Iron based superconductors: Pnictides versus chalcogenides

Abstract

a b s t r a c tWe present a brief review of the present day situation with studies of high-temperature superconductivity in iron pnictides and chalcogenides. Recent discovery of superconductivity with T c 4 30 K in A x Fe 2 À x/2 Se 2 (A ¼ K, Cs, Tl, etc) represents the major new step in the development of new concepts in the physics of Fe-based high-temperature superconductors. We compare LDA and ARPES data on the band structure and Fermi surfaces of novel superconductors and those of the previously studied FeAs superconductors, especially isostructural 122-superconductors like BaFe 2 As 2 . It appears that electronic structure of new superconductors is rather different from that of FeAs 122-systems. In particular, no nesting properties of electron and hole-like Fermi surfaces is observed, casting doubts on most popular theoretical schemes of Cooper pairing for these systems. Doping of novel materials is extremely important as a number of topological transitions of Fermi surface near the G point in the Brillouin zone are observed for different doping levels. The discovery of Fe vacancies ordering and antiferromagnetic (AFM) ordering at pretty high temperatures (T N 4 500 K), much exceeding superconducting T c makes these systems unique antiferromagnetic superconductors with highest T N observed up to now. This poses very difficult problems for theoretical understanding of superconductivity. We discuss the role of both vacancies and AFM ordering in transformations of band structure and Fermi surfaces, as well as their importance for superconductivity. In particular, we show that system remains metallic with unfolded Fermi surfaces quite similar to that in paramagnetic state. Superconducting transition temperature T c of new superconductors is discussed within the general picture of superconductivity in multiple band systems. It is demonstrated that both in FeAs-superconductors and in new FeSesystems the value of T c correlates with the value of the total density of states (DOSs) at the Fermi level.

Magnetic properties of novel FeSe(Te) superconductors

“…10 which clearly exhibits consecutive maximum and minimum. It was recently shown 38 , that superconducting transition temperatures T c of a number of iron-based superconductors correlate with the corresponding values of the density of states at the Fermi level, thus supporting the BCS-like pairing mechanism in these systems. Remarkably, that the presently calculated behavior of N (E F ) under pressure (the upper curve in Fig.…”

Interrelation of superconductivity and magnetism in FeSe1−xTex compounds. Pressure effects

“…in Ref. [8], for many Fe-based HTSCs the experimental values of superconducting transition temperatures are well described in the framework of the electron-phonon mechanism of pairing. The close interrelation of magnetism and superconductivity determines the importance of further studying of magnetic and superconducting properties and their evolution under variations of composition, pressure, etc.…”

Comparative study of electronic structure of new superconductors (Sr, Ca)Pd2As2 and related compound BaPd2As2

“…Also we made some effort to investigate FeSe based superconductors (chalcogenides) [12,13,14,15] and to compare them with pnictides [16]. Electronic structures of some related systems were studied during recent years: APt 3 P [17], BaFe 2 Se 3 [18], SrPt 2 As 2 [19].…”