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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.

The present systematic Raman scattering experiments reveal the phason and amplitudon of the charge density wave (CDW) mode in the charge stripes of La(2-x)Sr(x)CuO4. Only about 15% of the electronic density of states condenses into the CDW state. The symmetries of the CDW modes change by the rotation of the stripes at the insulator-metal transition. The energy of the phason is finite at 0.06 < or = x < or = 0.1 and zero at x = 0.035 and 0.115 < or = x < or = 0.135, which suggests that the CDW is commensurate at 0.06 < or = x < or = 0.1 and incommensurate otherwise. The zero-energy phason seems to reduce T(c) at x = 1/8.

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The individual k|| and k ⊥ stripe excitations in fluctuating spin-charge stripes have not been observed yet. Raman scattering has a unique selection rule that the combination of two electric field directions of incident and scattered light determines the observed symmetry. If we set, for example, two electric fields to two possible stripe directions, we can observe the fluctuating stripe as if it is static. Using the different symmetry selection rule between the B1g two-magnon scattering and the B1g and B2g isotropic electronic scattering, we succeeded to obtain the k|| and k ⊥ strip magnetic excitations separately in La2−xSrxCuO4. Only the k ⊥ stripe excitations appear in the wide-energy isotropic electronic Raman scattering, indicating that the charge transfer is restricted to the direction perpendicular to the fluctuating stripe. This surprising restriction is reminiscent of the Burgers vector of an edge dislocation in metal. The edge dislocation easily slides perpendicularly to an inserted stripe and causes ductility in metal. Hence charges at the edge of a stripe move together with the edge dislocation perpendicularly to the stripe, while other charges are localized. A looped edge dislocation has lower energy than a single edge dislocation. The superconducting coherence length is close to the inter-charge stripe distance at x ≤ 0.2. Therefore we conclude that Cooper pairs are formed at looped edge dislocations. The restricted charge transfer direction naturally explains the opening of a pseudogap around (0, π) for the stripe parallel to the b axis and the reconstruction of the Fermi surface to have a flat plane near (0, π). They break the four-fold rotational symmetry. Furthermore the systematic experiments revealed the carrier density dependence of the isotropic and anisotropic electronic excitations, the spin density wave and/or charge density wave gap near (π/2, π/2), and the strong coupling between the electronic states near (π/2, π/2) and the zone boundary phonons at (π, π).