First-principles investigations of structural properties, phonons, and vacancy ordering in YBa_2Cu_3O_(7-x)

摘要

We have made first-principles investigations into two aspects related to normal state as well as superconducting properties of high-T_c materials: On the one hand, raman-active zone-center phonons in YBa_2Cu_3O_7 and the pressure dependence of the respective frequencies. On the other hand, ordering of oxygen vacancies and its effect on the total energy and electronic structure. Concerning the vibrational properties we demonstrate that an overall agreement between theoretical and experimental phonon frequencies for the 15 raman-active modes at q=0 is only achievable on the basis of optimized structural parameters in the calculation. Keeping in this line of procedure also the experimentally determined linear pressure coefficients for the oxygen-dominated A_91g)-phonon frequencies are theoretically very well reproduced. Ordering of vacancies is studied by means of Monte-Carlo simulations with the interaction parameters extracted from our ab-initio total energy calculations. The equilibrium structure of YBa_2Cu_3O_(6.5) is found to be the observed ortho-II structure. The ordering of oxygens into the Cu-O chains is shown to increase the partial (located in the CuO_2 planes) density of electron states at the Fermi level.