Iron based high temperature superconductors

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Iron based high temperature superconductors

• K Haule, Rutgers University

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Technologically relevant
Wires fabricated by the powder-in-tube (PIT)
method
Zhaoshun Gao, et.al., arXiv 0806.2451
Jc up to 2105 A/cm2
(Hc2) up to 120 T
More three-dimensional than cuprates
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How it all started.
Published in Chemical journal (Journal of
American Chemical Society) Received January 2008,
published online Feb 2008
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And exploded.
more than 23 cond-mats in March 2008
gt260 preprints at the end of July
mostly from China!
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First family of SC
SmFxO1-xFeAs x0.2 d) Tc55K, cm/0803.3603 a3.933A, c8.4287A
PrFxO1-xFeAs c) Tc52K, cm/0803.4283 a3.985A, c8.595A
CeFxO1-xFeAs b) Tc41 K, cm/0803.3790 a3.996A, c8.648A
LaFxO1-xFeAs a) Tc26 K, JACS-2008 a4.036A, c8.739 A
La1-xSrxOFeAs Tc25K, cm/0803.3021, a4.035A, c 8.771A
Smaller c

1. Y. Kamihara et.al., Tokyo, JACS
2. X.H. Chen, et.al., Beijing,arXiv 0803.3790
3. Zhi-An Ren, Beijing, arXiv 0803.4283
4. Zhi-An Ren, Beijing, arXiv 0804.2053.

Rare earths
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Crystal StructureTetragonal I4/mmm
R O1-xFx FeAs R O1-x FeAs
F not important, vacancy fine

• 2D square lattice of Fe
• Fe - magnetic moment
• As-similar then O in cuprates

But As not in plane!
Perfect tetrahedra 109.47
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Variety of materials
CaFe2As2, (Tc12K _at_ 5.5GPa), Milton S.
Torikachvili, arXiv0807.0616v2
Li1-xFeAs, (Tc18K), X.C.Wang et.al.,
arXiv0806.4688
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What is the glue?
KH, J.H. Shim, G. Kotliar, cond/mat 0803.1279
(PRL. 100, 226402 (2008))
Phonons give Tclt1K
L. Boeri, O. V. Dolgov, A. A. Golubov
arXiv0803.2703 (PRL, 101, 026403 (2008))
llt0.21, Tclt0.8K
Not conventional superconductors!
Y. Kamihara et.al., J. Am. Chem. Soc. 130, 3296
(2008).
Huge spin susceptibility (50 x Pauli)
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Phase diagrams SmFeAsO
magneto-transport experiments
muon spin rotation
A. J. Drew et.al., arXiv0807.4876.
Very similar to cuprates, log(T) insulator due to
impurities
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Phase diagrams
Stoichiometric compound
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Common features of the parent compound
SmOFeAs
CaFe2As2 and Ca0.5Na0.5Fe2As2
Enormous normal state resistivities!
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Magnetic and structural PT
LaOFeAs
arXiv0806.3304v1
Clarina de la Cruz, Nature 453, 899 (2008).
In single crystals of 122 seems TM and TS close
or the same
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Fe magnetism ?
Weak structural distortion 150 K from
tetragonal to orthorombic
SDW (stripe AFM) at lower T
Neutrons by Clarina de la Cruz et.al, Nature
453, 899 (2008).
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Itinerancy Frustration
The undoped compound is metal (although very bad
one 1mWcm), hence moment is partially screened
Magnetic exchange interaction is very frustrated
(Qimiao Si, Elihu Abrahams, arXiv0804.2480)
Exchange interactions are such that J2J1/2, very
strong frustration, (KH, G. Kotliar, arXiv
0805.0722)
For the doped compound, LDA structural
optimization fails for non-magnetic state! (It
is very good if magnetism is assumed)
For non-magnetic state, LDA predicts 1.34Å
shorter FeAs distance (10.39 instead of
11.73). One of the largest failures of LDA.
Paramagnetic state must have (fluctuating) magneti
c moments not captured in LDA
T. Yildirim, arXiv 0807.3936
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Signatures of moments in 1111 compounds
Doped LaOFeAs
Susceptibility 50xlarger than Pauli LDA
T. Nomura et.al., 0804.3569
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Susceptibility of 122
Single crystal!
R. Klingeler et.al., arXiv0808.0708v1
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Nonmagnetic impurities not detrimental to SC

• Fe replaced by Co
• Impurities do not destroy SC (like Zn doping in
  cuprates)
• No signature of Curie-Weiss susc.

F.L. Ning et.al, arXiv0808.1420
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Band structure of LaOFeAs
LDA DOS
LDA Mostly iron bands at EF (correlations
important)
6 electrons in 5 Fe bands Filling 6/10 -gt large
spin
The 5-band Hubbard-type model As(p)-Fe(d)
hybridization weak
KH, J.H. Shim, G. Kotliar, cond/mat 0803.1279
(PRL. 100, 226402 (2008))
Hoppings available at http//www.physics.rutgers.e
du/haule/FeAs/
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DMFT for LaFxO1-xFeAs
LDADMFT LaOFeAs is at the verge of the
metal-insulator transition (for realistic U4eV,
J0.7eV) For a larger (U4.5, J0.7eV)
semiconducing insulator
Not a one band model all 5 bands important (for
Jgt0.3)
Need to create a singlet out of spin and orbit
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Importance of Hunds coupling
LaO1-0.1F0.1FeAs
Hubbard U is not the relevant parameter.
The Hunds coupling brings correlations!
Specific heat within LDADMFT for
LaO1-0.1F0.1FeAs at U4eV
For J0 there is negligible mass enhancement at
UW!
The coupling between the Fe magnetic moment and
the mean-field medium (As-p,neighbors Fe-d)
becomes ferromagnetic for large Hunds coupling!
KH, G. Kotliar, cond/mat 0803.1279
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ARPES on Ba0.6K0.4Fe2As2
good nesting
LDA(LAPW) calculation
C. Liu, et.al., arXiv 0806.3453
Gaps on the two FS around G are very different
H. Ding et.al., arXiv0807.0419
Large gap in the inner G and M
Small gap in the outer G pocket
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Anisotropy of the gap
Gap size
H. Ding et.al., arXiv0807.0419
Fermi surface
Negligible anisotropy. D-wave gap excluded!
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Other common possibilities
d cos(kx)-cos(ky)
ex-s cos(kx)cos(ky)
s-constant

-
G
-

M
Nodes in the gap
No nodes, but gap different sign on G and M
maybe
no
Most likely
Chubukov et.al., arXiv 0807.3735
and many other possibilities
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Conclusions

• Variety of materials with common SDW feature and
  SC
• 1111 LaOFeAs, CeOFeAs,SmOFeAs
• 122 BaFe2As2, CaFe2As2
• 111 LiFeAs
• 11 FeSe, FeTe
• Highest Tc55K achieved in SmFxO1-xFeAs
• Some similarities with cuprates, but also
  differences (Co doping)
• Correlations weaker than in cuprates (not doped
  Mott insulators)
• ARPES shows almost uniform gap on FS sheets
  (s-wave, extended s-wave,.)
• Other probes of gap symmetry are still
  controversial (1/T1T3, 1/l2exp/powerlaw,
  Cvexp/powerlaw)

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Optics in LaFeAsO
A.V. Boris et.al, arXiv 0806.1732
Neff (LDA) 0.37
Neff (exp) 0.1 Neff(DMFT)
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One more possibility
Fa Wang et.al., arXiv 0807.0498
Numerical Renormalization Group advanced way
of summing LOD
inter-pocket Josephson tunneling
The gap has the same sign on all FS pockets.
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DMFT for LaFxO1-xFeAs
In LaOFeAs semiconducting gap is opening Large
scattering rate at 116K
10 doping
T116 K
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LaOFeAs under pressure Tc43K
Hiroki Takahashi et.al., Nature 453, 376-378
(2008)
Very incoherent in normal state (large
resistivity)
High Tc