Why is the simplest Fe-based superconductor the strangest? - Drs. Mahmoud Abdel-Hafiez and Xiaojia Chen
A comprehensive study of the doping dependence phase diagram of FeSe-based superconductors is still required due to the lack of a clean and systematic means of doping control. By the in situ ARPES and a series of thermodynamic measurements, an international joint team co-led by Drs. Mahmoud Abdel-Hafiez and Xiaojia Chen from HPSTAR, realized a hump at elevated temperatures around 280K seems to be a standard feature of any degenerated semiconductors observed many times in various systems. This hump reflects the crossover between semiconducting and metallic behavior. Co doping at the Fe site, is found to decrease the superconducting transition temperature. Additionally, inelastic neutron-scattering experiments that reveal both stripe and Néel spinfluctuations over a wide energy range at 110 K. On entering the nematic phase, a substantial amount of spectral weight is transferred from the Néel to the stripe spin fluctuations. These stories are published in the July 19th edition of Nat. Commun. (doi:10.1038/ncomms12182) and in June 8th edition of Phys. Rev. B (93, 224508).
Recently, FeSe superconductors has attracted considerable interest because of its typical magnetism and fascinating superconducting properties. Although the superconducting transition temperature Tc of bulk FeSe (Tc=8 K) is low, it increases drastically under pressure (Tc=37 K), by carrier doping (Tc=40–48 K), or in the mono layer limit (Tc=65-109 K). The unique superconducting properties of FeSe are presumably related to its magnetism, which has also been shown to be uncommon. FeSe displays nematic, but not stripe magnetic order that is unexpected because nematic order has been argued to be the consequence of stripe magnetic order, and both break the C4 lattice symmetry.
Nat. Commun. 7, 12182 (2016)
Phys. Rev. B , 224508 (2016)
In iron pnictides, the stripe magnetic order invariably occurs at or immediately below the nematic-(tetragonal-to-orthorhombic) ordering temperature. Although previous works have shown that the nematic order could be driven by spin fluctuations without the requirement of magnetic order, the microscopic origin of the absence of the long-range stripe magnetic order in FeSe remains elusive.
To further elucidate the role of the Néel spin fluctuations in iron-based superconductivity, a detailed study of the pressure-/electron-doping dependence of the spin correlations in FeSe would be desirable.
Caption: left panel: Schematic representation of the stripe and Néel spin fluctuationsin the (H,K) plane. Middle panel: Dispersions of the stripe andNéel spin fluctuations in FeSe at 4K. The spectral weight transfer from theNéel (1,1) to stripe (1, 0) wavevector below 70meV on cooling to 4 K can beclearly seen. The vertical bars indicate the energy integration range. Thehorizontal bars at 15meV indicate the full-width at half-maximum of theGaussian fitting. The horizontal bars at other energies are the errors derivedby leastsquare fittings. The dashed lines are a guide to the eye. Right panel: The S concentration (x) dependence of the superconducting transition temperature,structural transition (Ts), and the T* obtained from magnetic, specific heatand electric resistivity data. The phase diagram highlights the suppression ofTs and the transition at T* by increasing the S concentration. The insetsummarizes the Co concentration dependence, in which the Tc decreases uponincreasing doping.