‘Tc’ ~ 70 K: Fe-Based SC 200 Tc (K) 100 1970 1980 1990 2000 2010 year 1UC FeSe on STO 100 ~ 70 K 55 K SmFeAsO 2012 LaFeAsO 2008 LaFePO 2006 1970 1980 1990 2000 2010 year

FeSe Monolayer on a SrTiO3 Substrate Tc = 65±5 K (100 K ?) Se Fe Se SrTiO3 I. Mazin, Nature Mater. 14, 755 (2015). Review Dung-Hai Lee, Chin. Phys. B 24, 117405 (2015).

FeSe Monolayer on a SrTiO3 Substrate Tc S. He, Q-K. Xue, X.J. Zhou et al., Nature Mater. 12, 605 (2013).

FeSe Monolayer on a SrTiO3 Substrate Tc Tc = 108 K ?? Science Bulletin 60 (14), 1301-1304 (2015) S. He, Q-K. Xue, X.J. Zhou et al., Nature Mater. 12, 605 (2013). Z. Zhang, D.-L. Feng, Yayu Wang et al., arXiv: 1507.00129 J.F. Ge, Q.K. Xue et al., Nature Mater. 14, 285 (2015).

Z. Zhang, K.A. Moler, D.L. Feng, Yayu Wang, arXiv: 1507.00129; Onset of Meissner effect in FeSe monolayer on Nb doped SrTiO3 substrate Z. Zhang, K.A. Moler, D.L. Feng, Yayu Wang, arXiv: 1507.00129; Science Bulletin 60 (14), 1301 (2015)

Low-energy mSR: Depth profile

Thickness dependence of Tc for FeSe films on STO 80 60 Tc (K) 40 electron doping applying pressure Y. Mizuguchi et al., Appl. Phys. Lett. 93, 152505 (2008). Medvedev et al., Nature Materials 8, 630 (2009). Margadonna et al., Phys. Rev. B 80, 064506 (2009). 20 1UC 2UC 3UC 4UC 50UC ∞UC FeSe Thickness

Pressure dependence of Tc for bulk FeSe J.P. Sun, T. Shibauchi et al., arXiv: 1512.06951; Nat. Commun. 7, 12146 (2016)

Thickness dependence of Tc for FeSe films on STO 80 60 Tc (K) 40 electron doping applying pressure Y. Mizuguchi et al., Appl. Phys. Lett. 93, 152505 (2008). Medvedev et al., Nature Materials 8, 630 (2009). Margadonna et al., Phys. Rev. B 80, 064506 (2009). 20 1UC 2UC 3UC 4UC 50UC ∞UC FeSe Thickness

Thickness dependence of Tc for FeSe films on STO 80 C. Tang, Q.-K. Xue et al., arXiv: 1509.08950; Phys. Rev. B92, 180507(R) (2015). 60 Y. Miyata, T. Takahashi et al., Nature Mater. 14, 775 (2015). Tc (K) C.H.P. Wen, D.-L. Feng et al., arXiv: 1508.05848; Nat. Commun. 7, 10840. 40 J. Shiogai et al., Nature Phys. 12, 42 (2016). 20 1UC 2UC 3UC 4UC 50UC ∞UC FeSe Thickness

SC of 2UC-FeSe (double-layer) on STO C. Tang, Q.-K. Xue et al., arXiv: 1509.08950; Phys. Rev. B92, 180507(R) (2015).

Electron Doping into FeSe Films on STO K deposition Electric field SrTiO3 MgO Y. Miyata et al., Nature Mater. 14, 775 (2015). J. Shiogai et al., Nature Phys. 12, 42 (2016).

Electron Doping into FeSe Films on STO K deposition Electric field Y. Miyata et al., Nature Mater. 14, 775 (2015). J. Shiogai et al., Nature Phys. 12, 42 (2016).

Electron Doping into FeSe Films on STO K deposition C.-L. Song, Q.-K. Xue et al., arXiv: 1511.02007; Phys. Rev. Lett. 116, 157001 (2016). Y. Miyata et al., Nature Mater. 14, 775 (2015).

Electron Doping into FeSe Films on STO I. Mazin, Nature Mater. 14, 755 (2015).

FeSe Monolayer on SrTiO3: Interface Effect (1) Tc = 65±5 K Se Fe Interface effects : Electron transfer (doping) from O vacancies in the STO surface layer (?) Doping/transfer is only to the bottom FeSe layer – Open question (?) Se SrTiO3 Se O O Dung-Hai Lee,Chin. Phys. B 24, 117405 (2015). Ti Ov Review O

Interface Atomic Structure: 1UC-FeSe on STO F. Li, Q.-K. Xue et al., arXiv: 1512.05203. (State Key Labo. of Low-Dimensional Quantum Physics, State Key Labo. of New Ceramics and Fine Processing, Tsinghua Univ.) Se O O Ti O OV Ti O O Sr TiO2-x

FeSe Monolayer on SrTiO3: Charge Transfer (?) Hao Ding, Q-K. Xue et al., arXiv: 1603.00999; Phys. Rev. Lett. 117, 067001 (2016). J. Shiogai et al., Nature Phys. 12, 42 (2016).

Enhanced Tc in Monolayer FeSe on STO 80 C. Tang, Q.-K. Xue et al., arXiv: 1509.08950; Phys. Rev. B92, 180507(R) (2015). 60 Y. Miyata, T. Takahashi et al., Nature Mater. 14, 775 (2015). Tc (K) C.H.P. Wen, D.-L. Feng et al., arXiv: 1508.05848; Nat. Commun. 7, 10840. 40 J. Shiogai et al., Nature Phys. 12, 42 (2016). 20 1UC 2UC 3UC 4UC 50UC ∞UC FeSe Thickness

FeSe Monolayer on a SrTiO3 Substrate Tc = 65±5 K Se Fe Interface effects : Electron transfer (doping) from O vacancies in STO (?) An additional pairing channel from a coupling with the bond-stretching O vibrations (W0 ~ 100 meV) in SrTiO3. Se SrTiO3 Se O O Dung-Hai Lee,Chin. Phys. B 24, 117405 (2015). Ti OV Review O

FeSe on STO: Interface Effect (2) Dung-Hai Lee,　Chin. Phys. B 24, 117405 (2015).

Proposed 1UC FeSe-TiO Complex Dung-Hai Lee,Chin. Phys. B 24, 117405 (2015).

Enhanced Tc in Monolayer FeSe on STO 80 Tc = 60 K-class SC cannot be achieved by bulk and thicker films of FeSe. 60 Tc (K) 40 20 1UC 2UC 3UC 4UC 50UC ∞UC FeSe Thickness

Summary of Tc and Gap in Bulk, Doped, and Monolayer FeSe S.N. Rebec, Z.-X.. Shen et al., arXiv: 1606.09358.

Interface Atomic Structure: 1UC-FeSe on STO F. Li, Q.-K. Xue et al., arXiv: 1512.05203. aFeSe = 3.86 Å　 (3.76 Å) hSe = 1.31±0.01 Å (1.45 Å ) dSe-Ti = 3.58 Å hSeopt = 1.38 Å a = 111.4°±0.9° (103.6°) a opt = 109.5° bulk FeSe

FeSe Monolayer on a SrTiO3 (110) Substrate

Small Electron Fermi Surface Pockets in FeSe L.P. Gor’kov, arXiv: 1510.03327; Phys. Rev. B 93, 060507 (2016).. B. Rosenstein et al., arXiv: 1601.07425. ● Validity of Migdal theorem - low EF The Fermi energy is small; EF ~ ħW0 EF ~ 60 meV ħW0 ~ 80-100 meV EF m* = m / [1 + m ln (EF/ħW0)]

s-Wave SC Gap in Monolayer FeSe on STO Q. Fan, D.-L. Feng et al., Nature Phys. 11, 946 (2015).

Exfoliated Monolayer Materials Anomalous charge transport

Exfoliated Monolayer Materials “Magnetic graphene” MPS3 (M: 3d-TM, Mn, Fe, Co, Ni) J.-G. Park (IBS-CCES, Seoul) Anomalous charge transport Bulk single crystal P.A. Joy & S. Vasudevan, PRB 46, 5425 (1992).

Families of Iron-Based Superconductors Sr O V As Fe FeSe Sr4V2O6Fe2As2 LaFeAsO BaFe2As2 LiFeAs Tcmax = 55 K Tcmax = 39 K Tcmax = 22 K Tcmax = 8 K Tcmax = 38 K

Families of Iron-Based Superconductors Sr O V As Fe FeSe Sr4V2O6Fe2As2 BaFe2As2 LaFeAsO LiFeAs KxFe2-ySe2 Ca4(Mg, Ti)3Fe2As2O8-y (Ca, La)Fe2As2 Tcmax = 55 K Tcmax = 39 K Tcmax = 22 K Tcmax = 8 K Tcmax = 38 K Tcmax = 55 K Tcmax = 45-49 K Tcmax = 48-49 K Tcmax = 47 K Already optimized !?

Why is Fe-As(Se) unique ? Tc vs Bond Angle 1UC FeSe FeSe

Tc vs Pn/Ch Height 1UC FeSe Y. Takano (NIMS)