Atomic-layer synthesis and imaging uncover broken inversion symmetry in La_2-xSr_xCuO₄ films

Materials with broken inversion symmetry are of great interest for their exotic electronic properties but are relatively rare. We show here that even if a material is inversion-symmetric in the bulk form, in epitaxial thin films of the same compound the crystallographic unit cell can be quite asymmetric. To demonstrate this, we have used atomic layer-by-layer molecular beam epitaxy to synthesize La_2-xSr_xCuO₄ films with Sr-dopant atoms deposited above, below, or on both sides of each CuO₂ layer. Surface x-ray diffraction experiments analyzed by the Coherent Bragg Rod Analysis (COBRA) method have been carried out in combination with energy-differential diffraction measurements to determine whether the actual Sr distribution coincided with the nominal one (that would be expected in absence of La-Sr interdiffusion). The differential approach minimizes the systematic errors, and the combination with COBRA is potentially capable to determine the concentration of atomic species on a monoatomic layer-by-layer basis. The results show that the concentration of Sr in La/Sr layers just above the CuO₂ layers is much larger than in layers just below them, irrespective of the deposition sequence, and drastically breaking the inversion symmetry.