXiv/0804.4678
Conclusions Conventional phonon-mechanism is
unlikely but so is Mott limit-induced repulsion
of the cuprate d-wave kind. We have something new
!!
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Emerging consensus (PCAR, ARPES, STM, mW,
)nodeless single in 1111, two s in
122, nodes ??
NMR sees nodal behavior ( T2 ) in 1111
Multiband superconductivity in Fe-pnictides !?
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Josephson Effect Between FeAs and Pb
X. Zhang et al., PRL 102, 147002 (2009)
Point Contact Junction

• I-V characteristics are resistively like a
  shunted junction
• Shapiro steps were observed under microwave
  irradiation

Strong indication of s-wave like SC state
courtesy of R. Greene
Ba1-xKxFe2As2/Pb
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Minimal Model of FeAs Layers III
V. Cvetkovic and ZT, EPL 85, 37002 (2009)
arXiv/0804.4678
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
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Interactions in FeAs I
V. Cvetkovic and ZT, EPL 85, 37002 (2009)
arXiv/0804.4678
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Interactions in FeAs II
V. Cvetkovic and ZT, EPL 85, 37002 (2009)
arXiv/0804.4678
V. Cvetkovic and ZT, arXiv/0808.3742
e-h
Typically, we find Ws is dominant ? Valley
density-wave(s) (VDW) in FeAs
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Valley Density-Wave (VDW) and SC in FeAs I
V. Cvetkovic and ZT, arXiv/0808.3742
V. Stanev, J. Kang, ZT, PRB 78, 184509 (2008)
Outcome combined SDW/CDW/ODW and structural
orders at q M ? VDW
Josephson terms in k-space (ccdd) play key
role in SC
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Valley Density-Wave (VDW) and SC in FeAs II
The condition for interband SC is actually
milder
but
RG calculations indicate, near a VDW state
A. V. Chubukov et al, PRB 78, 134512 (2008)
In Fe-pnictides interband superconductivity (s
or s- state) is a strong possibility (perhaps
with little help from phonons)
M. Parish, J. Hu, and B. A. Bernevig, PRB 78,
144514 (2008)
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Two Kinds of Interband Superconductivity
d
V. Stanev, J. Kang, ZT, PRB 78, 184509 (2008)
Interband pairing acts like Josephson coupling in
k-space. If G2 is repulsive ? antibound Cooper
pairs (sSC)
c
Type-II (intrinsic) interband SC
Type-I interband SC
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Unified Model of Valley Density-Wave (VDW) in
FeAs
V. Cvetkovic and ZT, arXiv/0808.3742
also, SO(6) Podolsky, Kee, Kim, arXiv/0812.2907
VDW in Fe-pnictides is a (nearly) highly
symmetric combination SDW/CDW/ODW
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Interplay of VDW and SC in FeAs I
This is true interband SC since U gt 0
different from U lt 0
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Interplay of VDW and SC in FeAs II
V. Cvetkovic and ZT, arXiv/0808.3742
Also, A. V. Chubukov et al, PRB 78, 134512 (2008)
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Conclusions

• Iron pnictides are semimetals turned
  superconductors
• Correlations are significant, hence a SDW in
  parent compounds, but weaker than in cuprates
• Superconducting gap has substantial s-wave
  character
• Both magnetism and superconductivity are
  intrinsically multiband in nature s interband
  SC is a likely possibility near a nesting-driven
  SDW

Zlatko Tesanovic, Johns Hopkins
University E-mail zbt_at_pha.jhu.edu
Web http//www.pha.jhu.edu/zbt