Spin-orbital interplay and topology in the nematic phase of iron pnictides

Abstract

The origin of the nematic state is an important puzzle to be solved in iron pnictides. Iron superconductors are multiorbital systems and these orbitals play an important role at low energy. The singular C4 symmetry of dzx and dyz orbitals has a profound influence at the Fermi surface since the Γ pocket has vortex structure in the orbital space and the X/Y electron pockets have yz/zx components respectively. We propose a low energy theory for the spin-nematic model derived from a multiorbital Hamiltonian. In the standard spin-nematic scenario the ellipticity of the electron pockets is a necessary condition for nematicity. In the present model nematicity is essentially due to the singular C4 symmetry of yz and zx orbitals. By analyzing the (π, 0) spin susceptibility in the nematic phase we find spontaneous generation of orbital splitting extending previous calculations in the magnetic phase. We also find that the (π, 0) spin susceptibility has an intrinsic anisotropic momentum dependence due to the non trivial topology of the Γ pocket.

Magnetic interactions in iron superconductors: A review

“…Different experiments seem to indicate that it is electronic in origin [45-47, 51, 54, 55, 67] but it is difficult to pinpoint between the spin [71,72] or orbital [36,73,74] degrees of freedom due to the spin-orbital entanglement. A recent effective model sensitive to the orbital degree of freedom finds that spin fluctuations generate orbital splitting [75]. The possible role of impurities is also under investigation [48,53,58,[76][77][78][79].…”

“…when the ordered states are well described by an instability of the Fermi surface and if the orbital content of the Fermi surface does not play a relevant role. Very recently a low energy model, which only keeps interactions in the spin-channel but retains the orbital character of the bands and the information about U and Hund's coupling, has been derived [75].…”

“…The renormalisation of the mass, which in a Fermi liquid description is the inverse of the quasiparticle weight, is ∼ 3, emphasizing the importance of correlations. Aside from a few exceptions [37,75,175,176] weak and strong coupling approaches ignore the physics emerging from the multi-orbital character of iron superconductors. The simplest generalisation of these models allows the coexistence of itinerant weakly correlated orbitals and strongly localised ones.…”

“…Recently an effective action has been proposed for the magnetic instability derived from a multi-orbital Hamiltonian [75]. The Landau coefficients depend on the orbital content, Hubbard and Hund's coupling.…”

Electronic origin of structural transition in 122 Fe based superconductors

“…A second scenario is that the C 4 symmetry is broken primarily by electronic interactions so that lattice degrees of freedom are secondary order parameters that passively follow the symmetry breaking induced by the electronic interactions hence the primary order parameter is electronic in origin. One such scenario is the spin-nematic transition whereby the spins of the two Fe sublattices phase-lock, this breaks C 4 symmetry, without developing any spontaneous magnetization, i.e., without breaking time reversal symmetry [18][19][20][21][22][23][24][25][26][27][28]. Another possible candidate in this scenario is ferro-orbital ordering [28][29][30][31][32][33], where below nematic transition either the occupations or the hopping matrix elements (or both) of the d xz and the d yz orbitals of Fe become inequivalent.…”