Novel mechanism for weak magnetization with high Curie temperature observed in H-adsorption on graphene.

To elucidate the physics associated with the magnetism observed in nominally nonmagnetic materials containing only sp-electrons, we have developed an extreme model to simulate the adsorption of H (in a straight-line form) on graphene. Our first principles calculations for the model result in a ferromagnetic ground state at a high temperature with a magnetic moment of one Bohr magneton per H atom. The removal of p  z -orbitals from sublattice B of graphene introduces p  z -vacancies. The p  z -vacancy-induced states are created not because of the variations in interatomic interactions but because of the p  z -orbital imbalance between two sublattices (A and B) of the conjugated p  z -orbital network. Therefore, some critical requirements should be satisfied to create these states (denoted as [Formula: see text]) to avoid further imbalances and to minimally affect the conjugated p  z -orbital network. The requirements for the creation of [Formula: see text] can be given as follows: (1) [Formula: see text] consists of p  z -orbitals of only the atoms in sublattice A, (2) the spatial wavefunction of [Formula: see text] is antisymmetric, and (3) in principle, [Formula: see text] extends over the entire crystal without decaying, unless other p  z -vacancies are encountered. Both the origin of spin polarization and the magnetic ordering of the model can be attributed to the aforementioned requirements.

Ferromagnetism in graphene traced to antisymmetric orbital combination of involved electronic states.

Based on first principles calculations, we reveal that the origin of ferromagnetism caused by [Formula: see text] electrons in graphene with vacancies can be traced to electrons partially filling [Formula: see text]-antibonding and [Formula: see text]-nonbonding states, which are induced by the vacancies and appear near the Fermi level. Because the spatial wavefunctions of both states are composed of atomic orbitals in an antisymmetric configuration, their spin wavefunctions should be symmetric according to the electron exchange antisymmetric principle, leading to electrons partially filling these states in spin polarization. Since this [Formula: see text] state originates not from interactions between the atoms but from the unpaired [Formula: see text] orbitals due to the removal of [Formula: see text] orbitals on the minority sublattice, the [Formula: see text] state is constrained, distributed on the atoms of the majority sublattice, and decays gradually from the vacancy as ∼[Formula: see text]. According to these characteristics, we concluded that the [Formula: see text] state plays a critical role in magnetic ordering in graphene with vacancies. If the vacancy concentration in graphene is large enough to cause the decay-length regions to overlap, constraining the [Formula: see text] orbital components as little as possible on the minority sublattice atoms in the overlap regions results in the vacancy-induced [Formula: see text] states being coherent. The coherent process in the overlap region leads to the wavefunctions in all the involved regions antisymmetrized, consequently causing ferromagnetism according to the electron exchange antisymmetric principle. This unusual mechanism concerned with the origin of [Formula: see text]-electron magnetism and magnetic ordering has never before been reported and is distinctly different from conventional mechanisms. Consequently, we can explain how such a weak magnetization with such a high critical temperature can be experimentally observed in proton-irradiated graphene.

Origins of distinctly different behaviors of Pd and Pt contacts on graphene.

Based on first-principles calculations, we propose an exchange-transfer mechanism to understand the distinctively different behaviors of Pd and Pt contacts on graphene. The feature of the mechanism is that the pi electrons on the graphene transferring to the Pd d_{xz} + d_{yz} orbital are largely compensated by the electrons from the Pd d_{z;{2}} orbital. This mechanism causes more interaction states and transmission channels between the Pd and graphene. Most importantly, the mechanism keeps enough pi electrons on the graphene. We show that a tensile strain in the Pd layer, necessary to match the graphene lattice, plays a key role in stimulating this exchange transfer when Pd covers on graphene, while a similar strain in the Pt layer does not cause such a mechanism.

Novel electronic structure induced by a highly strained oxide interface with incommensurate crystal fields.

The misfit oxide, Bi2Ba1.3K0.6Co2.1O7.94, made of alternating rocksalt-structured [BiO/BaO] layers and hexagonal CoO2 layers, was studied by angle-resolved photoemission spectroscopy, revealing the electronic structure of a highly strained oxide interface. We found that low-energy states are confined within individual sides of the interface, but scattered by the incommensurate crystal field from the other side. Furthermore, the high strain on the rocksalt layer induces large charge transfer to the CoO2 layer, and a novel effect, the interfacial enhancement of electron-phonon interactions, is discovered.

Low photoemission intensity near induced by the surface relaxed structure of CrO2(001).

Based on first principles calculations we find that CrO2(001) will form a relaxed structure at its surface, at which each surface Cr atom is surrounded by four oxygen atoms in a distorted tetrahedral configuration. This tetrahedral environment has important effects on the electronic structure, leading to an inversion of the t2g-eg splitting of Cr 3d orbitals. Two 3d electrons of the surface Cr ion will fully occupy the doublet eg, which becomes lower in energy than the t2g, leaving the t2g orbitals empty. The consequence is that the Fermi level lies in a gap between the eg and t2g for a local electronic structure at the surface. This finding is consistent with and explains the extremely low photoemission intensity near EF at CrO2(001) [Kämper et al., Phys. Rev. Lett. 59, 2788 (1987)].