Prediction of Quantum Anomalous Hall Effect in MBi and MSb (M:Ti, Zr, and Hf) Honeycombs.

The abounding possibilities of discovering novel materials has driven enhanced research effort in the field of materials physics. Only recently, the quantum anomalous hall effect (QAHE) was realized in magnetic topological insulators (TIs) albeit existing at extremely low temperatures. Here, we predict that MPn (M =Ti, Zr, and Hf; Pn =Sb and Bi) honeycombs are capable of possessing QAH insulating phases based on first-principles electronic structure calculations. We found that HfBi, HfSb, TiBi, and TiSb honeycomb systems possess QAHE with the largest band gap of 15 meV under the effect of tensile strain. In low-buckled HfBi honeycomb, we demonstrated the change of Chern number with increasing lattice constant. The band crossings occurred at low symmetry points. We also found that by varying the buckling distance we can induce a phase transition such that the band crossing between two Hf d-orbitals occurs along high-symmetry point K2. Moreover, edge states are demonstrated in buckled HfBi zigzag nanoribbons. This study contributes additional novel materials to the current pool of predicted QAH insulators which have promising applications in spintronics.

Dynamic scaling of island size distribution in submonolayer one-dimensional growth.

The dynamic scaling of the island size distribution (ISD) in the submonolayer growth regime of low-dimensional nanostructured systems is a long standing problem in epitaxial growth. In this study, kinetic Monte Carlo simulations of a realistic atomistic lattice-gas model describing the one-dimensional nucleation and growth of Al on Si(100):2×1 were performed to investigate the scaling behavior under varied growth conditions. Consistent with previous predictions, our results show that the shape of the scaled island size distribution can be altered by controlling the temperature and the C-defect density. The shifts in the scaled ISD are opposite to each other with temperature depending on the density of C-defects. For low C-defect density, a shift from a monomodal to a monotonically decreasing distribution as temperature increases is observed. We attribute the monomodal distribution to enhanced nucleation and aggregation whereas a monotonically decreasing distribution is attributed to restricted aggregation with defects playing only a minor role. At higher C-defect density, we show that the scaled ISD shift is from a monotonically decreasing to a monomodal distribution with increasing temperature. We argue that the reversal of the shift is due to competing effects introduced by high C-defect concentration. In addition, results show that the ISD is generally insensitive to flux variations and that at a high coverage regime the shift in the scaling behavior vanishes. Lastly, we posit that the shift in the scaled ISD indicates the departure of the island density's temperature dependence from predictions of classical nucleation theory.

Robust Large Gap Two-Dimensional Topological Insulators in Hydrogenated III-V Buckled Honeycombs.

A large gap two-dimensional (2D) topological insulator (TI), also known as a quantum spin Hall (QSH) insulator, is highly desirable for low-power-consuming electronic devices owing to its spin-polarized backscattering-free edge conducting channels. Although many freestanding films have been predicted to harbor the QSH phase, band topology of a film can be modified substantially when it is placed or grown on a substrate, making the materials realization of a 2D TI challenging. Here we report a first-principles study of possible QSH phases in 75 binary combinations of group III (B, Al, Ga, In, and Tl) and group V (N, P, As, Sb, and Bi) elements in the 2D buckled honeycomb structure, including hydrogenation on one or both sides of the films to simulate substrate effects. A total of six compounds (GaBi, InBi, TlBi, TlAs, TlSb, and TlN) are identified to be nontrivial in unhydrogenated case; whereas for hydrogenated case, only four (GaBi, InBi, TlBi, and TlSb) remains nontrivial. The band gap is found to be as large as 855 meV for the hydrogenated TlBi film, making this class of III-V materials suitable for room temperature applications. TlBi remains topologically nontrivial with a large band gap at various hydrogen coverages, indicating the robustness of its band topology against bonding effects of substrates.

Structural and electronic properties of hydrogen adsorptions on BC3 sheet and graphene: a comparative study.

We have systematically investigated the effect of hydrogen adsorption on a single BC3 sheet as well as graphene using first-principles calculations. Specifically, a comparative study of the energetically favorable atomic configurations for both H-adsorbed BC3 sheets and graphene at different hydrogen concentrations ranging from 1/32 to 4/32 ML and 1/8 to 1 ML was undertaken. The preferred hydrogen arrangement on the single BC3 sheet and graphene was found to have the same property as that of the adsorbed H atoms on the neighboring C atoms on the opposite sides of the sheet. Moreover, at low coverage of H, the pattern of hydrogen adsorption on the BC3 shows a proclivity toward formation on the same ring, contrasting their behavior on graphene where they tend to form the elongated zigzag chains instead. Lastly, both the hydrogenated BC3 sheet and graphene exhibit alternation of semiconducting and metallic properties as the H concentration is increased. These results suggest the possibility of manipulating the bandgaps in a single BC3 sheet and graphene by controlling the H concentrations on the BC3 sheet and graphene.

A kinetic Monte Carlo study on the role of defects and detachment in the formation and growth of In chains on Si(100).

Deposition on a Si(100) surface and subsequent self-assembly of In atoms into one-dimensional (1D) atomic chains at room temperature is investigated via kinetic Monte Carlo simulation of a suitable atomistic model. Model development is guided by recent experimental observations in which 1D In chains nucleate effectively exclusively at C-type defects, although In atoms can detach from chains. We find that a monotonically decreasing form of the scaled island size distribution (ISD) is consistent with a high defect density which facilitates persistent chain nucleation even at relatively high coverages. The predominance of heterogeneous nucleation may be attributed to several factors including low surface diffusion barriers, a high defect density, and relatively weak In-In binding.

Geometries and stabilities of Ag-doped Si n (n=1-13) clusters: a first-principles study.

The structures of AgSi(n) (n=1-13) clusters are investigated using first-principles calculations. Our studies suggest that AgSi(n) clusters with n=7 and 10 are relatively stable isomers and that these clusters prefer to be exohedral rather than endohedral. Moreover, doping leaves the inner core structure of the clusters largely intact. Additionally, the plot of fragmentation energies as a function of silicon atoms shows that the AgSi(n) are favored to dissociate into one Ag atom and Si(n) clusters. Alternative pathways exist for n>7 (except n=11) in which the Ag-Si cluster dissociates into a stable Si(7) and a smaller fragment AgSi(n-7). The AgSi(11) cluster dissociates into a stable Si(10) and a small fragment AgSi. Lastly, our analysis indicates that doping of Ag atom significantly decreases the gaps between the highest occupied molecular orbital and the lowest unoccupied molecular orbital for n>7.