FeSe and its relatives

Experimental facts

• Can be doped with holes and electrons ( e.g. Te, Rb)
• Nematic transition at 90K if undoped at ambient pressure
• Superconducting down to a single layer, where Tc = 8K at ambient pressure; 30K under pressure
  • electron doping can eliminate hole pocket at Gamma point; a problem for interband pairing mechanism
  • If placed on top of STO, Tc rises dramatically, 80K; Presumably due to interfacial phonon with strong e-ph interaction
    • s-wave gap, 20meV https://arxiv.org/pdf/1504.02185.pdf
    • STO oxygen isotope effect observed
  • review https://arxiv.org/pdf/1604.03566.pdf
• magnetic order can be stripes or bistripes
• "spin resonance" observed https://www.nature.com/articles/s41467-017-00162-x
• AF correlations decrease and stripe increase, below nematic transition https://www.nature.com/articles/ncomms12182
• strong orbital-dependent correlation effects in iron chalcogenides, ARPES https://www.nature.com/articles/ncomms8777, review https://www.nature.com/articles/s41535-017-0059-y
• orbital selective SC, STM http://science.sciencemag.org/content/357/6346/75

Theoretical considerations

• J1, J2 model is probably insufficient
• sign problem free MC is possible, http://engine.scichina.com/publisher/scp/journal/SB/61/12?slug=Browse, cooperative effect between several pairing mechanism (nematic, AF, phonons) https://doi.org/10.1007/s11434-016-1087-x
• Nematic possibly driven by S = 1 quantum fluctuations, https://www.nature.com/articles/nphys3456, see also https://journals.aps.org/prb/pdf/10.1103/PhysRevB.79.174409
• Electronic nematicity review, https://www.nature.com/articles/nphys2877
• Orbital selective physics
  • superconductivity https://arxiv.org/abs/1306.4184