Magnetism and its microscopic origin in iron-based high-temperature superconductors

Abstract

High-temperature superconductivity in the iron-based materials emerges from, or sometimes coexists with, their metallic or insulating parent compound states. This is surprising, as these undoped states exhibit dramatically different antiferromagnetic spin arrangements and Néel temperatures. Although there is a general consensus that magnetic interactions are important for superconductivity, much remains unknown concerning the microscopic origin of the magnetic states. In this review, we summarize the progress in this area, focusing on recent experimental and theoretical results, and their microscopic implications. We conclude that the parent compounds are in a state that is more complex than that implied by a simple Fermi surface nesting scenario, and a dual description including both itinerant and localized degrees of freedom is needed to properly describe these fascinating materials.S oon after the discovery of high critical-temperature (high-T c ) superconductivity in copper oxides 1 , neutron scattering studies revealed that the parent compounds of these superconductors have an antiferromagnetic (AF) ground state with a simple collinear spin structure ( Fig. 1a) 2,3 . Because the associated AF spin fluctuations may be responsible for electron pairing and superconductivity 4-6 , over the past 25 years a tremendous effort has focused on characterizing the interplay between magnetism and superconductivity in these materials 7 . In the undoped state, the parent compounds of copper oxide superconductors are Mott insulators and have exactly one valence fermion with spin 1/2 for each copper atom, leading to robust electronic correlations and localized magnetic moments 5,6 . Superconductivity emerges after introducing charge carriers that suppress the static AF order. Although the strong Coulomb repulsion in the parent compounds is screened by the doped charge carriers, the electronic correlations are certainly important for the physics of the doped cuprates, particularly in the underdoped regime 6 .Consider now the iron-based superconductors [8][9][10] . Several parent compounds of these materials, such as LaFeAsO, BaFe 2 As 2 , NaFeAs and FeTe, are not insulators but semimetals 11-14 . In these cases, electronic band structure calculations have revealed that their Fermi surfaces (FSs) are composed of nearly cylindrical hole and electron pockets at the (0,0) and M (1,0)/M (0,1) points, respectively 15,16 . The high density of states (DOS) resulting from the extended momentum space with nearly parallel FS between the hole and electron pockets leads to an enhancement of the particle-hole susceptibility. This suggests that FS nesting among those pockets could induce spin-density-wave (SDW) order at the in-plane AF wave vector Q AF = (1,0) with a collinear spin structure (Fig. 1b) 17 , much like the FS-nesting-induced SDW in pure chromium 18 . Neutron scattering experiments on LaFeAsO (ref. 19), BaFe 2 As 2 (ref. 20) and NaFeAs (ref. 21) have reported results compatible with the theoretically predicted AF spin structure...

Magnetic and pairing properties of a two-orbital model for the pnictide superconductors: a quantum Monte Carlo study

“…25 Throughout this work, the correlation strength is taken up to the intermediate range, i.e., U/ |t 1 | 2 for both undoped and doped cases, which is believed to be proper for the pnictides SCs. 26 In Eq. (2), H i 1 can be transformed 18 as…”

Nonmagnetic impurity resonance states as a test of superconducting pairing symmetry in CeCoIn5

“…When entering the superconducting state, the magnetic excitations spectra by inelastic neutron scattering show the appearance of a sharp spin resonance 10 . These finding underline the analogy to the cuprate high-temperature superconductors 14,15 and the new iron superconductors 16,17 , where AF spin fluctuations may actually mediate unconventional superconductivity.So far, the superconducting pairing symmetry in CeCoIn 5 has been discussed from both experimental and theoretical sides, it has not yet been determined unambiguously. Soon after the discovery of CeCoIn 5 material, its Fermi surface (FS) has been studied in detail by quantum oscillation, which consists of nearly cylindrical one and small ellipsoidal ones.…”

“…When entering the superconducting state, the magnetic excitations spectra by inelastic neutron scattering show the appearance of a sharp spin resonance 10 . These finding underline the analogy to the cuprate high-temperature superconductors 14,15 and the new iron superconductors 16,17 , where AF spin fluctuations may actually mediate unconventional superconductivity.…”

“…The recent Muon knight shift measurements also found that the magnetic field dependence of the reduction of the muon knight shift is not in proportion to √ H, which is roughly explained by the Fermi liquid relation and is inconsistent with simple expectation for a d-wave superconductors 29 . These facts likely reflect the multiband nature of superconductivity in CeCoIn 5 , similar to the story of Fe-based superconductors 16,17 .On the other hand, de Hass-van Alphen (dHvA) experiments in CeCoIn 5 clearly indicate the FS is three dimensional (3D) [30][31][32] . If quasi-2D FS (a large FS denoted as β-band in the dHvA experiments) is strictly cylindrical along the k z direction, the hot lines would be parallel to k z .…”

“…The recent Muon knight shift measurements also found that the magnetic field dependence of the reduction of the muon knight shift is not in proportion to √ H, which is roughly explained by the Fermi liquid relation and is inconsistent with simple expectation for a d-wave superconductors 29 . These facts likely reflect the multiband nature of superconductivity in CeCoIn 5 , similar to the story of Fe-based superconductors 16,17 .…”