Search

The introduction of ferromagnetic order in topological insulators in general breaks the time-reversal symmetry and a gap is opened in topological surface bands. Various studies have focused on gap-opened magnetic topological insulators, because such modified band structures provide a promising platform for observing exotic quantum physics. However, the role of antiferromagnetic order in topological insulators is still controversial. In this report, we demonstrate that it is possible to restore the topological surface states by effectively reducing the antiferromagnetic ordering in Gd-substituted Bi2Te3. We successfully control the magnetic impurities via thermal treatments in ultra-high vacuum condition and observe apparent restoration of topological surface band dispersions. The microscopic mechanism of atomic rearrangements and the restoration process of topological surface states are unraveled by the combination of scanning tunneling microscopy measurements and density functional theory calculations. This work provides an effective way to control the magnetic impurities which is strongly correlated with topological surface states.

Transition-metal containing metal phthalocyanine (MPc) molecules are representative molecular magnets and their adsorption on two-dimensional metallic surfaces has been extensively studied. In/Si(111)-4×1 is a prototypical quasi-one-dimensional metallic surface made up of alternating In nanowires and Si Seiwatz chains. Here, we investigated the adsorption of MPc (M = Mn, Fe, Co) on In/Si(111)-4×1 using density functional theory calculations. We found that there are three locally stable adsorption sites of MPc, two on In nanowires and one on Seiwatz chains. Also, the stable orientation of MPc is different depending on adsorption sites. Among three MPc’s, MnPc are found to carry a finite magnetic moment of 1.6 ~ 2.1 μB in its three almost-degenerate adsorption structures. The interaction between MPc molecules and the In/Si(111)-4×1 surface turns out to occur via the charge transfer, and the charge transfer channel varies for different adsorption sites: Transition-metal 3dz2 and two N 2pz orbitals on In nanowires vs. four N 2pz orbitals on Seiwatz chains.

The adsorption of acetonitrile (CH3CN) on Si(111)-(7 × 7) at a room temperature has been investigated using scanning tunneling microscopy (STM) and first-principles calculations. The site-specific ...

One-dimensional indium chains on Si(111) exhibit both temperature-and defect-induced perioddoubling (×2) structural transitions, and their natures have been focus of recent investigations. Using density functional perturbation theory calculations, we examined the vibrational properties of the room-temperature Si(111)In-4×1 structure. The phonon band structure revealed two unstable modes at the zone-boundary in the chain direction, which lead to a lattice instability towards a structure with parallel trimers. This lattice-instability-driven 4 × 2 structure is different from the low-temperature 4×2 (LT-4×2) structure (hexagon structure). The result suggests that the roomtemperature phase is neither a static 4×1 phase nor the dynamical fluctuation of the LT-4×2 phase. We demonstrate that the room-temperature phase is a dynamically fluctuating parallel-trimer 4×2 phase, and the fluctuations become suppressed near the defects to result in the defect-induced ×2 structure.

Macrocyclic crown ethers (CEs) have tunable cavity sizes and site-selective binding with metal ions, making the CE-ion complex a promising candidate as a two-dimensional (2D) electrolyte. In this work, density functional theory method is used to determine the energetically stable structures of 12-crown-4 ether (CE4) and 15-crown-5 ether (CE5) complexed with four cations: Li + , Na + , Mg 2 + , Ca 2 + . In addition to the CE-ion binding energies, the diffusion barriers for ion transport through the CE cavities are calculated. Among the complexes investigated, CE5 presents the lowest energy barrier for ion diffusion. The barriers for Li + travelling through a single CE5 and moving between two CE5s are 0.29 eV and 0.16 eV, respectively. Field-controlled modulation of the diffusion barrier is also demonstrated. By applying a 0.15 V/A electric field perpendicular to the plane of the CE, the diffusion barrier of Li + through one CE5 can be reduced from 0.29 to 0.20 eV to facilitate the ion transport.

We report density functional theory calculations on the vacancy-induced magnetic properties of the wurzite GaN $$(10\bar 10)$$ surface. Among various intrinsic vacancies on the surface, we focus on the monatomic Ga and N vacancies. We have found two stable geometries of a surface Ga vacancy and one stable geometry of a surface N vacancy. Two stable surface Ga vacancy structures are shown to have magnetic moments of 3 μ B , which is the same as for the bulk Ga vacancy. Differently from the bulk N vacancy carrying null magnetic moment, the monatomic surface N vacancy has a finite magnetic moment of 1 μ B . These results support the experimental claim that the roomtemperature ferromagnetism observed in inorganic nanoparticles is due to surface vacancies and their complexes. We also present simulated scanning tunneling microscope images for comparison with those obtained in future experimental studies.

We report density-functional-theory calculations on vacancies on the ZnO\((10\bar 10)\) surface with the on-site Coulomb interaction correction. Zn and O vacancies and three different configurations of ZnO divacancies were examined considering all the possible charge states. Using the calculated vacancy formation energies, we constructed the phase diagram by spanning the allowed range of electronic and ionic chemical potential. Differently from the bulk where the divacancy is not stabilized at all, one configuration of surface divacancies can be stable in a certain region of the chemical potentials. Among all the stable vacancies, only the neutral and the singly negatively-charged monatomic Zn vacancies exhibit a finite magnetic moment of 1 μB, which implies that the surface Zn vacancies may be responsible for the experimentally-observed room-temperature ferromagnetism in ZnO nanocrystals.

We have investigated the vibrational properties of orthorhombic PtSi by performing first-principles calculations. The calculated phonon band structure shows unstable phonon modes around the Γ-point. The ionic displacement vectors associated with the unstable Γ-point phonon mode suggest a structural modulation to a lower-symmetry phase, which was confirmed by comparing the theoretical equations of states of the modulated (P212121) and the unmodulated (Pnma) PtSi. The symmetry-reduced modulated orthorhombic PtSi shows a phonon band gap. We attribute the origin of the phonon band gap to the separation of the low-frequency Pt phonon modes and the high-frequency Si phonon modes due to the large atomic mass difference between Pt and Si.

We investigated the structural and the binding properties of black phosphorus (black-P) by employing density functional theory (DFT) calculations in combination with various implementations of the van der Waals (vdW) interaction. Both the binding energy curve of the two isolated puckered layers and the equation of states of the bulk black-P suggest that the conventional generalized gradient approximation (GGA) functional is incapable of describing the interlayer vdW interaction. From the comparison of the seven different vdW implementations, the appropriate vdW schemes in describing layer-structured black-P are found to be either the Grimme’s dispersion correction (DFT-D2) or the optB86b vdW denisty-functional approach.

We report density functional theory calculations for the bulk ZnO divacancy in hexagonal ZnO and compare the results with the monatomic bulk vacancies. We find that the ZnO divacancy could be viewed as a tightly-bound state of a doubly-negative Zn vacancy and the doubly-positive O vacancy from the structural and the electronic points of view. The singly-positive ZnO divacancy is found to carry a magnetic moment of 1µB, but it can only be stabilized in very high p-type samples. Otherwise, the non-magnetic neutral divacancy is stable. These results suggest that the ZnO divacancies in the bulk region cannot play a significant role in stabilizing the recently-observed ferromagnetism in ZnO nanoparticles.

Industrial demands for sustainable and renewable energy resources have inspired studies on photonic and electronic properties of graphitic-carbon nitride (g-C3N4) as a promising photocatalyst without precious metal. The absorption and the yield by metal-free pristine g-C3N4 are, however, still limited with hydrogen/oxygen evolution reaction (HER/OER) mostly around ultraviolet-light. Here, we propose the graphene-decorated g-C3N4 as a metal-free photocatalyst under visible-light, based on our atomic-scale measurements and calculations. The g-C3N4 nanosheets on highly-oriented pyrolytic graphite (HOPG) exhibit band-gaps appropriate for visible-light absorption and work-functions tuned for band alignments to supply electrons and holes for HER/OER. Scanning probe microscopy (SPM) measurements for local density of states (LDOS) in atomic scale and work-functions in nanometer scale with ab initio calculations confirmed the various electronic transitions for each nitrogen and carbon atom in different atomic registries. The graphene-decorated g-C3N4, therefore, could provide a breakthrough enabling the efficient water-splitting reactions under visible-light without precious metal.

We have performed spin-polarized density functional theory calculations on κ-carbides, (Fe, Mn)3AlC, to investigate both the effect of different exchange-correlation functionals and the effect of Mn substitution. Differently from our previous calculations using the Perdew-Wang functional, Fe2MnAlC is found to be the most stable crystal among crystalline κ-carbides by using the Perdew, Burke, and Ernzerhof functional. Supercell calculations to simulate low Mn concentration show that substitutional Mn atoms hardly interact with each other and suggest that a random alloy model can be applied. The stabilization of Fe2MnAlC and its enhanced magnetization are attributed to the formation of a -Mn-C- linear chain structure.

We perform spin-polarized density functional theory calculations for a Zn vacancy on the ZnO(10 $\bar 1$ 0) surface. Two stable configurations of the surface Zn vacancy are found, and the activation energy barrier is estimated to be ∼0.01 eV. The lower energy configuration has a newly formed surface Zn-O bond to restore the bulk-like structure on the surface. Due to the newly formed bond, the vacancy state in the band gap is characterized by a complicated hybridization of neighboring surface and subsurface atoms and by a more extended electron density. Despite such a hybridization, the surface Zn vacancy is found to have a robust magnetic moment of 1 μ B , implying that surface Zn vacancies may be responsible for the ferromagnetism observed in ZnO thin films and nanoparticles. Simulated scanning tunneling microscope images show that the two structures of the surface Zn vacancy can be distinguished in the filled-state images.

The initial adsorption of oxygen molecules on Si(001) is investigated at room temperature. The scanning tunneling microscopy images reveal a unique bright O2-induced feature. The very initial sticking coefficient of O2 below 0.04 Langmuir is measured to be ∼0.16. Upon thermal annealing at 250–600 °C, the bright O2-induced feature is destroyed, and the Si(001) surface is covered with dark depressions that seem to be oxidized structures with -Si-O-Si- bonds. This suggests that the observed bright O2-induced feature is an intermediate precursor state that may be either a silanone species or a molecular adsorption structure.

We investigated the structural and the binding properties of tetrahedral P4 molecular pairs by employing density functional theory (DFT) calculations using both the local density approximation (LDA) and the generalized gradient approximation (GGA). To consider the van der Waals interaction, we employed the GGA with dispersion correction, specifically Grimme’s approach (DFT-D2). This DFT-D2 in conjunction with the GGA (GGA-D2) well fixes the failure of the GGA in reproducing a reasonable binding energy curve. The binding energy and the equilibrium intermolecular distance are found to be in close correlation with the number of adjacent atomic pairs between two molecules. Application of the LDA and the GGA-D2 to a molecular crystal β-P4 and comparison with experiments confirms that the GGA-D2 should be used for a reliable description of P4-based molecular systems.

The Cl-saturated Si(001) surface is investigated by employing density functional theory calculations. From the electronic density of states, the low- and the high-bias filled-state images and the empty-state images are found to originate from Cl(3p) states directed in-plane, in parallel with the Si dimer row, and perpendicular to the surface (z), respectively. The bright triplet, which is a characteristic scanning tunneling microscopy (STM) feature of an unchlorinated bare Si dimer, is attributed to an occupied dangling bond state of the bare Si dimer that is hybridized with the neighboring Cl(3pz) orbital. The previously observed split-dimer feature is confirmed to be the dimer-bond oxidized structure. Finally, we suggest that the initial back-bond oxidation does not occur on the chlorinated Si(001) surface because it is energetically not favored compared with dimer-bond oxidation and because its STM simulation does not match experimental images.

We have performed ab initio total energy calculations to investigate the defect energetics and diffusion behavior of Xe in UO 2 and ThO 2 matrices. All calculations have been carried out using density functional theory within the generalized gradient approximation and applying the projector-augmented-wave method. Our results show that the formation and migration energies of vacancy defects are more than twice as high in ThO 2 compared with UO 2 . Another notable difference between the two oxides is the role played by an oxygen vacancy in the movement of a cation vacancy. An vacancy enhances the movement of a uranium vacancy by lowering its migration energy by about 1 eV, but a similar effect is not observed in ThO 2 . The different behavior of cation vacancies in the two oxides strongly affects the mobility of fission gases and leads to differences in their respective diffusion behavior. We suggest that the strong resistance against oxidation of ThO 2 prevents the creation and migration of defects, and results in a lower mobility of fission gases in ThO 2 as compared to UO 2 .

We have investigated the optimal growth conditions of carbon nanotubes (CNTs) using the chemical vapor deposition and the Ni nanoparticle arrays. The diameter of the CNT is shown to be controlled down to below 20 ㎚ by changing the size of Ni particle. The position and size of Ni particles are controlled continuously by using wafer-scale compatible methods such as lithography, ion-milling, and chemical etching. Using optimal growth conditions of temperature, carbon feedstock, and carrier gases, we have demonstrated that an individual CNT can be grown from each Ni nanoparticle with almost 100% probability over wide area of SiO₂/Si wafer. The position, diameter, and wall thickness of the CNT are shown to be controlled by adjusting the growth conditions.

There are many interests to achieve long-range magnetic order in topological insulators of Bi2Se3 or Bi2Te3 by doping magnetic transition metals such as Fe and Mn. The transition metals act as not only magnetic dopants but also electric dopants because they are usually divalent. However, if the doping elements are rare-earth metals such as Gd, which are trivalent, only magnetic moments can be introduced. We fabricated single crystals of Bi2-xGdxTe3 (0 ≤ × ≤ 0.2), in which we observed magnetic phase change from paramagnetic (PM) to antiferromagnetic (AFM) phase by increasing x. This PM-to-AFM phase transition agrees with the density functional theory calculations showing a weak and short-ranged Gd-Gd AFM coupling via the intervening Te ions. The critical point corresponding to the magnetic phase transition is x = 0.09, where large linear magnetoresistance and highly anisotropic Shubnikov-de Haas oscillations are observed. These results are discussed with two-dimensional properties of topological surface state electrons.

There are many interests to achieve long-range magnetic order in topological insulators of Bi2Se3 or Bi2Te3 by doping magnetic transition metals such as Fe and Mn. The transition metals act as not only magnetic dopants but also electric dopants because they are usually divalent. However, if the doping elements are rare-earth metals such as Gd, which are trivalent, only magnetic moments can be introduced. We fabricated single crystals of Bi2-xGdxTe3 (0 ≤ × ≤ 0.2), in which we observed magnetic phase change from paramagnetic (PM) to antiferromagnetic (AFM) phase by increasing x. This PM-to-AFM phase transition agrees with the density functional theory calculations showing a weak and short-ranged Gd-Gd AFM coupling via the intervening Te ions. The critical point corresponding to the magnetic phase transition is x = 0.09, where large linear magnetoresistance and highly anisotropic Shubnikov-de Haas oscillations are observed. These results are discussed with two-dimensional properties of topological surface state electrons.

The atomic and electronic structures and stability of native defects in a single-layer ${\mathrm{MoS}}_{2}$ are investigated, based on density-functional theory calculations. Native defects such as a S vacancy (${V}_{\mathrm{S}}$), a S interstitial (${\mathrm{S}}_{i}$), a Mo vacancy (${V}_{\mathrm{Mo}}$), and a Mo interstitial (${\mathrm{Mo}}_{i}$) are considered. The ${\mathrm{S}}_{i}$ is found to have S-adatom configuration on top of a host S atom, and the ${\mathrm{Mo}}_{i}$ has Mo-${\mathrm{Mo}}_{i}$ split interstitial configuration along the $c$ direction. The formation energies of the native defects in neutral and charged states are calculated. For the charged states, the artificial electrostatic interactions between image charges in supercells are eliminated by a supercell size scaling scheme and a correction scheme that uses a Gaussian model charge. It is found that the ${V}_{\mathrm{S}}$ has a low formation energy of 1.3--1.5 eV in the Mo-rich limit condition, and the ${\mathrm{S}}_{i}$ has 1.0 eV in the S-rich limit condition. The ${V}_{\mathrm{S}}$ is found to be a deep single acceptor with the (0/$\ensuremath{-}$) transition level at 1.7 eV above the valence-band maximum (VBM). The ${\mathrm{S}}_{i}$ is found to be an electrically neutral defect. The Mo-related native defects of ${V}_{\mathrm{Mo}}$ and ${\mathrm{Mo}}_{i}$ are found to be high in formation energy above 4 eV. The ${V}_{\mathrm{Mo}}$ is a deep single acceptor and the ${\mathrm{Mo}}_{i}$ is a deep single donor, of which the (0/$\ensuremath{-}$) acceptor and (+/0) donor transition levels are found at 1.1 and 0.3 eV above the VBM, respectively.

We discuss our new implementation of the Real-space Electronic Structure method for studying the atomic and electronic structure of infinite periodic as well as finite systems, based on density functional theory. This improved version which we call HARES (for High-performance-fortran Adaptive grid Real-space Electronic Structure) aims at making the method widely applicable and efficient, using high performance Fortran on parallel architectures. The scaling of various parts of a HARES calculation is analyzed and compared to that of plane-wave based methods. The new developments that lead to enhanced performance, and their parallel implementation, are presented in detail. We illustrate the application of HARES to the study of elemental crystalline solids, molecules and complex crystalline materials, such as blue bronze and zeolites., Comment: 17 two-column pages, including 9 figures, 5 tables. To appear in Computer Physics Communications. Several minor revisions based on feedback

We have determined the structure of AlSb and GaSb (001) surfaces prepared by molecular beam epitaxy under typical Sb-rich device growth conditions. Within the range of flux and temperature where the diffraction pattern is nominally $(1\ifmmode\times\else\texttimes\fi{}3)$, we find that there are actually three distinct, stable $(4\ifmmode\times\else\texttimes\fi{}3)$ surface reconstructions. The three structures differ from any previously proposed for these growth conditions, with two of the reconstructions incorporating mixed III-V dimers within the Sb surface layer. These heterodimers appear to play an important role in island nucleation and growth.

The Cr doping concentration dependence of the electronic properties including bonding characteristic and spin band gap of SiC dilute magnetic semiconductor is studied by employing first-principles pseudopotential plane-wave method based on density functional theory within generalized gradient approximation. For the cubic /spl beta/-SiC crystal, the calculated lattice constant is a/sub 0/=4.36 /spl Aring/ and the bulk modulus is B/sub 0/=208.14 GPa. The 64 and 8-atom supercell models are employed which corresponds to the impurity concentration of 3 and 25%, respectively. In the case of 3% impurity concentration, the Cr atoms substituting for C induces large stress on the neighboring Si atoms due to the significant difference in atomic size. On the other hand, in 25%-doped SiC:Cr, the adjacent Si atoms moved away from the Cr atom. Calculations of the magnetic moment and magnetization energy is done in which the latter is defined as the energy difference between the ferromagnetic phase and paramagnetic phase of SiC:Cr. Results show that the SiC:Cr has permanent magnetic moments of 2/spl mu//sub B/ regardless of the substitution site and the studied doping concentration. Also, calculation of density of states is done resulting in the determination of a wide spin band gap of 1.58 eV for the SiC:Cr.

We present an ab initio pseudopotential local density functional calculation for stoichiometric high-Tc cuprate YBa_2Cu_3O_7 using the plane-wave basis set. We have overcome well-known difficulties in applying pseudopotential methods to first-row elements, transition metals, and rare-earth materials by carefully generating norm-conserving pseudopotentials with excellent transferability and employing an extremely efficient iterative diagonalization scheme optimized for our purpose. The self-consistent band structures, the total and site-projected densities of states, the partial charges and their symmetry-decompositions, and some characteristic charge densities near E_f are presented. We compare our results with various existing (F)LAPW and (F)LMTO calculations and establish that the ab initio pseudopotential method is competitive with other methods in studying the electronic structure of such complicated materials as high-Tc cuprates. [8 postscript files in uuencoded compressed form]

By using a first-principles method employing the local density approximation plus Hubbard parameter approach, we study point defects in NiO and interactions between them. The defect states associated with nickel or oxygen vacancies are identified within the energy gap. It is found that nickel vacancies introduce shallow levels in the density of states for the spin direction opposite to that of the removed Ni atom, while the oxygen vacancy creates more localized in-gap states. The interaction profiles between vacancies indicate that specific defect arrangements are strongly favored for both nickel and oxygen vacancies. In the case of nickel vacancies, defect ordering in a simple-cubic style is found to be most stable, leading to a half-metallic behavior. The ionized oxygen vacancies also show a tendency toward clustering, more strongly than neutral pairs. The microscopic origin of vacancy clustering is understood based on overlap integrals between defect states.

We report on the growth and properties of high quality bicolor oxide superlattices, composed of two perovskites out of BaTiO3, CaTiO3, and SrTiO3. The artificially grown superlattices are structurally unique and have a macroscopically homogeneous phase, which is not feasible to recreate in bulk form. By artificial structuring, it is found that the polarization of such superlattices can be highly increased as compared to pseudo-binary ceramics with the same overall composition. Such strong enhancement in superlattice is attributed to newly-developed ionic motions of A-site cations at the hetero-interfaces due to the interfacial coupling of electrostatic and elastic interactions, which cannot be found in single phase materials., 15 pages, 4 figures

Impurities interact with a charge density wave (CDW) and affect the phase transitions in low-dimensional systems. By using scanning tunneling microscopy, we visualize the interaction between oxygen impurities and the CDW in indium atomic wires on Si(111), a prototypical one-dimensional electronic system, and unveil the microscopic mechanism of the intriguing O-induced increase of the transition temperature (Tc). Driven by the fluctuating CDW, the O atoms adopt an asymmetric structure. By adjusting the asymmetry, a pair of O impurities in close distance can pin the one-dimensional CDW, which develops into the two-dimensional domains. First-principles calculations showed that the asymmetric interstitially-incorporated O defects induce shear strains, which assists the formation of hexagon structure of the CDW phase. The cooperative interplay between the O impurities and the CDW is responsible for the enhancement of the CDW condensation and the consequent increase in Tc.

Initial adsorption configurations of $\mathrm{N}{\mathrm{H}}_{3}$ molecules on the Si(001) surface are studied by using scanning tunneling microscopy (STM) and ab initio pseudopotential calculations. In contrast to the previous consensus of single configuration, we demonstrate that there exist two distinct STM features. One is found to be the existing model of on-dimer configuration, that is characterized by the previously unreported ``U-shape'' empty-state feature. The other is identified to be $\mathrm{N}{\mathrm{H}}_{2}$ and H adsorbed at the same side of two adjacent dimers along the dimer row. The identification of all the adsorption configurations would be useful for the quantitative analysis of Si nitridation.

We report the discovery of pressure-induced superconductivity in a semimetallic magnetic material ${\mathrm{CeTe}}_{1.82}.$ The superconducting transition temperature ${T}_{c}=2.7\mathrm{K}$ (well below the magnetic ordering temperatures) under pressure $(g2\mathrm{kbar})$ is remarkably high, considering the relatively low carrier density due to a charge-density-wave transition associated with lattice modulation. The mixed magnetic structure of antiferromagnetism coexisting with ferromagnetism can provide a clue for this high ${T}_{c}.$ We discuss a possible theoretical model for the superconducting pairing mechanism.

We have investigated the Li/GaAs(110) surface by combining the low-energy electron diffraction (LEED) and the ab initio pseudopotential calculations. The potential energy surface for the single atomic adsorption shows the existence of two inequivalent adsorption sites. At 0.5 monolayer coverage of Li, a well defined $1\ifmmode\times\else\texttimes\fi{}2$ LEED pattern is observed. The atomic structure at this coverage is characterized by Li zigzag chains running in the $[11\ifmmode\bar\else\textasciimacron\fi{}0]$ direction. This structure is stabilized by electron transfer from the Li adatoms to the dangling bonds of certain surface Ga atoms. The simulated scanning tunneling microscopy images show linear (asymmetric zigzag) chains along the $[11\ifmmode\bar\else\textasciimacron\fi{}0]$ direction for positive (negative) sample biases.