铁硒超导薄膜的原位制 备和电子结构测量 谢斌平 复旦大学 杭州 2014 全国表面分析应用技术学术交流会及赛默飞世尔科技 XPS 应用及设备维护技术培训班

Transcription

1 铁硒超导薄膜的原位制 备和电子结构测量 谢斌平 复旦大学 杭州 2014 全国表面分析应用技术学术交流会及赛默飞世尔科技 XPS 应用及设备维护技术培训班

2 Outline 1. Experimental setup----ombe+mbe+arpes 2. Heterostructure Design and in-situ ARPES study FeSe/STO films S. Tan, D. L. Feng*, Nature Materials 12, (2013) 1ML FeSe/STO/KTO R. Peng, D. L. Feng*, Physical Review Letters 112, (2014) 1ML FeSe/BTO/KTO R. Peng, D. L. Feng*, arxiv: Summary

3 1. Experimental setup----ombe+mbe+arpes

4 The mechanism of complex materials Understand the microscopic electronic properties and the role of competing degrees of freedom Study the low-energy electronic excitations ARPES Velocity and direction of the electrons in solids

5 XPS/UPS/ESCA

6 Angle Resolved Photoemission Spectroscopy Binping Xie, et al. PRL 98, (2007) Multi-channel detection High energy and high angular resolution

7 Determination of the quasiparticle dispersion Energy distribution curves (EDC) Momentum distribution curves (MDC) Energy (ev) PEAK POSITION Dispersion PEAK WIDTH 1/τ scattering rate Momentum Complex lineshapes and background Fermi function cut-off Good fit with Lorentzian shape High sampling of BZ No Fermi function complications

8 Coupling to a collective mode: e.g. phonons λ electron-mode coupling constant λ Screening of electrons due to ions -> m* Ashcroft, Mermin Solid State Physics

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10 Fermi surface of iron based superconductors Type-1 Fe-HTS Type-2 Fe-HTS Y. Zhang et. al, Nature Materials, 10, (2011).

11 Fudan ARPES system 6-450K,UHV 5 mev total energy resolution 0.1degree angular resoution ±15º in one cut He-I, 21.2eV light 5.5eV, 7eV laser

12 Sample and materials storage 真空漏率低于 1*10-9 Pam 3 /s

13 OMBE+FeSe MBE+ARPES In-situ study of FeSe films CAFM?? SC?? Tuning the interface/strain Interface-> SC?? New FeSe films for pairing symmetry study

14 Oxide-MBE+MBE+ARPES ARPES UFO Oxide MBE FeSe MBE Transferring process: Under UHV Easy and quick to operate

15 Compact FeSe MBE system setup Maximum 7 K-cells Temp 1200C Working Temp C

16 氧化物分子束延逐层生长系统及技术 La1-xSrxMnO3 薄膜一个原胞层逐层生长 封东来等物理 2012

17 圆型多层叠套真空传输系统

18 FeSe/SrTiO 3 Thin Films

19 Experimental Detail Nb-SrTiO 3 substrate preparation Vacuum degas: 550 for 3H Se flux: 950 for 30min FeSe thin film MBE growth Fe Evaporation rate: 0.02Å/s Se Evaporation rate: 1.00Å/s Growth temperature: 490 Anneal temperature: 600 in situ ARPES measurement Photon energy: 21.2eV(Helium lamp); 7eV(Laser) Temperature:>25K

20 FeSe/Nb:STO------in-situ ARPES Confirming the ex-situ ARPES results Pairing-formation T record, even if it is a 2D BKT type 0.12 excessive electrons per Fe derived from the Luttinger volume.

21 Electronic Structure Evolution

22 Thickness Dependence of the Lattice Constant

23 Phase Diagram

24 FeSe/SrTiO 3 FeSe/SrTiO 3 /KTaO 3 Thin Films

25 FeSe/Nb:STO/KTO MBE growth KTO substrate a=3.989å Atomically flat surface Epitaxial Nb:STO Retains 2-dimensional character Stable RHEED oscillation Epitaxial FeSe

26 Epitaxial Strain effect The in-plane reciprocal vector (Q // ) of the Nb:STO film equals to that of KTO substrate The lattice constant of single-layer FeSe is about 3.99Å, which is extremely expanded

27 Pairing temperature is 5K enhanced to 70K in extremely tensile-strained FeSe Fermi surface structure of 1ML FeSe/Nb:STO/KTO Two elliptical electron pockets at M δ=0.12e - /Fe

28 Gap anisotropy Nodeless Anisotropic with minima at the crossing of pockets

29 FeSe/SrTiO 3 /KTaO 3 Pairing Temperature 5K enhancement Other interfacial effect than strain? FeSe/BaTiO 3 /KTaO 3 Thin Films

30 FeSe/BTO/KTO film: two types of lattices Nb:BTO 3x3 superstructure appear after Se flux Three sets of Fermi surfaces/leed structures are resolved FeSe BU --- a=3.989å Two sets of rotated FeSe BR a=3.78å

31 Substrate and a dependent band structure near Gamma Interfacial coupling electronic structure Larger a larger band mass of the parabolic band smaller separations between the two bands indicating stronger correlation effects with expanded lattice BTO and STO have different effects, although both terminated with TiO2 plane.

32 gap & Tg a=3.989å J enhancement plays some J enhancement role! But not plays dominating some role! a=3.78å or 3.99Å Tc differs just 5K, disastrous news for J models. a=3.78å or 3.99Å Tc differs just 5K, disastrous news for J models. 70K 75K Pairing T record Enhancing tensile strain slightly enhances Tc high Tc for all the single-layer films a=3.78å Spin fluctuation cannot account for all the physics in single-layer FeSe

33 Summary Magnetism There are CAF order in FeSe films if undoped, enhanced with tensile strain On superconductivity Paring temperature 65K T FeSe/SrTiO 3 70K T FeSe/SrTiO 3 /KTaO 3 75K T 1ML FeSe/BaTiO 3 /KTaO 3 Gap anisotropy in strained 1ML FeSe: s wave in heavily electron doped FeHTSs, strong constraints for theory The interfacial effects charge transfer-----high e doping in 1ML FeSe strain effect---enhance strain can slightly enhance Tc Interfacial is crucial for high Tc in FeSe! interfacial e-ph, nontrivial J dependence, screening effects? needs further investigation, more excitement expected!

34 Thank you! there are plenty of good stuff on the surface or at the interface!