1. BiS 2 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 Superconductors Long Island 6 th September 2013

2. BiS 2 –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: A 1g and B 2g are degenerate. Much more research is needed.

3. First observations in Bi 4 O 4 S 3 T c = 8.6K

4. Crystal structure for Bi 4 O 4 S 3 Tetragonal layered structure Charge reservoir

5. Physical properties Metallic But non-Fermi liquid Polycrystalline samples… Shielding fraction ~ 95% V.P.S. Awana, JACS (2012)

6. Electron doping: LaO(F)BiS 2 V.P.S. Awana et al. Sol. St. Comm. 157 (2013) 21 Electron doping

7. Superconducting dome K. Deguchi et al., EPL 101, 17004 (2013)

8. Electron doping: La 1-x M x OBiS 2 Electron doping M. B. Maple et al., PRB 87, 174512 (2013)

9. Hydrostatic pressure M. B. Maple et al., arXiv:1308.1072

10. Phonons?

11. Unconventional? Transport and magnetization measurements arXiv:1207.4955

12. Downfolding procedure Full DFT calculation for LaOBiS 2 6 Bi 6p, 12 S 3p, 6 O 2p H. Usui, K. Suzuki, and K. Kuroki PRB 86, 220501(R) (2012) 2 Bi 6p 2 S 3p 2 layers 2 Bi 6p 2 S 3p 1 layer

13. Band structure hole pockets Semiconductor to metal transition with electron doping

14. 2d view of the bands

15. Fermi surface nesting

16. RPA calculation for multi-band model Lindhard function (irreducible susceptibility) Bi 6p orbital NJP 11 (2009) 025016 and references [36] and [37].

17. Spin/charge susceptibility

18. Introducing interactions Coulomb intra-orbital Coulomb inter-orbital Hund’s coupling pair hopping inter-orbital Dyson-type equations: charge (orbital) spin 4x4 matrix involving all many-body terms.

19. RPA spin susceptibility

21. Obtaining the RPA pairing functions spin charge very pronounced peak at incommensurate k vector singlet pairing interaction projected pairing interaction eigenvalue problem, with matrix indexes k and k’ gap function g α associated to largest eigenvalue will have the highest T c

22. Pairing functions

23. Competition between A 1g and B 2g

24. Intra-band scattering

25. Nesting and pairing

26. Conclusions  Properties of new BiS 2 -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