Anisotropic spin fluctuations in detwinned FeSe

Abstract

Superconductivity in FeSe emerges from a nematic phase that breaks four-fold rotational symmetry in the iron plane. This phase may arise from orbital ordering, spin fluctuations, or hidden magnetic quadrupolar order. Here we use inelastic neutron scattering on a mosaic of single crystals of FeSe detwinned by mounting on a BaFe2As2 substrate to demonstrate that spin excitations are most intense at the antiferromagnetic wave vectors Q AF = (±1, 0) at low energies E = 611 meV in the normal state. This two-fold (C2) anisotropy is reduced at lower energies 3-5 meV, indicating a gapped four-fold (C4) mode. In the superconducting state, however, the strong nematic anisotropy is again reflected in the spin resonance (E = 3.7 meV) at QAF with incommensurate scattering around 5-6 meV. Our results highlight the extreme electronic anisotropy of the nematic phase of FeSe and are consistent with a highly anisotropic superconducting gap driven by spin fluctuations.

Strong local moment antiferromagnetic spin fluctuations in V-doped LiFeAs

“…Considering the different superconducting behavior in Ba 1−x K x Fe 2 As 2 and LiFe 1−x V x As, we argue that the inter-orbital scattering processes between d xz/yz and d xy orbitals increase the fluctuating moment but are detrimental to superconductivity [32]. These results are consistent with the notion that intra-orbital scattering processes between hole and electron pockets, particularly those involving d yz -d yz orbital characters, are good for superconductivity in different classes of iron-based superconductors [33][34][35][36][37].…”

Anisotropic effect of a magnetic field on the neutron spin resonance in FeSe

“…If the spin gap energy below the resonance decreases linearly with the applied field, we estimate that the lowest energy position of the spin gap to E = 0 meV in the c-axis and in-plane magnetic fields corresponds to fields of 12 and 30 T, respectively. These values are close to the measured upper critical fields (B c2 ) of 16 and 28 T. The field direc-tional dependence of the spin resonance is also consistent with that of the superfluid density from heat capacity measurements [60], implying that the resonance is associated with superconducting electrons arising from the orbital selective hole-electron quasi-particle excitations [42,43].…”

“…First, the compound is known to be extremely clean and has a relatively low resonance energy of E r = 3.6 meV [41]. Second, superconductivity in FeSe is orbital selective and occurs mostly through hole-electron Fermi surface nesting of quasi-particles with d yz orbital characters [42], resulting in a resonance only at the in-plane AF wave vector Q AF = (1, 0) [43]. Third, neutron polarization analysis of the resonance reveals that the mode is anisotropic in spin space and essentially c-axis polarized due to SOC [44], suggesting that a magnetic field cannot split the mode into triplets.…”

“…At zero field, superconductivity in FeSe induces a resonance at E r ≈ 3.6 meV and a spin gap of about 2.8 meV as shown from data obtained on PANDA [Figs. 1(e) and 1(f)] [41,43,44]. The effect of an 8.5 T in-plane magnetic field on the resonance and low-energy spin excitations is shown using data obtained on MACS.…”

Spin fluctuation anisotropy as a probe of orbital-selective hole-electron quasiparticle excitations in detwinned Ba(Fe1−xCox)2As2Tian1, Li2, Xu3 et al. 2019Phys. Rev. BSelf Cite145110View full textAdd to dashboardCite

“…The two-dimensional (2D) magnetic scattering images of Ba(Fe 0.946 Co 0.054 ) 2 As 2 in the (H, K) plane for different energy transfers below (T = 4.6 K) and above (T = 23 K) T c are shown in Figures 5(a) The effect of the partial detwinning ratio was corrected using the method developed in Ref. 33 . Consistent with Figs.…”