Compositional Evolution of the Anti-phase Stripe Superstructure in Bi2Sr2-xLaxCuO6 (0<x<=1.1) Revealed by Transmission Electron Microscopy

The detailed structural properties of La-doped Bi2Sr2-xLaxCuO6 (La-Bi2201, 0<=x<=1.1) have been studied by transmission electron microscopy (TEM). The well-known incommensurate supermodulation q1 in the Bi-based cuprates evolves from a monoclinic superstructure in the pristine Bi2201 phase to an orthogonal one in La-Bi2201(x=0.73) phase. The b-component of the modulation vector (q1) for x=0.25 sample is about 0.24b* and increases slightly to 0.246b* in x=0.84 sample, while it increases significantly to 0.286b* for x=1.10 sample. We have revealed a new anti-phase stripe superstructure in all the La-doped Bi2201 samples, giving rise to a new modulation with a vector q2. This q2 modulation is directly evolved from the orthogonal modulation q1 through an addition of the anti-phase structure so that its vector q2 equals to q1b/2. We also discussed the implication of these structural studies on the electronic structure by angle-resolved photoemission spectroscopy (ARPES) experiments in La-Bi2201.