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