Title: Valley DensityWave and Superconductivity in Iron Pnictides
1Valley Density-Wave and Superconductivity in Iron
Pnictides
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, arXiv/0804.4678
V. Cvetkovic and ZT, arXiv/0808.3742
- Iron Pnictides Semimetals turned
Superconductors - Superconducting Gap in FeAs (PCAR, ARPES, mw,
STM) - Minimal Model of FeAs planes Different from
CuO2!! - VDW and Multiband Superconductivity in FeAs
2PnictidesSemiconductors ? Semimetals ?
Superconductors
3Fe-pnictides Semimetals ? Superconductors
4Fe-pnictides Semimetals ? Superconductors
5Superconductivity in Fe
K. Shimizu et al. Nature 412, 316-318 (2001).
6Cu-oxides versus Fe-pnictides
However, there are also many differences! This
may add up to a new and interesting physics
7Key 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
8Fe-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
9Phase diagram of Fe-pnictides
Like CuO2, phase diagram of FeAs has SDW (AF) in
proximity to the SC state.
10ARPES
ARPES and dHvA see coherent (metallic) bands in
reasonable agreement with LDA.
11Minimal Model of FeAs layers I
V. Cvetkovic and ZT, arXiv/0804.4678
2 orbs Stanford-Berkeley, 3 orbs PAL,
- 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
12Hunds Rule Defeated
V. Cvetkovic and ZT, 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)
13Minimal Model of FeAs layers II
V. Cvetkovic and ZT, 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 ?
14Valley Density-Wave (VDW) in Fe-pnictides
V. Cvetkovic and ZT, arXiv/0804.4678
V. Cvetkovic and ZT, arXiv/0808.3742
Outcome CDW (structural), SDW (AF) and orbital
orders at q M
15Fictitious Superconductor ? VDW in
Fe-pnictides
(. . .)
16 The Coastline of the Fermi Sea
?
New REOFeAs SC Tc ? 55K
17What can ? tell us about superconducting state ?
Standard BCS theory works well in materials like
Nb, Sn or Hg. In Pb and more complex systems
(Va3Sn) one needs strong coupling theory (2?/Tc
? 4-6 )
18What can ? tell us about superconducting state ?
Our results for FeAs appear inconsistent with
these features
19Andreev Spectroscopy
Experimental setup Gold tip in contact with FeAs
SC
Gap value from Andreev peaks 2? ? 13.4
meV 2?/kBTC 3.68 (BCS) Extra features beyond
gap (contact specific) Slanted
background Always G(-V) gt G(V)
20 ? 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 Nodeless superconducting gap and no
sizeable pseudogap behavior. Very different from
high Tc cuprate superconductors !!
21 ? in FeAs superconductors II
V. Cvetkovic and ZT, 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
!!
22Emerging 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 !?
23Minimal Model of FeAs layers III
V. Cvetkovic and ZT, arXiv/0804.4678
FeAs are different from CuO2 Charge
carriers are more itinerant and less localized on
atomic sites. Multiband description is necessary,
unlike the effective single band model of cuprates
24Interactions in FeAs I
V. Cvetkovic and ZT, arXiv/0804.4678
25Interactions in FeAs II
V. Cvetkovic and ZT, 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
26Valley Density-Wave (VDW) and SC in FeAs I
V. Cvetkovic and ZT, arXiv/0808.3742
V. Stanev, J. Kang, ZT, arXiv/0809.0014
Outcome combined SDW/CDW/ODW and structural
orders at q M
Josephson terms in k-space (ccdd) play key
role in SC
27Valley Density-Wave (VDW) and SC in FeAs II
The condition for interband SC is actually
milder
but
RG calculation indicates, near a VDW state
In Fe-pnictides interband superconductivity (s
or s- state) is a strong possibility (perhaps
with little help from phonons)
M. Parish et al., PRB 78, 144514 (2008)
28Unified Model of Valley Density-Wave (VDW) in
FeAs
VDW in Fe-pnictides is a (nearly) highly
symmetric combination SDW/CDW/ODW
29VDW and SC in FeAs I
This is true interband SC since U gt G2
different from U lt G2
30VDW and SC in FeAs II
V. Cvetkovic and ZT, arXiv/0808.3742
Also, A. V. Chubukov et al, PRB 78, 134512 (2008)
31THE END
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