Coupling of stripes to lattice distortions in cuprates and nickelates

In recent X-ray diffraction experiments on orthorhombic nickelates, we have observed a significant coupling between the stripe correlations and the lattice distortion. In particular, the diagonal charge stripes of this insulating material exhibit a preferred orientation parallel to the orthorhombic a-axis, similar to the diagonal spin stripes in La2−xSrxCuO4 below the metal-insulator transition. In the light of this finding, we draw a comparison between the lightly doped parts of the phase diagrams of La2−xSrxCuO4 with low-temperature orthorhombic (LTO) structure, and La1.8−xEu0.2SrxCuO4 with low-temperature tetragonal (LTT) structure. In La1.8−xEu0.2SrxCuO4, a local minimum in the Sr dependence of the LTO/LTT transition temperature, as well as reduced spin-glass transition temperatures, may be indicators for the presence of diagonal charge correlations. Above the metal-insulator transition the LTT phase is suited to pin parallel stripes, while below that transition the LTT phase seems instead to compete with the formation of diagonal stripes.