Incommensurate magnetic fluctuations and Fermi surface topology in LiFeAs

Abstract

Using the angle-resolved photoemission spectroscopy (ARPES) data accumulated over the whole Brillouin zone (BZ) in LiFeAs we analyze the itinerant component of the dynamic spin susceptibility in this system in the normal and superconducting state. We identify the origin of the incommensurate magnetic inelastic neutron scattering (INS) intensity as scattering between the electron pockets, centered around the (π, π) point of the BZ and the large two-dimensional hole pocket, centered around the Γ-point of the BZ. As the magnitude of the superconducting gap within the large hole pocket is relatively small and angle dependent, we interpret the INS data in the superconducting state as a renormalization of the particle-hole continuum rather than a true spin exciton. Our comparison indicates that the INS data can be reasonably well described by both the sign changing symmetry of the superconducting gap between electron and hole pockets as well as sign preserving gap, depending on the assumptions made for the fermionic damping.The relation between unconventional superconductivity and magnetism is one of the most interesting topics in condensed-matter physics. For example, in most of the iron-based superconductors superconductivity occurs in close vicinity to an antiferromagnetic (AF) state[1-3]. Moreover, superconductivity emerges when antiferromagnetic order in parent compounds is suppressed, either by electron/hole doping or disorder. In addition, shortrange AF spin excitations are still present in the normal state of the doped systems and also become resonant in the superconducting state at energies below twice the superconducting gap magnitude, 2∆ 0 [4]. This resonant enhancement is believed to be a signature of a certain phase structure of the superconducting gap as the paramagnetic spin response of the Bogolyubov quasiparticles at the antiferromagnetic wave vector Q AF is sensitive to the anomalous coherence factor 1 − ∆ k ∆ k+Q AF |∆ k ||∆ k+Q AF | . Once the superconducting gap at parts of the Fermi surface, connected by Q AF , changes sign, the spin response acquires an additional enhancement at Ω ≤ 2∆ 0 , which is a hallmark of unconventional superconductivity. The observation of the spin resonance in many iron-based superconductors provides strong evidence for the so-called s +− -wave symmetry of the superconducting gap, where the gap structure changes sign between electron and hole pockets [5][6][7]. Note that this does not exclude the gap on each pocket to have a strong angular variation and even accidental nodal lines, allowed by A 1g symmetry [3]. The angular variation of the gap, measured in ARPES [8], is inconsistent with idealized lattice version of s +− , but can be modeled by taking into account interaction effects. While the behavior, described above, is observed in the majority of the iron-based superconductors, there are some notable exceptions. Perhaps the most interesting one is the stoichiometric LiFeAs, which superconducts at T c =17 K without any doping [9][10][11]. In addition, LiFeAs shows ne...

Superconductivity from repulsion in LiFeAs: Novels-wave symmetry and potential time-reversal symmetry breaking

“…If (π, π) spin fluctuations were strong enough, state B would be the most natural candidate. However, there is still no complete experimental evidence that spin fluctuations with large momentum transfer are strong 24 . There is indirect evidence that spin-fluctuations with a small momentum transfer may also be present -ferromagnetic fluctuations in the nonsuperconducting Li 1−y Fe 1+y As with y ≤ 0.04 are quite strong 46 .…”

Fine structure of the incommensurate antiferromagnetic fluctuations in single-crystalline LiFeAs studied by inelastic neutron scattering

“…In Ref. 21 the authors have identified the INS signal to be connected to scattering between the outer hole pocket and the electron pockets by using a simple tight-binding fit to the ARPES data, while involvement of the inner hole pocket was concluded in Ref. 20.…”

Kinks in theσBand of Graphene Induced by Electron-Phonon Coupling

“…They could even have an effect on transport properties and electronic instabilities when the band edges (almost) coincide with the Fermi energy. This is the case in several classes of materials, such as in iron pnictide superconductors [25,26], transition metal silicides [27], or transition metal dichalcogenites [28]. …”