Spin excitations and the Fermi surface of superconducting FeS

Abstract

High-temperature superconductivity occurs near antiferromagnetic instabilities and the nematic state. Debate remains on the origin of nematic order in FeSe and its relation with superconductivity. Here, we use transport, neutron scattering and Fermi surface measurements to demonstrate that hydrothermo grown superconducting FeS, an isostructure of FeSe, is a tetragonal paramagnet without nematic order and with a quasiparticle mass significantly reduced from that of FeSe. Only stripe-type spin excitations are observed up to 100 meV. No direct coupling between spin excitations and superconductivity in FeS is found, suggesting that FeS is less correlated and the nematic order in FeSe is due to competing checkerboard and stripe spin fluctuations.npj Quantum Materials (2017) 2:14 ; doi:10.1038/s41535-017-0019-6 INTRODUCTION High-transition temperature superconductivity in copper oxides and iron-based materials occurs near checkerboard and stripe antiferromagnetic (AF) instabilities, respectively. [1][2][3] Although there is also ample evidence for the existence of a nematic order, where a translationally invariant metallic phase spontaneously breaks rotational symmetry, 4-8 and for a nematic quantum critical point near optimal superconductivity in iron-based superconductors, 9, 10 much remains unclear concerning its microscopic origin and its relationship to superconductivity. 2,3 In particular, recent debates focus on whether nematic order in superconducting FeSe below the tetragonal-to-orthorhombic transition temperature T s = 91 K without static AF order 11-13 is due to competing magnetic instabilities or orbital ordering.14-22 Here, we use transport, neutron scattering and Fermi surface measurements to demonstrate that superconducting FeS, an isostructure of FeSe, 23, 24 is a tetragonal paramagnet without nematic order and with a quasiparticle mass significantly reduced from that of FeSe. Our neutron scattering experiments in the energy regime below 100 meV reveal only stripe-type spin fluctuations in FeS that are not directly coupled to superconductivity. These properties suggest that FeS is a weakly correlated analog of FeSe and, moreover, that the nematic order in FeSe is due to the frustrated magnetic interactions underlying the competing checkerboard and stripe spin fluctuations. 16-18A key to understanding the physics of the iron-based superconductors is to determine the role played by magnetism and by electronic nematicity to superconductivity. [1][2][3][5][6][7] In a typical AF ordered iron-pnictide, a tetragonal-to-orthorhombic lattice distortion T s occurs at temperatures above or at the AF ordering temperature T N , 2 and the nematic phase is observed in the paramagnetic orthorhombic phase between T s and T N . [5][6][7] Although iron chalcogenide FeSe single crystals [ Fig. 1a, b] also undergo a nematic transition at T s and become superconducting at T c = 9.3 K, 11 the low-temperature static AF ordered phase is absent. 12,13 This has fueled debates concerning the role of AF order and spin fluctuations...

Selective orbital reconstruction in tetragonal FeS: A density functional dynamical mean-field theory study

“…Our results in Fig. 3 seems to confirm very recent measurements demonstrating that the quasiparticle mass is significantly reduced from that of FeSe in hydro-thermo grown FeS superconductor51. Interestingly, this work suggests that metallic FeS is less correlated compared to FeSe, which is consistent with our results showing that the on-site Coulomb interaction U in metallic FeS is at least 0.4 eV smaller than that of tetragonal FeSe as in Fig.…”

Suppression of electronic correlations by chemical pressure from FeSe to FeS

“…While the size of the quasi-two dimensional Fermi surface increases with chemical pressure, the most important change is the increase in Fermi velocities (and bandwidths) ( Fig.3d), which reflects the reduction of the electronic correlations. These findings agree with the reduction of the effective masses determined from quantum oscillations in FeSe 1−x Se x outside the nematic phase [19,34] and FeS [20,21]. Furthermore, the low temperature resistivity shows a T 2 Fermi-liquid-like behavior for FeS, in contrast to the other compositions closer to 0.05 the nematic phase, as shown in Fig.3(e) and also reported in Ref.…”

“…However, sulfur substitution in FeSe 1−x S x completely suppresses the nematic state for x ≥ 0.18(1) [6,18,19], without promoting a high-T c superconducting phase or stabilizing a magnetic order, in contrast to applied pressure [14,15]. The end member of this series, the tetragonal FeS, with a lower T c ≈ 4 K, is suggested to be less correlated [20,21], emphasizing the important role of chemical pressure in tuning electronic ground states and the strength of electronic correlations.…”

Effect of electron correlations on spin excitation bandwidth in Ba0.75K0.25Fe2As2Murai1, Suzuki2, Ideta3 et al. 2018Phys. Rev. B8140View full textAdd to dashboardCite

“…A similar discussion can be used to understand the material-and doping-dependent trends of spin-excitation bandwidth in FeSCs, as has been shown using the dynamical mean field theory (DMFT). [51][52][53] Our results thus demonstrate that it is possible to model the spin excitations of FeSCs by incorporating aspects of the low-energy quasiparticle renormalization that affect both single-and two-particle quantities. In addition, the consistency of the mass renormalization factors determined by independent INS and ARPES measurements highlights the potential capability of INS for characterizing the strength of electron correlations.…”