Spin-Density-Wave Gap with Dirac Nodes and Two-Magnon Raman Scattering in BaFe₂As₂

Abstract

Journal of the Physical Society of Japan FULL PAPERSRaman selection rules for electronic and magnetic excitations in BaFe 2 As 2 were theoretically investigated and applied them to the separate detection of the nodal and anti-nodal gap excitations at the spin density wave (SDW) transition and the separate detection of the nearest and the next nearest neighbor exchange interaction energies.Raman spectra are composed of magnetic excitations with gradually decreasing intensity toward far above the SDW transition temperature (T SDW ) and electronic excitations induced by the Brillouin zone folding below T SDW . The SDW gap has Dirac nodes, be- and change into the gap structure by the first order transition at T SDW , while those from the nodal region gradually change into the SDW state. Magnetic excitations are observed as a very broad peak in all polarization configurations. The selection rule for two-magnon scattering from the stripe spin structure was obtained. Applying it to the two-magnon Raman spectra it is found that the magnetic exchange interaction energies are not presented by the short-range superexchange model, but the second derivative of the total energy of the stripe spin structure with respect to the moment directions.The selection rule and the peak energy are expressed by the two-magnon scattering process in an insulator, but the large spectral weight above twice the maximum spin wave energy is difficult to explain by the decayed spin wave. It may be explained by the electronic scattering of itinerant carriers with the magnetic self-energy in the localized spin picture or the particle-hole excitation model in the itinerant spin picture.The magnetic scattering spectra are compared to the insulating and metallic cuprate superconductors whose spins are believed to be localized.

Magnetic interactions in iron superconductors: A review

“…B 1g and B 2g polarisations are sensitive to anisotropic responses along the Fe-Fe bond and Fe-As bond respectively. A strong signal, which cannot be explained by the orthorombic distortion alone, appears just in B 1g Raman response [241,267] when undergoing the magnetostructural transition . In Ref.…”

Phonon, two-magnon, and electronic Raman scattering of Fe1+yTe1−xSeOkazaki1, Sugai2, Niitaka3 et al. 2011Phys. Rev. B

“…In the Raman spectra of BaFe 2 As 2 (Ba122), similar broad peak and weak edge structures have been observed. 36,37 The weak edge structures evolve below the spin-densitywave (SDW) transition temperature T SDW around 400 and 800 cm −1 for Ba122. In the optical conductivity σ(ω) of Ba122, also, double-peak structures develop below T SDW and this feature is considered to be related to the SDW gap.…”

“…The energy of two-magnon scattering is also similar between Fe 1.074 Te and Ba122. 36,37 This may be surprising at first glance, because the magnetic structure is different between Fe 1.074 Te and Ba122. While Ba122 has a singlestripe AFM structure with the AFM wavevector Q AF = (π, π), Fe 1+y Te has a double-stripe AFM structure with Q AF = (δπ, 0).…”

Coupling of the AsA1gphonon to magnetism in iron pnictides

“…36 Raman response represents a powerful tool for investigating the properties of lattice dynamics. 1,3,5,8,[37][38][39][40][41][42][43] A significant narrowing of the A 1g As phonon linewidth at the onset of magnetism has been reported, whereas both softening and hardening of the phonon frequency with decreasing temperature have been observed. 1,39,41 Crucial information is also encoded in the intensity of the phonon resonances.…”

“…When undergoing the magnetostructural transition, a strong phonon signal emerges in the B 1g Raman response but not in B 2g . 8,42 In 122 compounds, the A 1g intensity shows a strong enhancement in the magnetic state. 1,3,8,42 In spite of this, in BaFe 2 As 2 , the B 1g intensity is about 1.5 times larger than the A 1g intensity.…”