Atomsitic Simulation Of Doped LaMnO₃ And La₂CuO₄

In this thesis, we review the research history and some results of mangnites and the hole-stripes in La₂-xSrxCuO₄. The brief introduction of the theoretic background of atomistic simulation is also given.With systemic atomistic simulation, we investigate the atomic distribution and local structure in Ca/Sr doped LaMnO3. In La_1/3Ca_2/3MnO₃, it is found that the doped Ca²⁺ ions have a tendency to be ordering. Apart from Wigner-crystal and bi-stripe charge ordering models proposed by other researchers, we find a new possible and energetically favourable charge ordering model, layer-stripe model, in which La³⁺ and Mn³⁺ ions forming a LaMnO3-like local structure whereas Ca²⁺ and Mn4+ forming a CaMnO3-like local structure. It is also found that in these three stripe models, especially in the layer-stripe model, the local structure is very different from its average structure. The layer-stripe model can coexist with other charge ordering models in La_1-xCa_xMnO₃ (x = 1/3, 1/2 and 2/3), and may be a general phenomenon in these compounds. In La1-xSrxMnO3 (x≤1/4), our simulation reveals a tendency of Sr clustering rather than random Sr distribution. The local structures in the vicinity of the La and Sr clusters are different. In La_2-xSr_xCuO₄ (x≤1/3), we verify two types of domain-wall hole-stripes, and find several other possible hole-stripe models, which are half-filled or quarter-filled. Our simulated results are similar to the experimental and other theoretical results, indicating that charge-ordering may be a general phenomenon in transition metal oxides.We also investigate the lattice distortion and MnO6 octahedra distortion in Ca/Sr/Co/Ni-doped LaMnO3 and in La_0.75Ca_0.25MnO₃/La_0.89Sr_0.11MnO₃ under pressure. With these structural transiton, we estimate bandwidth (W) and calculate Jahn-Teller energy (EJT). The caculated EJT accords well with the experimental data. We calculate lattice strains with lattice parameters and find that there is a quantitative relation between Curie temperature (TC) and lattiece strains. We also demonstrate the quantitative relation among TC, W and EJT. It is also found that about 75% enhancement of TC is contributed by EJT and the rest by W. Therefore, we propose that EJT plays a main role, whereas W plays a much less but not negligible role on TC. Our calculated TC from two methods agrees well with the experimental results.