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BiS2 compounds: Properties, effective low-energy models and RPA results
George Martins (Oakland University)Adriana Moreo (Oak Ridge and Univ. Tennessee)Elbio Dagotto (Oak Ridge and Univ. Tennessee)
Supported by DOE through Oak Ridge Nat. Lab.
Martins, Moreo, and Dagotto PRB 87, 0811021(R) (2013)
International Workshop on Recent Developments in Fe-based HTc SuperconductorsLong Island 6th September 2013
BiS2 –based family of superconductors
Basic properties• Experimental results (brief summary)• Band structure (2-band effective model)
Phonons or unconventional? RPA results Fermi surface nesting:
• Peculiar pairing properties: A1g and B2g are degenerate.
• Much more research is needed.
First observations in Bi4O4S3
Tc = 8.6K
Crystal structure for Bi4O4S3
Tetragonal layered structure
Charge reservoir
Physical properties
MetallicBut non-Fermi liquid
Polycrystalline samples…
Shielding fraction ~ 95%
V.P.S. Awana, JACS (2012)
Electron doping: LaO(F)BiS2
V.P.S. Awana et al.Sol. St. Comm. 157 (2013) 21
0.5 0.5 2
, , Pr, ,
Ln O F Bi S
Ln La Ce Nd Yb
10.6, 3.0, 5.6, 4.3, 5.3cT K
2O F
Electron doping
Superconducting dome
K. Deguchi et al., EPL 101, 17004 (2013)
Electron doping: La1-xMxOBiS2
4
4
4
4
Th
Hf
Zr
Ti
3La
Electron doping
M. B. Maple et al., PRB 87, 174512 (2013)
Hydrostatic pressure
M. B. Maple et al., arXiv:1308.1072
Phonons?
Unconventional?
Transport and magnetization measurementsB0.096Bi
arXiv:1207.4955
0T
14T
Downfolding procedure
Full DFT calculation for LaOBiS2
6 Bi 6p, 12 S 3p, 6 O 2p
H. Usui, K. Suzuki, and K. Kuroki PRB 86, 220501(R) (2012)
2 Bi 6p2 S 3p2 layers
2 Bi 6p2 S 3p1 layer
Band structure
hole pocketsSemiconductor to metal transition with electron doping
2d view of the bands
Fermi surface nesting
RPA calculation for multi-band model
,
1, , ,
n
pqst n sp n qt n
k i
q i G k i G k q i iN
,s p
sp n
n
a k a kG k i
i E k
sa k s k
Lindhard function (irreducible susceptibility)
Bi 6p orbital
NJP 11 (2009) 025016 and references [36] and [37].
s
Spin/charge susceptibility
Introducing interactions
Coulombintra-orbital
Coulombinter-orbital
Hund’scoupling
pair hoppinginter-orbital
Dyson-type equations:
charge(orbital)
spin
4x4 matrix involving all many-body terms.
3
4V U J J 2
JJ
RPA spin susceptibility
1.65U
0.3J
U
1.0n
RPA spin susceptibility
1.65U
0.3J
U
1.0n
Obtaining the RPA pairing functions
spin charge
very pronounced peak at incommensurate k vector
singlet pairinginteraction
projected pairing interaction
eigenvalue problem, with matrix indexes
k and k’
gap function gα associated to largest eigenvalue will have the highest Tc
Pairing functions
Competition between A1g and B2g
Intra-band scattering
Nesting and pairing
Conclusions
Properties of new BiS2-based SC family was presented:• Layered material • Semiconducting /metallic• 2 6p bands minimum model• Phonons or unconventional?
RPA results show:• Magnetic fluctuations dominate• Pairing is highly dependent on nesting• A1g (no nodes) and B2g (nodal) are degenearte