Symmetry of spin excitation spectra in the tetragonal paramagnetic and superconducting phases of 122-ferropnictides

Abstract

We study the symmetry of spin excitation spectra in 122-ferropnictide superconductors by comparing the results of first-principles calculations with inelastic neutron scattering (INS) measurements on BaFe 1.85 Co 0.15 As 2 and BaFe 1.91 Ni 0.09 As 2 samples that exhibit neither static magnetic phases nor structural phase transitions. In both the normal and superconducting (SC) states, the spectrum lacks the threedimensional (3D) 4 2 /m screw symmetry around the ( 1 2 1 2 L) axis that is implied by the I4/mmm space group. This is manifest both in the in-plane anisotropy of the normal-and SC-state spin dynamics and in the out-ofplane dispersion of the spin-resonance mode. We show that this effect originates from the higher symmetry of the magnetic Fe-sublattice with respect to the crystal itself, hence the INS signal inherits the symmetry of the unfolded Brillouin zone (BZ) of the Fe-sublattice. The in-plane anisotropy is temperature-independent and can be qualitatively reproduced in normal-state density-functional-theory calculations without invoking a symmetry-broken ("nematic") ground state that was previously proposed as an explanation for this effect. Below the SC transition, the energy of the magnetic resonant mode ω res , as well as its intensity and the SC spin gap inherit the normal-state intensity modulation along the out-of-plane direction L with a period twice larger than expected from the body-centered-tetragonal BZ symmetry. The amplitude of this modulation decreases at higher doping, providing an analogy to the splitting between even and odd resonant modes in bilayer cuprates. Combining our and previous data, we show that at odd L a universal linear relationship ħ hω res ≈ 4.3 k B T c holds for all the studied Fe-based superconductors, independent of their carrier type. Its validity down to the lowest doping levels is consistent with weaker electron correlations in ferropnictides as compared to the underdoped cuprates.

Effect of Li-deficiency impurities on the electron-overdoped LiFeAs superconductor

“…This nesting condition is consistent with previous work on LiFeAs [21] and our own measurements. These results are also in agreement with the Fermi surface nesting interpretation of the low-energy spin excitations in the electron- [30,31] and hole-doped [39,40] BaFe 2 As 2 , and thus suggest that the stoichiometric LiFeAs is an intrinsically electron-overdoped superconductor. This is also consistent with the fact that further electron-doping in the SC LiFeAs via Ni and Co substition can systematically reduce T c [11].…”

“…The quasiparticle excitations between the mismatched hole and electron Fermi surfaces due to the self electron-doping should produce incommensurate spin fluctuations along the direction transverse to the AF ordering wave vector Q = (1, 0) (Figs. 1d and 1e) consistent with the calculated spin susceptibility χ ′′ (Q, ω) based on a random phase approximation of a three-dimensional 5-orbital tight-binding model for BaFe 2 As 2 [30,31,38]. Experimentally, the transverse incommensurate spin fluctuations with δ K ≈ 0.1 were found at E = 7 meV for the electron overdoped BaFe 2−x Ni x As 2 at x = 0.15 [31].…”

“…We find that a few percent Li-deficiency can completely suppress superconductivity and change transport properties but without significant effect on the sizes of the Fermi surfaces [20] and incommensurate spin excitations [21]. By comparing our results with previous work on the SC LiFeAs [20,21], NaFe 1−x Co x As [26][27][28], BaFe 2−x T x As 2 [29][30][31], and LaFe 1−y Zn y AsO 1−x F x [32], we conclude that the stoichiometric LiFeAs is an intrinsically electronoverdoped superconductor similar to NaFe 1−x Co x with x ≈ 0.065, and that Li deficiencies affect its SC properties similar to the Zn impurity effects in the electronoverdoped iron pnictide superconductors [32]. These results naturally explain the absence of the static AF order in Li 1−x FeAs, and why superconductivity in LiFeAs is so sensitive to the Li-deficiency.…”

Polarized neutron scattering studies of magnetic excitations in electron-overdoped superconducting BaFe1.85Ni0.15As

“…Near the optimally electron-doped superconductor BaFe 2−x Ni x As 2 at x = 0.1 (T c = 20 K), the static AF order is suppressed 12 . However, short-range spin excitations persist and couple directly to superconductivity via a collective magnetic excitation termed the neutron spin resonance [12][13][14][15][16][17] . As a function of Ni-doping, the energy of the resonance is associated with both the superconducting electronic gap ∆ and k B T c , thus indicating its direct connection with superconductivity 18 .…”

“…Although the resonance appears to be a common feature amongst different classes of unconventional superconductors including high-T c copper oxides [19][20][21][22][23] , heavy Fermions 24,25 , and iron-based materials [12][13][14][15][16][17][26][27][28] , much remains unknown about its microscopic origin. Assuming that the resonance is a spin-1 singlet-to-triplet excitation of the Cooper pairs 29 , it should be possible to split it into three peaks under the influence of a magnetic field via the Zeeman effect by an amount ∆E = ±gµ B B, where g = 2 is the Lande factor and B is the magnitude of the field [30][31][32][33] .…”