J. G. Che

Possible faceting transformation of Mo from (111) to {110}

Abstract It is well-documented that the (111) surfaces of Mo and W are unstable upon the adsorption of ultra-thin overlayers of some fcc metals and transform to {112} type facets. Using first principles total energy calculations, we found that it is also thermodynamically allowed,for Mo(111) to facet to the {110} orientations upon the adsorption of a coating layer of Pd and Pt. An overlayer coverage of about one physical monolayer would be needed to drive the transformation.

First principles studies of overlayer-induced faceting

Abstract Using first principles total energy calculations, we studied the overlayer growth mode and the substrate stability when ultra-thin overlayers of various metals are grown on Mo(111) substrates. In close agreement with experimental results, we found that the growth modes are Stranski-Krastanov, and the overlayers of some metals, can induce the substrate to facet. The faceting is basically driven by the enhancement of the surface energy anisotropy due to adsorption, and the interaction with the substrate are stronger for faceting agents than for non-faceting agents. In some cases it is energetically favorable for the substrate to transform to facets of more than one orientation, leading to the possible coexistence of two types of pyramidal facets.

A route to multiferroics by non-d0 cation B in magnetic perovskites

Based on first-principles calculations on BaTcO3, we explored a route to promoting ferroelectricity by non-d0 cation B in magnetic perovskites, leading to the co-existence of magnetic and dielectric ordering in a single phase. The ground state of the perovskite BaTcO3 was found to be an insulator with G-type antiferromagnetic order. A tensile strain in BaTcO3 not only inverted the lowest unoccupied states dominated from t2g to eg, driving the ferroelectric distortion, but also enlarged this driven force due to the induced narrower gap as its divisor. The tensile strain simultaneously weakened the repulsive force when Tc distorted to one apex of the O-octahedron in BaTcO3. Most importantly, since the physics of our finding is not Tc specific, the route is generally applicable to other magnetic perovskites.

Could the relaxed and the O-deficient CrO2(100) surface retain half-metallicity?

We report the results of a first principles study on the clean and the reduced CrO2(100) surface. It is shown that both surfaces retain the half-metallicity because they share a common feature: the surface Cr atom also behaves like an isolated ion. Thus, the half-metallicity of the CrO2 surfaces can be qualitatively understood by a simple model based on Hund’s rule.

A phase diagram for band inversion of topological materials as a function of interactions between two involved bands

Basing on first-principles calculations, we predicate that Bi on a graphene derivative, g-C14N3, which involves a 3 × 3 unit cell of graphene with four C atoms substituted by three N atoms, is a topological insulator with a gap of 50 meV. With the help of maximally localized Wannier functions, we find that its band inversion gap can be determined by examining a pair of interaction parameters between the two involved bands. Accordingly, a phase diagram for band inversion of topological materials as a function of the interactions is obtained. The conclusion also holds for Sb, Ir and Rh on g-C14N3. These materials are topological nontrivial either insulator or semimetal, indicating that g-C14N3 is a good platform for conceiving topological materials.

The adsorption of Ag and Sb on Ag(111) surface

The adsorption of Ag and Sb atoms on Ag(111) surface was studied by using the extended Huckel theory (EHT) and a cluster model. The possible mechanisms for the layer-by-layer growth of Ag on Ag(111) with Sb as surfactant are discussed. The results are in qualitative agreement with the previous experimental and theoretical results.

Effects of charged surface states on Sb ad-dimer rotation on Si(001)

Based on first-principles calculations, we identify a two-stage process for the rotation of Sb ad-dimer on Si(001). The bond between Sb and Si, which is broken over transition states in the rotation pathway, undergoes changes from σ bonding to π bonding near the transition states. Being close to the Fermi level, these π states are sensitive to an external electric field, and thus may play a role in understanding the reversible rotation of the Sb ad-dimer between two orthogonal orientated states on Si(001) observed by scanning tunneling microscopy.