Effects of Strain on Electronic Properties of Graphene

We present first-principles calculations of electronic properties of graphene under uniaxial and isotropic strains, respectively. The semi-metallic nature is shown to persist up to a very large uniaxial strain of 30% except a very narrow strain range where a tiny energy gap opens. As the uniaxial strain increases along a certain direction, the Fermi velocity parallel to it decreases quickly and vanishes eventually, whereas the Fermi velocity perpendicular to it increases by as much as 25%. Thus, the low energy properties with small uniaxial strains can be described by the generalized Weyl's equation while massless and massive electrons coexist with large ones. The work function is also predicted to increase substantially as both the uniaxial and isotropic strain increases. Hence, the homogeneous strain in graphene can be regarded as the effective electronic scalar potential.Comment: 4 pages, 6 figures; Published versio

Proximity-induced giant spin-orbit interaction in epitaxial graphene on topological insulator

Heterostructures of Dirac materials such as graphene and topological insulators provide interesting platforms to explore exotic quantum states of electrons in solids. Here we study the electronic structure of graphene-Sb2Te3 heterostructure using density functional theory and tight-binding methods. We show that the epitaxial graphene on Sb2Te3 turns into quantum spin-Hall phase due to its proximity to the topological insulating Sb2Te3. It is found that the epitaxial graphene develops a giant spin-orbit gap of about ~20 meV, which is about three orders of magnitude larger than that of pristine graphene. We discuss the origin of such enhancement of the spin-orbit interaction and possible outcomes of the spin-Hall phase in graphene

Study of vacancy ordering and the boson peak in metastable cubic Ge-Sb-Te using machine learning potentials

The mechanism of the vacancy ordering in metastable cubic Ge-Sb-Te (c-GST) that underlies the ultrafast phase-change dynamics and prominent thermoelectric properties remains elusive. Achieving a comprehensive understanding of the vacancy-ordering process at an atomic level is challenging because of enormous computational demands required to simulate disordered structures on large temporal and spatial scales. In this study, we investigate the vacancy ordering in c-GST by performing large-scale molecular dynamics simulations using machine learning potentials. The initial c-GST structure with randomly distributed vacancies rearranges to develop a semi-ordered cubic structure with layer-like ordered vacancies after annealing at 700~K for 100~ns. The vacancy ordering significantly affects the lattice dynamical properties of c-GST. In the initial structure with fully disordered vacancies, we observe a boson peak, usually associated with amorphous solids, that consists of localized modes at $\sim$0.575~THz. As vacancies become ordered, the boson peak disappears and the Debye-Waller thermal \textit{B} factor of Te decreases substantially. This finding indicates that the c-GST undergoes a transition from amorphous-like to crystalline-like solid state by thermal annealing in low-frequency dynamics.Comment: 8 pages, 1 Table of Contents figure, 7 main figures, Supplemental Materia

Ab initio studies of structural and electronic properties of the crystalline Ge(2)Sb(2)Te(5)

We study the atomic structure and the electronic and optical properties of Ge(2)Sb(2)Te(5) in two different crystalline states of cubic and hexagonal structures with the use of ab initio pseudopotential density functional method. It is found that electronic and atomic structures are very sensitive to the layer sequence in the two phases. The proximity of vacancy layer to Ge layer leads to the splitting of Ge-Te bond length, which, in turn, affects the electronic and optical properties. The effect of Te d orbitals is also investigated with respect to structural propertiesopen181

Crossover between multipole Coulomb and Kubas interactions in hydrogen adsorption on metal-graphene complexes

The hydrogen adsorption on alkaline-earth metal dispersed in doped graphenes was studied through ab initio calculations. Substitutional doping in graphenes is explored to control the ionic state of the metal atoms that plays a crucial role for dispersion and hydrogen adsorption. It was found that the adsorption behavior, particularly in Ca-dispersed graphene complexes, exhibits a crossover between the multipole Coulomb and Kubas-type (or orbital) interactions as the ionic state of Ca and the number of adsorbed hydrogen molecules change. The level exchange in s and d orbitals of Ca is responsible for the crossover. This finding enables the optimization of hydrogen adsorption and metal dispersion in graphitic materials, which is useful for developing solid hydrogen storage and efficient catalysts.open403

Effect of vacancy defects in graphene on metal anchoring and hydrogen adsorption

The dispersion of transition and alkaline-earth metals on defective graphenes is studied using first-principles calculations. The effect of vacancy defects on binding properties of metal atoms to the graphene and with hydrogen molecules is particularly investigated. It is shown that vacancy defects enhance efficiently the metal binding energy and thus its dispersion, particularly for alkaline-earth metals. Mg on vacancy defects shows a substantial increase in its binding energy and hydrogen uptake capacity. Among metals considered, Ca-vacancy complexes are found to exhibit the most favorable hydrogen adsorption characteristics in terms of the binding energy and the capacity.open363

Topological phase transitions in group IV-VI semiconductors by phonons

The topological insulator has an intriguing electronic structure in that it has nontrivial topology enforcing the helical Dirac fermionic states at interfaces to the band insulators. Protected by the time-reversal symmetry and finite band gaps in the bulk, the topology is immune to external nonmagnetic perturbations. One essential question is whether elementary excitations in solids like phonons can trigger a transition in the topological property of the electronic structures. Here we investigate the development of topological insulating phases in IV-VI compounds under dynamic lattice deformations using first-principles calculations. Unlike the static state of topological phases at equilibrium conditions, we show that nontrivial topological phases are induced in the compounds by the dynamic lattice deformations from selective phonon modes. Calculations of the time-reversal polarization show that the Z2 invariant of the compounds is flipped by the selective phonon modes and that the compounds exhibit oscillating topological phases upon dynamic lattice deformations.ope