Characterization of Structure and Electronic States of La0.5Ca0.25MnO3 surface By First-Principles Simulation

Perovskite oxides (perovskites) are promising candidates for oxidation catalysis in solid-state fuel cells because of their high chemical activity on the surface and variety in composition. However, direct observation of their surface structures and properties has been difficult due to the lack of easy-cleavage plane. Research on the clean surfaces of perovskites has been demanded.

Recently, some of the coauthors of this presentation have succeeded in epitaxial growth of La_0.75Ca_0.25MnO₃ (LCMO) thin films and their scanning tunneling microscope (STM) obsearvation at low temperature (see Fig. 1). They found that the LCMO surface is reconstructed and shows a zigzag pattern with √2×√2 periodicity. They also revealed via scanning tunneling spectroscopy that the surface is insulating. In order to identify its atomic structure and mechanism of its insulating nature, we have carried out first-principle simulations with the LCMO slab models.

We use the Vienna ab-initio simulation package (VASP). The strong Coulomb interaction among 3d electrons in Mn ions is taken into acount by local spin density approximation (LSDA) + U method. We adopted 28 doping models of the La_0.75Ca_0.25MnO₃ four-layered slab and analyzed atomic and electronic strucutres of 3 typical models (see Fig. 2). Since the samples are made on SrTiO₃(100) substrate in the experiments, we fixed the lattice constants and atomic positions of undermost layer to those determined by experiments. STM images were simulated using the local density of states (LDOS) at 1.5eV below the Fermi level and at 2 Å from the surface.

Simulated STM image for the most stable model (See Fig. 3) shows the same zigzag tendency as the experimental ones. This trend can be understood by the reconstruction of the coordinated O atoms around Mn. Some of the unstable doping models do not show the zigzag tendency because of large surface distortion due to the Ca dopant near the surface.

The stable doping model also reproduces the insulating nature of surface. On the other hand, layer projected density of states show no band gap for the inner layers of the slabs. Structural analysis together with the charge analysis suggests that this difference is led by the surface reconstruction around the Mn atoms.