High-temperature superconductivity in iron-based layered iron compounds

Abstract

We present a review of basic experimental facts on the new class of high -temperature superconductors -iron based layered compounds like REOFeAs (RE=La,Ce,Nd,Pr,Sm...), AFe2As2 (A=Ba,Sr...), AFeAs (A=Li,...) and FeSe(Te). We discuss electronic structure, including the role of correlations, spectrum and role of collective excitations (phonons, spin waves), as well as the main models, describing possible types of magnetic ordering and Cooper pairing in these compounds 1 . ContentsConclusion: end of cuprate monopoly 38 What is in common between iron based and cuprate superconductors?38 What is different between iron based and cuprate superconductors? 39References 39• There exists a relatively well defined (at least in the part of the Brillouin zone) Fermi surface, in this sense these systems are metals.• However, appropriate stoichiometric compounds are antiferromagnetic insulators -superconductivity is realized close to the Mott metal -insulator phase transition (controlled by composition) induced by strong electronic correlations.• The strong anisotropy of all electronic properties is observed -conductivity (and superconductivity) is realized mainly within CuO 2 layers (quasi two-dimensionality!).At the same time, many things are still not understood. Accordingly, we may list What do we not really know about cuprates:• Mechanism of Cooper pairing (a "glue" leading to formation of Cooper pairs).Possible variants:1. Electron -phonon mechanism.2 Under pressure Tc of HgBa 2 Ca 2 Cu 3 O 8+δ reaches ∼ 150K. BASIC EXPERIMENTAL FACTS ON NEW SUPERCONDUCTORS Electrical properties and superconductivity REOF eAs (RE = La, Ce, P r, N d, Sm, ...) systemDiscovery of superconductivity with T c = 26K in LaO 1−x F x F eAs (x = 0.05 − 0.12) [15] was preceded by the studies of electrical properties of a number of oxypnictides like LaOM P n (M = M n, F e, Co, N i and P n = P, As) highlighted by discovery of superconductivity in LaOF eP with T c ∼ 5K [24] LaON iP T c ∼ 3K [25], which has not attracted much attention from HTSC community. This situation has changed sharply after Ref.[15] has appeared and shortly afterwards a lot of papers followed (see e.g. [26,27,28,29,30,31,32,33,34,35]), where this discovery was confirmed and substitution of lanthanum by a number of other rare -earths, according to a simple chemical formula (RE) +3 O −2 Fe +2 As −3 , has lead to more than doubling of T c up to the values of order of 55K in systems based upon N dOF eAs and SmOF eAs, with electron doping via addition of fluorine or creating oxygen deficit, or hole doping achieved by partial substitution of the rare -earth (e.g. La by Sr) [36]. Note also Ref. [37], where the record values of T c ∼ 55K were achieved by partial substitution of Gd in GdOF eAs by Th, which, according to the authors, also corresponds to electron doping. In these early works different measurements of electrical and thermodynamic properties were performed on polycrystalline samples.In Fig. 1 (a), taken from Ref.[33], we show typical temperature dependences of electric resistivi...

Superconductivity inNdFe1−xCoxAsO(

“…6 Both electron-and hole-doped superconductors exist. 2,3 Most temperature-composition phase diagrams show a dome-shaped SC region and suggest the coexistence of magnetic order and superconductivity for underdoped samples. [7][8][9][10][11] An exception is the phase diagram of LaFeAsO 1−x F x where a discontinuous transition into the superconducting state is reported.…”

“…The magnetism is therefore not due to superexchange but to Fermi surface nesting in the two-dimensional band structure, which results in a spin-density wave ͑SDW͒ state. 4,5 The parent materials are rendered superconducting ͑SC͒ via chemical doping 2,3 or in some cases application of hydrostatic pressure. 6 Both electron-and hole-doped superconductors exist.…”

Electronic band structure and Fermi surface of new low-temperature Ni-based superconductors: 3.3K (Ni2P2)(Sr4Sc2O6) and 2.7K (Ni2As2)(Sr4Sc2O6) from first principles

“…The newly synthesized SCs (Ni 2 P 2 )(Sr 4 observed for related layered phases [8][9][10]. Besides, the (Ni 2 As 2 ) blocks are thicker than the (Ni 2 P 2 ) blocks, whereas the (Sr 4 Figure 1 shows the band structures of (Ni 2 P 2 )(Sr 4 Sc 2 O 6 ) and (Ni 2 As 2 )(Sr 4 Sc 2 O 6 )…”

“…In the band structure of (Ni 2 P 2 )(Sr 4 Sc 2 O 6 ), two lowest bands lying around - Sm, Dy, Gd, and A are alkali earth metals Ca, Sr, and Ba), for which at least three different competing types of magnetic fluctuations have been predicted, reviews [8][9][10][11][12][13].…”

“…The discovery of superconductivity in the layered iron pnictides (so-called 1111, 122, and 111 phases) [1][2][3][4][5][6] and iron chalcogenides Fe-Se(Te) [7] stimulated much activity in search for new related superconductors (SCs), reviews [8][9][10][11][12][13]. Recently, a new group of more complex five-component phases, namely iron pnictide oxides with general stoichiometry (Fe 2 Pn 2 )(A 4 M 2 O 6 ) (where Pn are pnictogens, and A and M are alkaline-earth and transition metals, respectively) has been discovered, and some physical properties of (Fe 2 Pn 2 )(A 4 M 2 O 6 ) phases have been examined both experimentally and theoretically, review [14].…”