Magnetic order close to superconductivity in the iron-based layered LaO1-xF x FeAs systems

Abstract

Following the discovery of long-range antiferromagnetic order in the parent compounds of high-transition-temperature (high-T(c)) copper oxides, there have been efforts to understand the role of magnetism in the superconductivity that occurs when mobile 'electrons' or 'holes' are doped into the antiferromagnetic parent compounds. Superconductivity in the newly discovered rare-earth iron-based oxide systems ROFeAs (R, rare-earth metal) also arises from either electron or hole doping of their non-superconducting parent compounds. The parent material LaOFeAs is metallic but shows anomalies near 150 K in both resistivity and d.c. magnetic susceptibility. Although optical conductivity and theoretical calculations suggest that LaOFeAs exhibits a spin-density-wave (SDW) instability that is suppressed by doping with electrons to induce superconductivity, there has been no direct evidence of SDW order. Here we report neutron-scattering experiments that demonstrate that LaOFeAs undergoes an abrupt structural distortion below 155 K, changing the symmetry from tetragonal (space group P4/nmm) to monoclinic (space group P112/n) at low temperatures, and then, at approximately 137 K, develops long-range SDW-type antiferromagnetic order with a small moment but simple magnetic structure. Doping the system with fluorine suppresses both the magnetic order and the structural distortion in favour of superconductivity. Therefore, like high-T(c) copper oxides, the superconducting regime in these iron-based materials occurs in close proximity to a long-range-ordered antiferromagnetic ground state.

Synthesis and physical properties of FeSe1/2Te1/2 superconductor

“…The parent compounds of the FeAs based superconductors do not exhibit superconductivity but possess stripe type commensurate antiferromagnetic spin order accompanied by a structural transition at around 150 K. 7,8 The superconductivity was obtained by either fluorine doping at the oxygen site or by the deficiency of oxygen ions in the system. [4][5][6][7][8][9] The highest T c = 55 K has been reported in SmFeAS͑O 1−x F x ͒. 10 Very recently, another iron based superconductor FeSe was reported with a T c of 8 K. 1 FeSe is the simplest-structured among the iron-based superconductors.…”

A Unified Theory for the Cuprates, Iron-Based and Similar Superconducting Systems: Application for Spin and Charge Excitations in the Hole-Doped Cuprates

“…Within a two-dimensional approximation, the lagron spectrum of the FeSCs is expected to differ from that of the cuprates, presented in Fig. 1, by replacing the minima positions from satellite points around Q, to satellite points around the two possible SDW wave vectors in the parent compounds: Q 1 = (π/a)x and Q 2 = (π/a)ŷ [7,8] ( or Q 1 = (π/2a)(x +ŷ) and Q 2 = (π/2a)(x −ŷ) [9]), or points close to them. Similarly to the cuprates [48][49][50][51], stripe-like inhomogeneities characterized by modulations due to the differences between the satellite points and Q 1 or Q 2 , could exist also in the FeSCs.…”

“…Their Bose condensation is manifested, at low doping levels, in AF order, in the cuprates [40], and in a structural distortion and magnetic order, characterized by a spin-density wave (SDW), in the FeSCs [7][8][9]. At higher doping levels the Bose condensation of svivons is manifested in static or dynamical inhomogeneities, based on modulations of the low-doping order.…”

“…Consequently, the intrasite Coulomb and exchange integrals, corresponding to Wannier functions of the hybridized orbitals of the entire bands which determine the Fermi-surface (FS), magnetic moments, etc., may be not large in the FeSCs [36], resulting in itineracy and largely reduced magnetic moments [7][8][9]. On the other hand, due to the dominantly Fe(3d) nature of the states at E F , their intrasite integrals are rather large [36] and a large-U approach should be applied to study the physical properties (e.g.…”

“…Similarly to the cuprates [6], the FeSCs are derived from an undoped "parent" compound which is generally magnetically ordered [7][8][9] at low temperatures and becomes SC under electron-or hole-doping. Also, both systems are characterized by a layered structure and a quasi-twodimensional electronic structure [10][11][12][13][14][15][16].…”