Low-energy spin excitations in (Li0.8Fe0.2)ODFeSe superconductor studied with inelastic neutron scattering

Abstract

We report an inelastic neutron scattering study of single crystals of (Li0.8Fe0.2)ODFeSe. Temperature-dependent low-energy spin excitations are observed near Q = (0.5, 0.27, 0.5) and equivalent wave vectors symmetrically surrounding Q = (0.5, 0.5, 0.5) in the 1-Fe Brillouin zone, consistent with a Fermi-surface-nesting description. The excitations are broadly distributed in energy, ranging from 16 to 35 meV. Upon cooling below the superconducting critical temperature (Tc), magnetic response below twice the superconducting gap 2∆SC exhibits an abrupt enhancement, consistent with the notion of spin resonance, whereas the response at higher energies increases more gradually with only a weak anomaly at Tc. Our results suggest that (Li0.8Fe0.2)ODFeSe might be on the verge of a crossover between different Cooper-pairing channels with distinct symmetries.A pivotal issue concerning the Cooper-pairing mechanism in the Fe-based superconductors (FeSCs) is the pairing symmetry, which is commonly regarded as an important thread for distinguishing among different theoretical proposals. Pairing mediated by electron-phonon interactions [1] and/or orbital fluctuations [2] is expected to occur in the plain s-wave (s ++ ) channel, whereas pairing mediated by spin fluctuations [3] is expected to have sign-reversal behaviors in the gap function and occur in the extended s-wave (s +− ) or the d-wave channel [4][5][6][7]. When hole pockets are present at the Γ point, it has been reasonably well established that the predominant pairing symmetry is s +− [8,9], which favors the unconventional pairing mechanism associated with spin fluctuations. But because the FeSCs are multi-orbital systems with multiple magnetic exchange interactions that are comparable in strength [10], the preference on one pairing channel over another may further depend on the Fermi-surface (FS) topology [5,11]. Therefore, the assumption that a universal pairing mechanism applies to all FeSCs requires a more stringent test, namely, pairing symmetry needs to be determined and compared with theory for systems with very different band filling and FS topologies.An important case of distinct FS topology was first established in alkali-metal intercalated Fe selenides [12], which have no hole pocket at the Γ point as a result of heavy electron doping [13][14][15]. Later it became clear that the electron doping can be achieved with various methods, including intercalation into bulk FeSe [16][17][18][19][20][21], epitaxial growth of a single atomic layer of FeSe on a charge-transferring SrTiO 3 substrate (FeSe/STO) [22][23][24], and surface dosing of FeSe with potassium [25][26][27], all of which end up with similar electronic structures. It is intriguing that the absence of hole pockets at the Γ point is empirically linked to the much higher values of T c than in bulk FeSe [28]. The associated pairing symmetry is considered to be of great theoretical importance but has remained unsettled [11,[29][30][31][32][33][34][35], in part because of seemingly contradictory results fro...

Resilient Nodeless d -Wave Superconductivity in Monolayer FeSe

“…DFT reveals that the only realistic possibility is fluctuations associated with CB-AFM order [21][22][23][24]. This is consistent with experiment [25,26]. In particular, in bulk 3D FeSe (with a low T c = 8 K), CB-AFM and stripe magnetic fluctuations are both observed, and the onset of nematic order suppresses the CB-AFM fluctuations [25].…”

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

“…Observable spin excitations should be brightest along the floating ring at weak mixing between the 3d xz and 3d yz orbitals. Similar low-energy spin resonances around Q AF have been observed recently in the superconducting phase of intercalated FeSe by inelastic neutron scattering [20][21][22]. Such experiments are then consistent with the hSDW state proposed here for heavily electron-doped FeSe.…”

“…Recent calculations by one of the authors also find electron-type Fermi surface pockets at the corner of the two-iron Brillouin zone when electrons are added to the local moments [18]. Furthermore, in the previous section, we have pointed out that the low-energy spin excitations predicted by the local-moment model in the hidden magnetic order phase resemble the 'floating' ring of spin-excitations that has been observed recently in heavily electron-doped FeSe by inelastic neutron scattering [20][21][22]. The extended two-orbital Hubbard model therefore is promising phenomenologically.…”

“…They vanish in energy at the Néel wave number (π/a, π/a) with an acoustic dispersion. We argue in the Discussion section that degeneracy of these hidden Goldstone modes with true spin excitations results in a ring of low-energy magnetic excitations similar to what is observed by inelastic neutron scattering in intercalated FeSe [20][21][22].…”

“…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.…”