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David Mandrus, Oak Ridge National Lab.
BaFe2As2: A Model Platform for Unconventional Superconductivity
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David Mandrus Brian Sales Rongying Jin (now at LSU)
Correlated Electron Materials Group
Michael McGuireWigner Fellow
Athena SefatWigner Fellow
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Presentation_name
Collaborators
ORNL: A. Christianson, M. Lumsden, S. E. Nagler (neutrons); J. Howe (electron microscopy); A. Payzant (X-rays); D. Christen (flux dynamics); M. Pan (tunneling); D. J. Singh (theory)
NHML/FSU: Larbalestier group, Tozer group
UCSD: Maple group (pressure), Basov group (IR)
McMaster: Imai group (NMR), Y. Mozharivskyj (crystallography)
Julich/Liege: R. Hermann (Mossbauer)
UTK: Keppens group (elastic properties), Mannella group (X-ray spectroscopy, ARPES), Egami group (PDF), Plummer group (STM)
LANL: F. Ronning, R. Movshovich, E. D. Bauer, J. D. Thompson
Houston: S. Pan group
LSU: Plummer Group, Zhang group, Jin group
UIUC: L. Greene group
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Heavy FermionSuperconductors CupratesIron-based
Superconductors
Chemically very different, but evidence points to similar SC mechanism
Pnictides are the Missing Link Between Cuprates and HFs
HF physics + SCOxide physics + SC
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S. Sanna, et al. PRL 93, 207001
G. Knebel, et al. PRB 74, 020501 (2006)
Canfield group,arXiv: 0905.4894
J. Zhao, et al. Nature Materials 7, 953 (2008)
H. Luetkens, et al. Nature Materials 8, 305 (2009)
Great Similarity of Phase Diagrams
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Greven group,Cond-mat: 0903.2291
Y. J. Uemura,Cond-mat: 0903.2758
Er ~ 4kBTc
Er/2Δ ~ 0.64
Universal Behavior of Spin Resonance
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Motivation and Perspective
Evidence points to a common mechanism underlying HF, Pnictide, and Cuprate SC
Focus on materials-specific properties
Work toward a predictive understanding
Doping studies are important in this regard
122 Materials show great chemical flexibility
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Some of Known Fe compounds (Before 1991) with the ThCr2Si2 Structure
EuFe2As2 KFe2As2 BaFe2As2 SrFe2As2 DyFe2B2 HoFe2B2 TmFe2B2 BaFe2P2
CaFe2P2 CeFe2Ge2 ErFe2B2 LuFe2B2 YFe2B2 CeFe2P2 GdFe2B2 TbFe2B2
CeFe2Si2 DyFe2Si2 ErFe2Ge2 EuFe2P2 DyFe2Ge2 ErFe2Si2 EuFe2Si2 LaFe2Ge2
LaFe2P2 SmFe2Ge2 UFe2Ge2 LaFe2Si2 NdFe2Si2 TlFe2Se2 ThFe2Si2 YFe2Si2UFe2P2 GdFe2Ge2 NdFe2Ge2 TbFe2Ge2 YbFe2Ge2 LuFe2Si2 PrFe2Si2 SmFe2Si2TmFe2Si2 YbFe2Si2 PrFe2Ge2 ThFe2Ge2 HoFe2Si2 SrFe2P2 TbFe2Si2 TlFe2S2
UFe2Si2 ZrFe2Si2
Crystals with ThCr2Si2 Structure(BaFe1.84Co0.16As2)
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M. Rotter, et al. PRL 101, 107006 (2008)
ThCr2Si2 structure I4/mmm
Layers of edge-sharing FeAs4 tetrahedra
Coupled SPT-AFM transition ~140 K
BaFe2As2 Basic Properties
D. Johrendt, R. Poettgen 0902.1085
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Managed by UT-Battellefor the Department of Energy
Magnetic Order
+-+-
+-+-
+-+ -
+-+ -
1 D Chains of parallel spin Fe atoms.
Y. Su, et al. PRB 79, 064504 (2009)
BaFe2As2
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BaFe2As2 Properties
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BaFe2As2 is not very anisotropic
ρc/ρab ≈ 5
M. A. Tanatar, et al. PRB 79, 134528 (2009) (Canfield group)
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Surface of BaFe2As2
T = 4.3 K
In collaboration with E. W. Plummer group & S.H. Pan group
Cleaved surface of BaFe2As2 is As terminated
Ba layer destroyed, some random Ba atoms observed on surface
No surface reconstruction
Some evidence of orbital order since only one DOS of one type of As was detected
Bulk As-Fe and As-Ba interlayer distancesare 1.3437 A and 1.8926 A respectively
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PRL 101, 107006 (2008)
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M. Rotter, et al. Angew. Chem. Int. Ed. 47, 7949 (2008)
Ba1-xKxFe2As2
Phase Diagram
J. T. Park, et al. PRL 102, 117006 (2009)Stuttgart group—Keimer, Hinkov
Electronically phase separated ona scale of ~65 nm
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Presentation_name
• Electron rather than hole doping
• Superconductivity robust to in-plane disorder
• Evidence for s symmetry superconducting state
• Very “clean” system experimentally—crystals quite homogeneous
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Ba(Fe1-xCox)2As2 Phase Diagram
C. Lester, et al. PRB 79, 114523 (2009)I.R. Fisher, S. M. Hayden groups
D. K. Pratt, et al. Cond-mat 0903.2833P. C. Canfield, R. J. McQueeney groups
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“No superconducting phase separation”--S.-H. Pan
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Resonance in Single Crystal BaCoResonance in Single Crystal BaCo0.160.16FeFe1.841.84AsAs22
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Neutron Scattering on Underdoped BaFe1.92Co0.08As2
Tc = 11 K
Drop in intensity of Bragg Peak below Tc
Resonance obeseved at 4.5 meV = 4.7 kBTc
Spectral weight transferred from Bragg peaks to resonance?
Scattering is 3D vs. 2D in optimally doped
A. D. Christianson, et al. Cond-mat 0904.0767
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advantages of RUS: all elastic constants can be obtained in one measurement
small samples (mm3)
The University of Tennessee
1.0x106 1.1x106 1.1x106 1.1x106 1.2x1060.0
0.5
1.0
1.5
2.0
2.5
Ampl
itude
(V)
Frequency (Hz)
Veerle Keppens
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0
T
Mo
du
lus
0T
Strain-Order Parameter Coupling at Phase Transitions
Bi-Linear coupling:Fc = βQε
Quadratic coupling:Fc = βQ2ε
Mo
du
lus
θ−−=
Tacc 0
c
c
TThcc
TTcc
<−=
>=
β
20
0
2
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00.10.20.30.40.50.60.70.80.9
50 100 150 200 250 300 350
T(K)
f(MH
z)
BaFe2As2
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0 50 100 150 200 250 300
0.27
0.28
0.29
0.30
0.31
0.32
0.33
0 50 100 150 200 250 3000.6
0.7
0.8
0.9
Squ
ared
Fre
quen
cy (
MH
z2 )
Temperature (K)
BaFe2As2
S
quar
ed F
requ
ency
(MH
z2 )
Temperature (K)
BaFe1.8Co0.2As2
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Susceptibility not Curie-Weiss -- looks like Cr
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EF is only 20-40 meV abovethe bottom of the electron bands
Usual assumption that kBT << EF is not valid
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D. Mandrus, et al. PRB 51, 4763 (1995)
Sharp Features in the DOS and a variable carrier concentration can give you a susceptibility that increases with T
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B. C. Sales, et al. Cond-mat: 0906.2134
This simple model, with only somewhat unreasonable parameters, can describe Hall, Seebeck coefficient, and susceptibility as a function of T and doping.
N0 = 1.4 x 1021/cm3
EH = 250 K
me* = 17 m0
mh* = 30 m0
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Concluding Speculation
Peak in T.C. ResonanceCorrelates With
J. G. Checkelsky, et al. 0811.4668(Ong group) Lumsden, et al.
PRL 102 107005 (2009)
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Discussion Questions
1) What causes the structural and magnetic transitions to separate in Ba(Fe1-xCox)2As2
2) How do we explain the temperature dependence of the susceptibility?
3) Are spin fluctuations causing the elastic constants to soften?