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R. H. Miwa

Publications and source records attributed to R. H. Miwa.

28 records · Page 2Linked to original sources
arXiv2015

Pyridine intercalated Bi$_2$Se$_3$ heterostructures: controlling the topologically protected states

We use ab initio simulations to investigate the incorporation of pyridine molecules (C$_5$H$_5$N) in the van der Waals gaps of Bi$_2$Se$_3$. The intercalated pyridine molecules increase the separation distance between the Bi$_2$Se$_3$ quintuple layers (QLs), suppressing the parity inversion of the electronic states at the $Γ$-point. We find that the intercalated region becomes a trivial insulator. By combining the pristine Bi$_2$Se$_3$ region with the one intercalated by the molecules, we have a non-trivial/trivial heterojunction characterized by the presence of (topologically protected) metallic states at the interfacial region. Next we apply an external compressive pressure to the system, and the results are (i) a decrease on the separation distance between the QLs intercalated by pyridine molecules, and (ii) the metallic states are shifted toward the bulk region, turning the system back to insulator. That is, through a suitable tuning of the external pressure in Bi$_2$Se$_3$, intercalated by pyridine molecules, we can control its topological properties; turning-on and -off the topologically protected metallic states lying at the non-trivial/trivial interface.

cond-mat.mtrl-sci↗
arXiv2015

Periodic arrays of intercalated atoms in twisted bilayer graphene: an \it{ ab initio} investigation

We have performed an \it {ab initio} investigation of transition metals (TMs = Mo, Ru, Co, and Pt) embedded in twisted bilayer graphene (tBG) layers. Our total energy results reveal that, triggered by the misalignment between the graphene layers, Mo and Ru atoms may form a quasi-periodic (triangular) array of intercalated atoms. In contrast, the formation of those structures is not expected for the other TMs, Co and Pt atoms. The net magnetic moment (m) of Mo and Ru atoms may be quenched upon intercalation, depending on the stacking region (AA or AB). For instance, we find a magnetic moment of 0.3 $μ_{\rm B}$ (1.8 $μ_{\rm B}$) for Ru atoms intercalated between the AA (AB) regions of the stacked twisted layers. Through simulated scanning tunneling microscopy (STM) images, we verify that the presence of intercalated TMs can be identified by the formation of bright (hexagonal) spots lying on the graphene surface.

cond-mat.mes-hall↗
arXiv2014

Topological Phases in Triangular Lattices of Ru Adsorbed on Graphene: ab-initio calculations

We have performed an ab initio investigation of the electronic properties of the graphene sheet adsorbed by Ru adatoms (Ru/graphene). For a particular set of triangular arrays of Ru adatoms, we find the formation of four (spin-polarized) Dirac cones attributed to a suitable overlap between two hexagonal lattices: one composed by the C sites of the graphene sheet, and the other formed by the surface potential induced by the Ru adatoms. Upon the presence of spin-orbit coupling (SOC) nontrivial band gaps take place at the Dirac cones promoting several topological phases. Depending on the Ru concentration, the system can be topologically characterized among the phases i) Quantum Spin Hall (QSH), ii) Quantum Anomalous Hall (QAH), iii) metal iv) or trivial insulator. For each concentration, the topological phase is characterized by the ab-initio calculation of the Chern number.

cond-mat.mes-hall↗
arXiv2011

Theoretical Study of Carbon Clusters in Silicon Carbide Nanowires

Using first-principles methods we performed a theoretical study of carbon clusters in silicon carbide nanowires. We examined small clusters with carbon interstitials and antisites in hydrogen-passivated SiC nanowires growth along the [100] and [111] directions. The formation energies of these clusters were calculated as a function of the carbon concentration. We verified that the energetic stability of the carbon defects in SiC nanowires depends strongly on the composition of the nanowire surface: the energetically most favorable configuration in carbon-coated [100] SiC nanowire is not expected to occur in silicon-coated [100] SiC nanowire. The binding energies of some aggregates were also obtained, and they indicate that the formation of carbon clusters in SiC nanowires is energetically favored.

cond-mat.mtrl-sci↗
arXiv2011

Spin-texture and magnetic anisotropy of Co adsorbed Bi$_2$Se$_3$ topological insulator surfaces

Based upon first-principles methods, we investigate the magnetic anisotropy and the spin-texture of Co adatoms embedded in the topmost Se network of the topological insulator Bi$_2$Se$_3$ surface. We find the formation of energetically stable magnetic moment perpendicular to the surface plane, S$_z$. Our results for the pristine Bi$_2$Se$_3$ surface indicate the presence of helical spin-texture not only in the massless surface Dirac states, but also surface states resonant within the valence band present spin-texture. On the other hand, upon the presence of Co adatoms we find that the out-of-plane surface magnetism represents the dominant spin state (S$_z$), while the planar spin components, S$_x$ and S$_y$, are almost suppressed.

cond-mat.mes-hall↗
arXiv2011

Doping of graphene adsorbed on the a-SiO$_2$ surface

We have performed an {\it ab initio} theoretical investigation of graphene sheet adsorbed on amorphous SiO$_2$ surface (G/a-SiO$_2$). We find that graphene adsorbs on the a-SiO$_2$ surface through van der Waals interactions. The inhomogeneous topology of the a-SiO$_2$ clean surface promotes a total charge density displacement on the adsorbed graphene sheet, giving rise to electron-rich as well as hole-rich regions on the graphene. Such anisotropic distribution of the charge density may contribute to the reduction of the electronic mobility in G/a-SiO$_2$ systems. Furthermore, the adsorbed graphene sheet exhibits a net total charge density gain. In this case, the graphene sheet becomes n-type doped, however, with no formation of chemical bonds at the graphene--SiO$_2$ interface. The electronic charge transfer from a-SiO$_2$ to the graphene sheet occurs upon the formation of a partially occupied level lying above the Dirac point. We find that such partially occupied level comes from the three-fold coordinated oxygen atoms in the a-SiO$_2$ substrate.

cond-mat.mtrl-sci↗
arXiv2011

Hydrogenated grain boundaries in graphene

We have investigated by means of first principles calculations the structural and electronic properties of hydrogenated graphene structures with distinct grain boundary defects. Our total energy results reveal that the adsorption of a single H is more stable at grain boundary defect. The electronic structure of the grains boundaries upon hydrogen adsorption have been examined. Further total energy calculations indicate that the adsorption of two H on two neighbor carbons, forming a basic unit of graphane, is more stable at the defect region. Therefore, we expect that these extended defects would work as a nucleation region for the formation of a narrow graphane strip embedded in graphene region.

cond-mat.mes-hall↗
arXiv2006

EL2--like defects in InP nanowires

We have performed an {\it ab initio} total energy investigation, within the density functional theory (DFT), of antisite defects in InP nanowires (InP$NW$s) grown along the [111] direction. Our total energy results indicate that, (i) P antisites (P$_{\rm In}$) are the most likely antisite defect compared with In antisites (In$_{\rm P}$), and (ii) the formation energies of P and In antisites do not depend on the $NW$ diameter. In particular, thin InP$NW$s, diameters of $\sim$13 Å, the P$_{\rm In}$ antisite exhibits a trigonal symmetry, lying at 0.15 Åfrom the $T_d$ site, followed by a metastable configuration with P$_{\rm In}$ in an interstitial position (1.15 Åfrom the $T_d$ site). We find a P$_{\rm In}$--P dissociation energy of 0.33 eV, and there is no EL2--like center for such a thin InP$NW$. However, EL2--like defects occur by increasing the $NW$ diameter. For diameters of $\sim$18 Å, the P$_{\rm In}$--P dissociation energy increases to 0.53 eV, which is 0.34 eV lower compared with the P$_{\rm In}$--P dissociation energy for InP bulk phase, 0.87 eV. We mapped the atomic displacements and calculated the relaxation energy, Franck--Condon shift, upon single excitation of P$_{\rm In}$ induced states in InP$NW$. The formation (or not) of EL2--like defects, P$_{\rm In}$--P dissociation energy barrier, and the Franck--Condon (FC) energy shift, can be tuned by the $NW$ diameter.

cond-mat.mtrl-sci↗
arXiv2006

Fe adatoms along Bi lines on H/Si(001): Patterning atomic magnetic chains

The stability, electronic and magnetic properties of Fe atoms adsorbed on the self-assembled Bi-dimer lines nanostructure on the H/Si(001) surface are addressed by spin-density functional calculations. Our results show that Fe adatoms are much more stable on sites closer to the Bi nanolines being able to form one-dimensional atomic arrays. The most stable structure occurs on a missing dimer line aside the Bi dimers, which corresponds to an array with distances between Fe adatoms of about 8 Ang. In this array the irons are coupled antiferromagnetically with spin magnetic moment of about 1.5 Bohr magnetons per Fe atom, whereas the coupling exchange interactions is found to be of 14.4 meV. We also estimate a large magnetic anisotropy energy for the Fe adatom of about 3 meV/atom. In addition, the electronic band structure of the Fe array at the most stable structure shows a magnetic half-metal behavior.

cond-mat.mtrl-sci↗