Introduction to Fe-based Superconductors
Fe-based superconductors have brought tremendous interests and surprises to the condensed matter physics communities ever since its first observation in 2008. Besides cuprates, Fe-based superconductors have become the second high temperature superconductor families. A comprehensive study on Fe-based superconductors, especially establishing the common physical properties in both cuprates and Fe-based superconductors will provide the key information for the pairing mechanism of high Tc superconductivity.
1.Lattice structures
Figure1. Presentations of lattice structure from different iron based superconducting families and the unusual AFM structure.
2.Phase diagram
Similar to cuprates, the SC region in most of the Fe-based superconductor phase diagrams also locates near an antiferromagnetic phase which indicates that magnetic order might be important for the realization of high temperature superconductivity.
Figure 2. the phase diagram of iron-based superconductor and cuprate superconductors. Left shows the cuprate phase diagram and Right shows the iron based superconductor phase diagram.
1）Multiple Nodeless Superconducting Gaps in (Ba0.6K0.4)Fe2As2 Superconductor from Angle-Resolved Photoemission Spectroscopy
After discovery of iron based superconductors in 2008, all kinds of experiment methods were used to reveal their rich physical characters, but the difficulty of growth of single crystals limited the momentum resolved experiment results from ARPES. Cooperation with single growth group, we got the rare single crystals. Then we finished first ARPES report on superconducting state of Fe-based superconductors in the world. This paper firstly presents band structure of (BaK)Fe2As2 and Fermi surface dependent superconducting gaps: the inner pocket around Γpoint shows bigger nodeless superconducting gap about 12meV, the out one shows smaller nodeless superconducting gap.
Figure 3. the Fermi surface of optimal (BaK)Fe2As2 and band structure along the high symmetry line in the BZ above and below the superconducting transition temperature.
2）Reorganization of band structure and unique Fermi surface in iron-based superconductors
We firstly reported the band structure Reorgnization in the iron-based superconductors. We also found the unique Fermi surface topology around the Brillouin zone corner M point. There is two-strong-spots structure alignment along the high symmetry line Γ-M around M point and it is likely from the Dirac-cone like electronic structure. We also carried out detailed examination on the parent compound BaFe2As2. We found this unique Fermi surface topology around M point was related to the magnetic ordering and it showed some kind of unconventional SDW order.
Figure 4. Fermi surface of (SrK)Fe2As2 and the Constant energy surface at different binding energy around M
Figure 5. The Fermi surface of parent compound BaFe2As2
Guodong Liu et al. Phys. Rev. B 80 (2009) 134519
3）Unusual Electronic Structure and Observation of Dispersion Kink in CeFeAsO Parent Compound of FeAs-based Superconductors
This is the first comprehensive high-resolution ARPES measurement on CeFeAsO, a parent compound of FeAs-based high temperature superconductors with a magnetic-structural transition at ~150 K. In the magnetic-ordering state, four holelike Fermi surface sheets are observed near Γ(0,0), and the Fermi surface near M(±π,±π) shows a tiny electron-like pocket at M surrounded by four strong spots. The unusual Fermi surface topology deviates strongly from the band structure calculations. The electronic signature of the magnetic-structural transition shows up in the dramatic change of the quasi-particle scattering rate. A dispersion kink at ~25 meV is observed for the first time in the parent compound of Fe-based superconductors.
Figure 6. the Fermi surface and band structures of parent CeFeAsO and the presentation of scattering velocity of quasiparticle above and below the magnetic-structure transition temperature
4）Distinct Fermi Surface Topology and Nodeless Superconducting Gap in a (Tl0.58Rb0.42)Fe1.72Se2 Superconductor
The Fermi surface topology of (Tl0.58Rb0.42)Fe1.72Se2 consists of two electron-like Fermi surface sheets around the Γ point which is distinct from that in all other iron-based superconductors reported so far. The Fermi surface around the M point shows a nearly isotropic superconducting gap of ~12 meV. The large Fermi surface near the Γ point also shows a nearly isotropic superconducting gap of ~15 meV, while no superconducting gap opening is clearly observed for the inner tiny Fermi surface. Our observed new Fermi surface topology and its associated superconducting gap will provide key insights and constraints into the understanding of the superconductivity mechanism in iron-based superconductors.
Figure7. the Fermi surface of optimal (TlRb)FeSe
5）Electronic Origin of High Temperature Superconductivity in Single-Layer FeSe Superconductor
This is the first ARPES measurement on the single-layer FeSe film on STO. The Fermi surface topology of the superconducting single-layer FeSe film is different from other Fe-based superconductors; it consists only of electron pockets near the zone corner without indication of any Fermi surface around the zone center. Our observation of large and nearly isotropic superconducting gap in this strictly two-dimensional system rules out existence of node in the superconducting gap. These results have established a clear case that such a simple electronic structure is compatible with high Tc superconductivity in iron-based superconductors.
Figure8. the Fermi surface of different iron-based superconductors. a, single layer FeSe . b (TlRb)Fe2Se2 c optimal (BaK)Fe2As2 d, the Fermi surface of β-FeSe from band structure calculation.
6）Phase diagram and electronic indication of high-temperature superconductivity at 65K in single-layer FeSe films
The recent discovery of possible high-temperature superconductivity in single-layer FeSe films has generated signicant experimental and theoretical interest. We report the phase diagram for an FeSe monolayer grown on a SrTiO3 substrate by tuning the charge carrier concentration over a wide range through an extensive annealing procedure. We identify two distinct phases that compete during the annealing process: the electronic structure of the phase at low doping (N phase) bears a clear resemblance to the antiferromagnetic parent compound of the Fe-based superconductors, whereas the superconducting phase(S phase) emerges with the increase in doping and the suppression of the N phase. By optimizing the carrier concentration, we observe strong indications of superconductivity with a transition temperature of 65±5K. These observations not only provide the key information for the superconducting mechanism but also work as a guide to search for higher Tc in Fe-based superconductors.
Figure9. the phase diagram of single layer FeSe on SrTiO3.