Structure of spin excitations in heavily electron-doped Li0.8Fe0.2ODFeSe superconductors

Abstract

Heavily electron-doped iron-selenide high-transition-temperature (high-T c ) superconductors, which have no hole Fermi pockets, but have a notably high T c , have challenged the prevailing s ± pairing scenario originally proposed for iron pnictides containing both electron and hole pockets. The microscopic mechanism underlying the enhanced superconductivity in heavily electron-doped iron-selenide remains unclear. Here, we used neutron scattering to study the spin excitations of the heavily electron-doped iron-selenide material Li 0.8 Fe 0.2 ODFeSe (T c = 41 K). Our data revealed nearly ring-shaped magnetic resonant excitations surrounding (π, π) at ∼21 meV. As the energy increased, the spin excitations assumed a diamond shape, and they dispersed outward until the energy reached ∼60 meV and then inward at higher energies. The observed energy-dependent momentum structure and twisted dispersion of spin excitations near (π, π) are analogous to those of hole-doped cuprates in several aspects, thus implying that such spin excitations are essential for the remarkably high T c in these materials.

Sign reversal of the order parameter in (Li1−xFex)OHFe1−yZnySe

“…At present, our results do not allow us to distinguish between the two. The latter has an advantage of having a clear candidate for pairing glue, namely spin fluctuations resulting from inter-pocket nesting in the unfolded zone (note that such fluctuations were observed, at q = {π, π/2} (refs [33][34][35] and calculated, at q = {π, 0.625π} (ref. 14) in K x Fe 2−y Se 2 or other intercalated system, which would have naturally led to a state depicted in Fig.…”

Fermi surface pockets in electron-doped iron superconductor by Lifshitz transition

“…In addition, recent inelastic neutron scattering experiments on intercalated FeSe find a ring of low-energy magnetic excitations centered at the wave number (π/a, π/a) associated with Néel order over the square lattice of iron atoms in a single layer [20][21][22]. Here, a denotes the lattice constant.…”

Probing the superconducting gap structure of (Li1−xFex)OHFeSe

“…26,27 Meanwhile the in-plane superfluid density obtained from muon-spin rotation (µSR) measurements is consistent with either one or two gaps, but very different behavior is seen in the out-of-plane component, which shows a much more rapid drop of the superfluid density with temperature. 28 Furthermore, inelastic neutron scattering (INS) measurements give evidence for the presence of a spin resonance peak, 29,30 consistent with a sign change of the order parameter across the Fermi surface, while a lack of a sign change was suggested from an STS study, on the basis of quasi-particle interference (QPI) results and the effect of impurities.…”

“…From a recent STM study it was proposed on the basis of QPI measurements, as well as examining the effects of impurities, that there is no sign change of the superconducting gap across the Fermi surface 26 , in which case such a two gap s-wave model readily explains the data. However INS measurements show evidence for a spin resonance peak, 29,30 which indicates that there is a sign change of the order parameter between regions of the Fermi surface connected by the resonance wave vector. This would be incompatible with two-gap s-wave superconductivity with no sign change and are also difficult to account for with the s ± state proposed for many iron arsenide superconductors, due to both the lack of a hole pocket at the zone center and a different nesting wave vector for the spin resonance.…”