Realistic Theoretical Study of Long Range Order in Half-Doped Manganites (Paperback)

The exploration of distinct orders and their mutual interplay in systems with transition metal elements is of significant importance to condensed matter physicists, as these orders may provide a way to tune material properties, giving rise to a wide practical application of these materials in industry. Usually these orders involve spin, orbital, charge and lattice degrees of freedom, providing diverse phase diagrams and interesting physical properties, such as superconductivity, thermopower, colossal magnetoresistance (CMR) etc. To have a clear understanding of the problem, one needs to have a deep understanding of the origin and stability of these systems, including the role of leading mechanisms and their mutual coupling. Currently these issues are under great debate in the literature. Manganese oxides, such as La1-xCa xMnO3 have attracted considerable attention due to the observed CMR effect at certain levels of doping (x ≈ 0.2--0.4). To gain a deeper microscopic insight on the issue, we developed a novel, three-step approach within Wannier states framework, that allows one to map out most relevant mechanisms and evaluate their relative importance. Based on the LDA+U results, as a starting point, we derive a low-energy Hamiltonian, that includes all relevant interactions. In the second step we construct Wannier states from relevant e g states close to the Fermi level via a unitary transformation and subject to certain imposed constraints. We use these states as a basis of our further calculations. In the third step, we perform a self-consistent mapping of the low-energy Hamiltonian to the LDA+U results expressed in terms of the same Wannier states basis. As a result, the Hamiltonian parameters are evaluated. These parameters are used to evaluate the contribution of various interaction terms in the Hamiltonian separately. In addition, this framework allows natural solution of a current discrepancy between the weak charge disproportion in doped manganites and the traditional Goodenough Mn3+/Mn4+ picture. Comparison of results for the doped system to the parental undoped LaMnO3 system is crucial for evaluating the role of charge degree of freedom. Finally, we explore the possibility of using a biorthogonal wavelet basis set in density functional calculations. An alternative method, based on a direct minimization is suggested for the search for the ground state.