Superconducting pairing and the pseudogap in nematic striped La2-xSrxCuO4

Abstract

The individual k\parallel and k\perp stripe excitations in fluctuating spin-charge stripes have not been observed yet. In Raman scattering if we set, for example, incident and scattered light polarizations 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\parallel and k\perp strip magnetic excitations separately in La2-xSrxCuO4. Only the k\perp stripe excitations appear in the wide-energy isotropic electronic Raman scattering, indicating that the charge transfer is restricted to the direction perpendicular to the stripe. This is the same as the Burgers vector of an edge dislocation which easily slides perpendicularly to the stripe. Hence charges at the edge dislocation move together with the 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 \le 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).