Nematic pairing from orbital-selective spin fluctuations in FeSe

Abstract

FeSe is an intriguing iron-based superconductor. It presents an unusual nematic state without magnetism and can be tuned to increase the critical superconducting temperature. Recently it has been observed a noteworthy anisotropy of the superconducting gaps. Its explanation is intimately related to the understanding of the nematic transition itself. Here we show that the spin-nematic scenario driven by orbital-selective spin-fluctuations provides a simple scheme to understand both phenomena. The pairing mediated by anisotropic spin modes is not only orbital selective but also nematic, leading to stronger pair scattering across the hole and X electron pocket. The delicate balance between orbital ordering and nematic pairing points also to a marked kz dependence of the hole-gap anisotropy.

Itinerant approach to magnetic neutron scattering of FeSe: Effect of orbital selectivity

“…Since the discovery of new cold vapor deposition methods allowed for the growth of high-quality stoichiometric crystals several years ago, theories have been gradually improved as higher resolution experiments were performed. Recently, several theories 45,73,[82][83][84][85] in addition to the present approach 30,59 have appeared which study selfenergy effects in FeSe, and some also present calculations for the structure of the superconducting gap measured by QPI and ARPES 73,[86][87][88] . In many respects, these ideas parallel our own, although they do not all invoke the concept of quasiparticle weight renormalization.…”

Anisotropic spin fluctuations in detwinned FeSe

“…In addition to having a weak peak around E ≈ 3.2 meV, we find that the scattering changes from well-defined commensurate peaks centered around Q AF below E = 3.625 ± 0. Although these results on twinned FeSe suggest that spin fluctuations play an important role in the superconductivity of FeSe, they provide no information on the possible orbital selective nature of the fluctuations that may lead to a highly anisotropic electron pairing state [19,31,[35][36][37][38]. From STM quasiparticle interference measurements on a single domain (detwinned) FeSe, where the Fermi surface geometry of electronic bands can be determined in the nematic phase, sign-reversed superconducting gaps are found at the hole [Γ or Q = (0, 0)] and electron [X or Q AF = (1, 0)] Fermi surface states derived from d yz orbitals of the Fe atoms along the orthorhombic a o -axis direction [Figs.…”

Crossband versus intraband pairing in superconductors: Signatures and consequences of the interplay

“…To date, multiband electronic structure is proven to be of crucial importance in rather versatile superconducting systems, such as MgB 2 [10], iron-based compounds [11][12][13][14][15][16], superconducting nanostructures [17][18][19][20][21], 2D electron gases at interfaces [22][23][24], metal-organic superconductors [25][26][27], etc. In such multiband superconductors, the pairing interaction and the proximity/hybridization of two or more bands can result in the formation of Cooper pairs with electrons originating from different bands, a phenomenon termed "cross-band pairing" or simply "crosspairing".…”

“…where Ω is the average energy scale of the effective interaction, and ζ i (k) = i (k) − µ with chemical potential µ. In g ij,kl =   g 11,11 g 11,22 g 11,(12) g 22,11 g 22,22 g 22,(12) g (12),11 g (12),22 g (12), (12)   , the upper left 2×2 inner matrix corresponds to the well established SMW case [37], and the third row and column include the crosspairing (where (12) indicates symmetrization under given indices, so that e.g., g (12),(12) = g 12,12 + g 21,21 ). In the interaction matrix the effective attraction between electrons is given by its diagonal elements, and the off-diagonal ones describe the Josephson-like coupling between intraband and cross-band condensates.…”