In-plane anisotropy of transport coefficients in electronic nematic states: Universal origin of nematicity in Fe-based superconductors

Abstract

The origin of the electronic nematicity and its remarkable material dependence are famous longstanding unsolved issues in Fe-based superconductors. To attack these issues, we focus on the in-plane anisotropy of the resistivity: In the nematic state in FeSe, the relation ρ x > ρ y holds, where ρ x(y) is the resistivity along the longer (shorter) Fe-Fe axis. In contrast, the opposite anisotropy ρ x < ρ y is realized in other undoped Fe-based superconductors. Such nontrivial material dependence is naturally explained in terms of the strongly orbitaldependent inelastic quasiparticle scattering realized in the orbital-ordered state. The opposite anisotropy between FeSe (ρ x > ρ y ) and other undoped compounds (ρ x < ρ y ) reflects the difference in the number of hole pockets. We also explain the large in-plane anisotropy of the thermoelectric power in the nematic state.

Elastoresistance measurements on CaKFe$_4$As$_4$ and KCa$_2$Fe$_4$As$_4$F$_2$ with the Fe site of $C_{2v}$ symmetry

“…Finally, it is highly debated whether the in-plane re-sistivity anisotropy in iron-based superconductors is due to anisotropic scattering or anisotropic Fermi surace [27,30,[63][64][65][66][67][68][69][70][71][72][73][74][75]. If the AFQ-order-like arrangement of the d xz and d yz orbitals in C 2v Fe-site compounds [43] suppresses Fermi surface deformation due to an applied strain, the present observation of enhanced elastoresistance in CaKFe 4 As 4 and KCa 2 Fe 4 As 4 F 2 may suggest the dominant role of anisotropic scattering.…”

Nematicity, magnetism and superconductivity in FeSe

“…Figure 20. Transport anisotropy in FeSe [220] and orbital order from nearest neighbor Coulomb interactions [221] Despite the fact that FeSe is a metal, Heisenberg models of localized spins have been applied to understand its properties. Ab initio studies suggest that the electronic structure of FeSe leads to various competing magnetic states and ultimately does not allow for long range magnetic order.…”

“…Furthermore, the transport anisotropy in the nematic phase has been argued to be due to the positions of cold spots on the Fermi surface which are determined by the spin fluctuations, see Fig. 20(a-e) [220].…”

Resistivity anisotropy from the multiorbital Boltzmann equation in nematic FeSe

“…Accounting for such a difference is far from being straightforward, since the dc conductivity of a multiband metal as iron pnictides is controlled by a delicate balance among Fermi velocities, density of states and scattering rates in the various pockets. In such a situation different theoretical proposals pointed out alternatively a prominent role either of the scattering-rate anisotropy [37][38][39][40][41][42] or of the Fermi-surface deformation [43][44][45]. The former approach relies mainly on the calculation of the inelastic scattering rate due to the exchange of spin fluctuations, whose anisotropy is ascribed either to the spin-nematic nature of the spin fluctuations [37,41,46] or to a secondary effect of orbital ordering [42].…”

“…In such a situation different theoretical proposals pointed out alternatively a prominent role either of the scattering-rate anisotropy [37][38][39][40][41][42] or of the Fermi-surface deformation [43][44][45]. The former approach relies mainly on the calculation of the inelastic scattering rate due to the exchange of spin fluctuations, whose anisotropy is ascribed either to the spin-nematic nature of the spin fluctuations [37,41,46] or to a secondary effect of orbital ordering [42]. The predominant role of the Fermi-surface deformation was instead motivated mainly by the analysis of the nematic anisotropy at finite frequency [44,45,47], which involves in principle both the scattering-rate and the plasma-frequency anisotropy.…”