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The gas-phase reaction mechanism of magnesium amide (Mg(NH2)2) and lithium hydride (LiH) to produce H2 is theoretically investigated using ab initio electronic structure and total energy calculations based on the density functional theory. The reaction with the addition of KH which has been reported to lower the activation energy is also considered. We identify intermediate complexes and transition states in the reaction pathways. Detailed processes are analyzed to show how each reactant (Mg(NH2)2 or LiH) contributes a H atom to produce a H2 molecule and KH induces a modified reaction mechanism with a lower reaction energy.

Metal-doped graphene produces magnetic moments that have potential application in spintronics. Here we use density function theory computational methods to show how the magnetic interaction between metal atoms doped in graphene can be controlled by the degree of flexure in a graphene membrane. Bending graphene by flexing causes the distance between two substitutional Fe atoms covalently bonded in graphene to gradually increase and these results in the magnetic moment disappearing at a critical strain value. At the critical strain, a carbon atom can enter between the two Fe atoms and blocks the interaction between relevant orbitals of Fe atoms to quench the magnetic moment. The control of interactions between doped atoms by exploiting the mechanical flexibility of graphene is a unique approach to manipulating the magnetic properties and opens up new opportunities for mechanical-magnetic 2D device systems.

Dirac line node (DLN) semimetals are a class of topological semimetals that feature band-crossing lines in momentum space. We study the type-I and type-II classification of DLN semimetals by developing a criterion that determines the type using band velocities. Using first-principles calculations, we also predict that Na3N under an epitaxial tensile strain realizes a type-II DLN semimetal with vanishing spin-orbit coupling (SOC), characterized by the Berry phase that is Z2-quantized in the presence of inversion and time-reversal symmetries. The surface energy spectrum is calculated to demonstrate the topological phase, and the type-II nature is demonstrated by calculating the band velocities. We also develop a tight-binding model and a low-energy effective Hamiltonian that describe the low-energy electronic structure of strained Na3N. The occurrence of a DLN in Na3N under strain is captured in the optical conductivity, which we propose as a means to experimentally confirm the type-II class of the DLN semimetal., 14 pages, 9 figure

A compressed stiff film on a soft substrate may exhibit wrinkles and, under increased compressive strain, post-buckling instabilities as well. We numerically analyze wrinkling behaviors of graphene attached on a polydimethylsiloxane (PDMS) substrate under lateral compression. The finite element method is used to simulate the equilibrium shape of the wrinkles as a function of compressive strain. Two-dimensional stretching and bending properties of graphene are obtained by density functional theory analysis, which are then converted to equivalent elastic properties of a continuum film with finite effective thickness. The PDMS is described using an Ogden or a neo-Hookean material model. Wrinkles first appear at extremely small strain. As the lateral compression increases, due to the nonlinear elasticity of the PDMS, sequential period-doubling bifurcations of the wrinkle mode are activated until the bifurcation stops and the film folds. We show that the bifurcations are consequences of a delicate balance between the deformations of the film and the substrate to minimize the total energy.

Because topological surface states of a single-crystal topological insulator can exist on all surfaces with different crystal orientations enclosing the crystal, mutual interactions among those states contiguous to each other through edges can lead to unique phenomena inconceivable in normal insulators. Here we show, based on a first-principles approach, that the difference in the work function between adjacent surfaces with different crystal-face orientations generates a built-in electric field around facet edges of a prototypical topological insulator such as Bi2Se3. Owing to the topological magnetoelectric coupling for a given broken time-reversal symmetry in the crystal, the electric field, in turn, forces effective magnetic dipoles to accumulate along the edges, realizing the facet-edge magnetic ordering. We demonstrate that the predicted magnetic ordering is in fact a manifestation of the axion electrodynamics in real solids.

Phonon softening of graphene under tensile strain is investigated based on ab initio density functional theory calculations. We demonstrate that, over a wide range of tensile strain directions, the strain-induced enhancement of phonon softening can give rise to phonon instabilities resulting in a mechanical failure of graphene. It is shown that there are two types of instabilities associated with phonons near K and Γ points, respectively, which induce symmetry-breaking structural distortions, and both of them lead to mechanical failure prior to the elastic failure commonly expected when the structural symmetry is retained.

A crack and its propagation is a challenging multiscale materials phenomenon of broad interest, from nanoscience to exogeology. Particularly in fracture mechanics, periodicities are of high scientific interest. However, a full understanding of this phenomenon across various physical scales is lacking. Here, we demonstrate periodic interlayer-mediated thin film crack propagation and discuss the governing conditions resulting in their periodicity as being universal. We show strong confinement of thin film cracks and arbitrary steering of their propagation by inserting a predefined thin interlayer, composed of either a polymer, metal, or even atomically thin graphene, between the substrate and the brittle thin film. The thin interlayer-mediated controllability arises from local modification of the effective mechanical properties of the crack medium. Numerical calculations incorporating basic fracture mechanics principles well model our experimental results. We believe that previous studies of periodic cracks in SiN films, self-de-bonding sol-gel films, and even drying colloidal films, along with this study, share the same physical origins but with differing physical boundary conditions. This finding provides a simple analogy for various periodic crack systems that exist in nature, not only for thin film cracks but also for cracks ranging in scale.

The Rashba effect is spin degeneracy lift originated from spin-orbit coupling under inversion symmetry breaking and has been intensively studied for spintronics applications. However, easily implementable methods and corresponding materials for directional controls of Rashba splitting are still lacking. Here, we propose organic-inorganic hybrid metal halide perovskites as 3D Rashba systems driven by bulk ferroelectricity. In these materials, it is shown that the helical direction of the angular momentum texture in the Rashba band can be controlled by external electric fields via ferroelectric switching. Our tight-binding analysis and first-principles calculations indicate that S = 1/2 and J = 1/2 Rashba bands directly coupled to ferroelectric polarization emerge at the valence and conduction band edges, respectively. The coexistence of two contrasting Rashba bands having different compositions of the spin and orbital angular momentum is a distinctive feature of these materials. With recent experimental evidence for the ferroelectric response, the halide perovskites will be, to our knowledge, the first practical realization of the ferroelectric-coupled Rashba effect, suggesting novel applications to spintronic devices.

We present a minimal but crucial microscopic theory for epitaxial graphene and graphene nanoribbons on the 4 H -SiC(0001) surface – prototypical materials to explore physical properties of graphene in a large scale. Coarse-grained model Hamiltonians are constructed based on the atomic and electronic structures of the systems from first-principles calculations. From the theory, we unambiguously uncover origins of several intriguing experimental observations such as broken-symmetry states around the Dirac points and new energy bands arising throughout the Brillouin zone, thereby establishing the role of substrates in modifying electronic properties of graphene. We also predict that armchair graphene nanoribbons on the surface have a single energy gap of 0.2 eV when their widths are over 15 nm, in sharp contrast to their usual family behavior.

In the two-dimensional electron gas (2DEG) emerging at the transition metal oxide surface and interface, it has been pointed out that the Rashba spin-orbit interaction, the momentum-dependent spin splitting due to broken inversion symmetry and atomic spin-orbit coupling, can have profound effects on electronic ordering in the spin, orbit, and charge channels, and may help give rise to exotic phenomena such as ferromagnetism-superconductivity coexistence and topological superconductivity. Although a large Rashba splitting is expected to improve experimental accessibility of such phenomena, it has not been understood how we can maximally enhance this splitting. Here, we present a promising route to realize significant Rashba-type band splitting using a thin film heterostructure. Based on first-principles methods and analytic model analyses, a tantalate monolayer on BaHfO$_3$ is shown to host two-dimensional bands originating from Ta $t_{2g}$ states with strong Rashba spin splittings - up to nearly 10% of the bandwidth - at both the band minima and saddle points due to the maximal breaking of the inversion symmetry. Such 2DEG band structure makes this oxide heterostructure a promising platform for realizing both a topological superconductor which hosts Majorana fermions and the electron correlation physics with strong spin-orbit coupling., Comment: 11 pages, 7 figures

We report first-principles calculations of atomic and electronic structures of epitaxial single-layer graphene on Si-terminated 4H-SiC(0 0 0 1) surface under homogeneous transverse electric fields. We find that atomic positions are insensitive to applied electric fields, but the electronic band structures of the graphene layer are shifted in energy, depending strongly on the applied electric fields, while those of the buffer layer are almost unchanged. This effect finally results in field-induced closing of the energy gap at the Dirac energy point and recovery of the conic feature of the low-energy band structures of free-standing graphene, which are verified and analyzed further with a tight-binding model consisting of the single-layer and the buffer-layer graphene only. The recovery of conical dispersion of the single-layer graphene and ambipolar field-effect behavior, despite the band-gap closure under electric field, makes epitaxial single-layer graphene one of the promising alternatives to current state-of-the-art transistors for radiofrequency applications.

We present a theoretical model capable of identifying multiple adsorption sites in a gas storage material with different binding energies and different amounts of adsorbed molecules. By applying this model to experimental data on the hydrogen storage in MOFs, we extract hydrogen adsorption properties of MOFs with coexisting strong and weak adsorption sites.

The feasibility to store hydrogen in calcium-decorated metal organic frameworks (MOFs) is explored by using first-principles electronic structure calculations. We show that substitution of boron atoms into the benzene ring of the MOF linker substantially enhances the Ca binding energy to the linker as well as the H 2 binding energy to Ca. The Kubas interaction between H 2 molecules and Ca added in the MOF gives rise to a large number of bound H 2 's (8H 2 's per linker) with the binding energy of 20 kJ/mol, which makes the system suitable for reversible hydrogen storage under ambient conditions.

A field effect transistor (FET) measurement of a SWNT shows a transition from a metallic one to a p-type semiconductor after helical wrapping of DNA. Water is found to be critical to activate this metal-semiconductor transition in the SWNT-ssDNA hybrid. Raman spectroscopy confirms the same change in electrical behavior. According to our ab initio calculations, a band gap can open up in a metallic SWNT with wrapped ssDNA in the presence of water molecules due to charge transfer., Comment: 13 pages, 6 figures

We investigate the electronic structures and electron emission properties of alkali-doped boron-nitride nanotubes (BNNTs) using density-functional theory calculations. We find that the nearly free-electron (NFE) state of the BNNT couples with the alkali atom states, giving rise to metallic states near the Fermi level. Unlike the cases of potassium-doped carbon nanotubes, not only the s but the d orbital state substantially takes part in the hybridization, and the resulting metallic states preserve the free-electron-like energy dispersion. Through first-principles electron dynamic simulations under applied fields, it is shown that the alkali-doped BNNT can generate an emission current 2 orders of magnitude larger than the carbon nanotube. The nodeless wave function at the Fermi level, together with the lowered work function, constitutes the major advantage of the alkali-doped BNNT in electron emission. We propose that the alkali-doped BNNT should be an excellent electron emitter in terms of the large emission current as well as its chemical and mechanical stability.

Saturated hydrocarbons, though relatively inert in nature, may become more reactive by functionalization. Using first-principles computational methods, we perform a systematic search for functional groups to be attached to saturated hydrocarbons for high-capacity storage of molecular hydrogen. When a saturated hydrocarbon such as propane is functionalized with OH, NH 2 , or SH, a transition metal atom may be strongly bound to each functional group. We find that hydrogen molecules are adsorbed on transition metal atoms with the binding energy suitable for reversible processes (adsorption and desorption) under practical hydrogen storage conditions. We also show that the functional groups may help reduce the tendency for transition metal clustering. The maximum gravimetric storage densities range from 8.7 wt% to 12.6 wt% depending on different materials. We explain the mechanism of multi-dihydrogen adsorption on a transition metal atom by generalizing the Kubas model.

Using first-principles density-functional electronic structure calculations, we propose functionalized organic molecules decorated with titanium atoms as high-capacity hydrogen storage materials. We study six kinds of functional groups which form complexes with Ti atoms and find that each complex is capable of binding up to six H2 molecules. Among such complexes, Ti-decorated ethane-1,2-diol can store H2’s with the maximum gravimetric density of 13 wt% and, under ambient conditions, a practically usable capacity of 5.5 wt%. We also present various forms of storage materials which are obtained by modifying well-known nanomaterials using Ti-functional group complexes.

Understanding of the electronic structure and the electrical transport properties on the nanoscale becomes increasingly important for the development of the next-generation nanodevices. We have developed a first-principles pseudopotential method to calculate the quantum conductance as well as the self-consistent charge distributions of nanostructures and studied the electronic structure and quantum conductance of carbon nanotubes with impurities or defects. Even if the carbon nanotube is metallic instead of semiconducting, boron and nitrogen atoms create acceptor-like and donor-like states which act as scattering centers for conducting electrons. Various defect geometries such as Stone–Wales defects are considered which give rise to interesting localized states and the corresponding conductance characteristics. These localized states are in resonance with the extended states of the metallic nanotube and form quasi-bound states with broadened energy levels leading to novel conductance behaviors. For semiconducting carbon nanotubes, it is shown that various defects located at the junction of two different tubes can produce both shallow and deep defect levels. Theoretical predictions are closely compared with recent scanning tunneling microscopy and scanning tunneling spectroscopy data.

A microscopic model for the resistance drift in the phase-change memory is proposed based on the first-principles results on the compressed amorphous Ge2Sb2Te5. First, it is shown that the residual pressure in the phase-change memory cell can be significant due to the density change accompanying the phase transformation. Our previous first-principles calculations showed that the energy gap is reduced and the density of localized in-gap states increases as the cell is pressurized. This indicates that the compressed amorphous Ge2Sb2Te5 is more conducting than those made under stress-free conditions. In addition, the crystallization dynamics was also accelerated under compressive stress. Based on these theoretical results, we propose a mechanism for the resistance drift in which the relaxation process in the amorphous Ge2Sb2Te5 corresponds to the growth of the crystalline nuclei inside the amorphous matrix, thereby lowering the internal stress. Our model can consistently explain several experimental observations such as the dependence of the drift exponent on the amorphous size.

We present an atomic resolution structural study of covalently bonded dopant pairs in the lattice of monolayer graphene. Two iron (Fe) metal atoms that are covalently bonded within the graphene lattice are observed and their interaction with each other is investigated. The two metal atom dopants can form small paired clusters of varied geometry within graphene vacancy defects. The two Fe atoms are created within a 10 nm diameter predefined location in graphene by manipulating a focused electron beam (80 kV) on the surface of graphene containing an intentionally deposited Fe precursor reservoir. Aberration-corrected transmission electron microscopy at 80 kV has been used to investigate the atomic structure and real time dynamics of Fe dimers embedded in graphene vacancies. Four different stable structures have been observed; two variants of an Fe dimer in a graphene trivacancy, an Fe dimer embedded in two adjacent monovacancies and an Fe dimer trapped by a quadvacancy. According to spin-sensitive DFT calculations, these dimer structures all possess magnetic moments of either 2.00 or 4.00 μB. The dimer structures were found to evolve from an initial single Fe atom dopant trapped in a graphene vacancy.

Silicene is a two-dimensional honeycomb lattice made of silicon atoms, which is considered to be a new Dirac fermion system. Based on first-principles calculations, we examine the possibility of the formation of solitonlike topological domain walls (DWs) in silicene. We show that the DWs between regions of distinct ground states of the buckled geometry should bind electrons when a uniform electric field is applied in the perpendicular direction to the sheet. The topological origin of the electron confinement is demonstrated based on numerical calculations of the valley-specific Hall conductivities, and possible experimental signatures of the quantum valley Hall effects are discussed using simulated scanning tunneling microscopy images. Our results strongly suggest that silicene could be an ideal host for the quantum valley Hall effect, thus providing a pathway to the valleytronics in silicon-based technology.

The effects of vacancies on mechanical properties of the transition metal carbides and nitrides are studied using the ab initio pseudopotential approach. Calculated shear elastic stiffness and electronic structures show that the vacancy produces entirely different effects on the mechanical strength of groups IVb nitrides and Vb carbides. It is found that the occupation of shear-unstable metallic dd bonding states changes essentially in an opposite way for the carbides and nitrides in the presence of vacancies, resulting in different responses to shear stress. Our study provides an atomistic understanding of the anomaly in hardness for these substoichiometric materials.

A family of crystal structures of carbon composed of alternating ${\mathrm{sp}}^{2}$ and ${\mathrm{sp}}^{3}$ bonds is investigated. Graphitic strips are connected by ${\mathrm{sp}}^{3}$ bonds to form an array of hexagonal pillars exhibiting a honeycomb lattice in the perpendicular plane. The electronic structure and elastic properties of this family of structures are calculated using an ab initio pseudopotential as well as the environment-dependent tight-binding method. Their electronic structure has a similar size dependence to zigzag nanotubes; they are metallic if twice the strip width is a multiple of three hexagonal units, and otherwise semiconducting with a wider range of the band gap than for carbon nanotubes. The structural stability is studied and compared with other carbon structures.

The current-degradation mechanism of a fully sealed, carbon-nanotube field emission display is investigated experimentally and theoretically. From residual gas analysis, it is strongly evidenced that CH3 radicals from the organic materials in the paste deteriorate emission properties. Based on ab initio methods, it is found that CH3 radicals can increase electrical resistance of the nanotube and suppress emission currents. In addition, molecular dynamics simulations demonstrate that thermal destruction of CH3-attached nanotubes occurs at lower temperatures than for pristine nanotubes. Our results suggest that the material selection of the paste is crucial for extending the lifetime of nanotube-based field emission displays.

We perform ab initio pseudopotential calculations for a nanocontact between a metallic carbon nanotube and a copper chain. We find that the on-top position is the most stable geometry of the copper chain on the nanotube and a local energy gap of $\ensuremath{\lesssim}0.1 \mathrm{eV}$ opens as the mirror symmetry of the nanotube is broken by the presence of copper. A weak ionic bonding is formed between the tube and the copper chain and the charge density in the conducting channel around the Fermi level is reduced at the contact region. Therefore, the electronic transport across the contact occurs essentially through the tunneling process. Appearance of such a locally semiconducting property in the intrinsically metallic carbon nanotube may explain the unusual low-temperature current-voltage characteristics exhibiting a huge contact resistance or a quantum dot behavior.

We solve the Green's equation exactly for the armchair-type carbon nanotubes with one π-electron tight-binding Hamiltonian and obtain analytic expressions for the conductance of the nanotube with local perturbations. Our results explicitly show the dependence of the conductance on the Fermi level, the tubular radius, and the strength of the perturbation.

We develop a method to incorporate the Kleinman-Bylander--type ab initio pseudopotential in the calculation of conductance in quantum wires using the Landauer formalism. This method is computationally efficient and mathematically stable; it does not involve singularity in inverting the transfer matrix. We also describe the ab initio nonlocal pseudopotential method to calculate the complex band structure that is required in the wave-function matching between the resistive material and the realistic metal probe. We present, as an example, the calculated conductance of the $(10,10)$ carbon nanotube with a pentagon-heptagon-pair defect in the low-temperature limit.

We have performed ab initio pseudopotential calculations within the local density approximation to investigate the structural phase transition of ZnTe under pressure. Zn 3d and Te 4d orbitals are treated as part of the valence states in order to describe the structural properties of ZnTe accurately. By calculating the total energy, atomic force, and stress tensors, we theoretically determine the structural phase transition of ZnTe from the zinc-blende (ZB) to the cinnabar to the distorted NaCl structure under increasing pressure. Calculational results are compared in detail with the recent experimental data obtained using angle-dispersive techniques and an image-plate detector. We also demonstrate that rotation of bonds toward lower-symmetry positions occurs at the critical pressure to relieve excessive strain. Detailed electronic structures of each phase are also presented.

In an effort to understand the behavior of hard materials in the presence of vacancies or impurities, we study the structural and electronic properties of diamond with specified vacancies. In our model, five carbon atoms in the shape of a centered tetrahedron are missing and 12 carbon atoms surrounding the vacancy are replaced by nitrogen or boron atoms. The bulk modulus of the material with nitrogen substitution is greater than that without substitution around vacancy, although it is still slightly smaller than that of vacancy-free diamond. Boron substitution results in a substantial relaxation of atomic positions and a large reduction in bulk modulus. Born’s criterion for mechanical stability is satisfied in both cases. The calculated electronic structures suggest that the doping of N atoms around the vacancy as modeled here fails to generate conduction electrons while that of B atoms successfully produces conducting holes. @S0163-1829~97!02324-2#

Density functional theory calculations are used to investigate the electronic structures of localized states at reconstructed armchair graphene edges. We consider graphene nanoribbons with two different edge types and obtain the energy band structures and charge densities of the edge states. By examining the imaginary part of the wavevector in the forbidden energy region, we reveal the decay behavior of the wavefunctions in graphene. The complex band structures of graphene in the armchair and zigzag directions are presented in both tight-binding and first-principles frameworks., Comment: 7 pages, 7 figures

We study by photoluminescence excitation the heretofore unsolved puzzle of a significant charge transfer over a thick ~100 to 1500 A ! AlxGa12xAs barrier in GaAs/AlxGa12xAs asymmetric double quantum wells, which the normally considered tunneling cannot account for. This phenomenon is completely general, observed in all the samples grown under standard growth conditions ~;600 °C!, that originated from many different sources. The existence of such leakage is also confirmed by time-resolved photoluminescence experiments. However, when the alloy barrier is replaced by an equivalent GaAs/AlAs digital alloy, or by AlAs, the leak largely disappears. In addition, a GaAs barrier separating two shallow In xGa12xAs quantum wells permits only relatively small transfer. The leak has a weak dependence on the barrier thickness at x50.3, but is a very strong function of x around x50.3. We argue that there is no way to explain all of the observed phenomena simultaneously other than by the existence of intrinsic structural inhomogeneities in the alloy. Essentially, there may exist low potential channels in the alloy barrier created by microscopic clustering of like molecules, through which percolationlike transport occurs. This picture is supported by a three-dimensional quantummechanical model calculation. Our work pins down the dynamical implications of the partial ordering and clustering in AlxGa12xAs and related semiconductor ternary alloys. The scope of this paper is exclusively for the transport over thick AlxGa12xAs barriers ( x,0.35) and does not include the relatively small but still non-negligible transport over thick homogeneous barriers such as GaAs, AlAs, and AlAs/GaAs digital alloys, and AlxGa12xAs (x.0.35). We contend that transport over these thick, homogeneous barriers may be owing to other mechanisms such as dipole-dipole transfer, photon reabsorption, nonequilibrium distribution of carriers, and polariton transport that are also unrelated to conventional tunneling. @S0163-1829~96!02144-3#

Ab initio pseudopotential calculations are performed for the electronic structure of the low-${\mathit{T}}_{\mathit{c}}$ intermetallics ${\mathrm{LuNi}}_{2}$${\mathrm{B}}_{2}$C and the related nonsuperconducting compound LuNiBC. Electronic structures of the two compounds are compared in great detail, especially in terms of the Fermi surfaces and the symmetry-decomposed density of states (DOS) near the Fermi level. The estimated electron-phonon coupling constant \ensuremath{\lambda} (0.8\char21{}1.1) from the heat-capacity data as well as from the calculated DOS at ${\mathit{E}}_{\mathit{F}}$ indicates that ${\mathit{T}}_{\mathit{c}}$ of ${\mathrm{LuNi}}_{2}$${\mathrm{B}}_{2}$C is reasonably well explained by the conventional Bardeen-Cooper-Schrieffer mechanism with intermediate coupling strength. The relatively high ${\mathit{T}}_{\mathit{c}}$ arises from the large DOS at the Fermi level. Absence of superconductivity in LuNiBC may be understood to be due to the reduced DOS at ${\mathit{E}}_{\mathit{F}}$. Unlike the high-${\mathit{T}}_{\mathit{c}}$ cuprates, the low-${\mathit{T}}_{\mathit{c}}$ ${\mathrm{LuNi}}_{2}$${\mathrm{B}}_{2}$C does not have the half-filled \ensuremath{\sigma}-antibonding bands and its electronic structure is almost three dimensional despite the layered atomic structure.

We perform ab initio pseudopotential total-energy calculations for ZnS, ZnSe, and ZnTe. Unlike in most previous calculations, we include Zn 3d orbitals (and, in case of ZnTe, Te 4d orbitals as well) as part of the valence states in order to study the behavior of the d electrons and their influence on energy levels. The results for the structural and electronic properties are in better agreement with experimental data than in the case where d electrons are considered as part of the core states. The role of the d electrons in determining the bonding strength of the material and the character of the states near the band gap is analyzed and discussed.

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]

We investigate the roles of the pseudospin and the valley degeneracy in electron scattering at graphene edges. It is found that they are strongly correlated with charge density modulations of short-wavelength oscillations and slowly decaying beat patterns in the electronic density profile. Theoretical analyses using nearest-neighbor tight-binding methods and first-principles density-functional theory calculations agree well with our experimental data from scanning tunneling microscopy. The armchair edge shows almost perfect intervalley scattering with pseudospin invariance regardless of the presence of the hydrogen atom at the edge, whereas the zigzag edge only allows for intravalley scattering with the change in the pseudospin orientation. The effect of structural defects at the graphene edges is also discussed.

A divacancy (DV) is one of the most abundant and most important defects in irradiated graphene, which modifies electronic and chemical properties of graphene. In this paper, we present ab initio calculations to study the dynamics and stability of DVs in graphene. Divacancies in graphene have various reconstructed structures, such as triple pentagon-triple heptagon (555-777) and pentagon-octagon-pentagon (5-8-5) patterns. A direct observation of the structural transformations between these reconstructions was recorded in transmission electron microscope images reported by Girit et al. in Science 323, 1705 (2009). We clarify the atomic structures of DVs observed in the experiment and investigate the atomic processes and energetics for the observed dynamical motions in great detail. It is found that a series of Stone--Wales-type transformations are responsible for the migration and structural transformations of DVs and that a pentagon-heptagon-heptagon-pentagon (5-7-7-5) defect appearing as an intermediate structure during the dynamical process plays an important role in the transformations of DVs.

By using the first-principles calculations, geometric, electronic, and magnetic properties of metallocene deposited on graphene are systematically investigated. Among all the metallocenes studied, only cobaltocene exhibits obvious charge transfer. Relatively delocalized ${e}_{1}$ orbitals of cobaltocene are responsible for $n$-type doping of graphene or nanoribbons, as well as for the spin-polarized current along the cobaltocene chains. We also propose that, based on our total energy calculations, cobaltocene may be used as a sensor to detect pentagonal defects in graphene by taking advantage of the rotation of cobaltocene's axis when located above pentagonal defects.

In metallic carbon nanotubes with defects, the electric current flow is expected to have characteristic spatial patterns depending on the nature of the defects. Here, we show, using first-principles transport calculations, that locally rotating loop currents in nanometer scale can be generated near defects in carbon nanotubes by quantum interference of conducting and quasi-bound states of electrons. The loop currents appear at energies near transmission dips, having opposite directions at lower- and higher-energy sides of the transmission dips and disappearing exactly at the centers of the dips. Temporal modulations of gate voltage around a transmission dip can produce oscillating magnetic dipoles, inducing magnetic fields that reflect characteristics of defects. This generation of loop currents and magnetic dipoles by quantum interference can generally occur in any nanostructure and it is potentially useful for novel electronic and magnetic nanodevices.

We have investigated the spectral broadening in the near-resonance fluorescence spectrum of a single rubidium atom trapped in a three-dimensional (3D) optical lattice in a strong Lamb-Dicke regime. Besides the strong Rayleigh peak, the spectrum exhibited weak Stokes and anti-Stokes Raman sidebands. The line width of the Rayleigh peak for low potential depths was well explained by matter-wave tunneling between the first-two lowest vibrational states of 3D anisotropic harmonic potentials of adjacent local minima of the optical lattice.

We developed two mid‐infrared quantum cascade structures based on InAs quantum dashes. The dashes were embedded either in AlInGaAs lattice‐matched to InP or in tensile‐strained AlInAs. The devices emit between 7 and 11 μm and are a step forward in the development of quantum cascade lasers based on 3‐D confined active regions.

All the nonpolar a‐plane GaN epilayers were grown on nominally on‐axis r‐plane sapphire substrate in metalorganic chemical vapor deposition reactor. The growth rate of a‐plane GaN epilayers was controlled by TMGa flow rate, f[TMGa]. The GaN epilayer grown at high growth mode reveals striated morphology with triangular‐shaped pits with sharp corner, suggesting a relatively higher growth rate on the (11_00) m‐plane facet than (112_0) a‐plane facet. In case of low growth rate, the reason for high difference of ω tilt offset was considered by higher compressive uniaxial strain of m‐axis. Room temperature photoluminescence measurement revealed that the band‐edge emission intensity increased at high TMGa flow rate, which consider reduction in the nonradiative recombination centers.

We have observed the excitonic wave‐packet motion through spectrally resolved four‐wave mixing measurements in a layered semiconductor GaSe, and demonstrated that the four‐wave mixing spectroscopy is a useful tool to observe the wave‐packet motion. We have found that the wave‐packet motion in a quasi‐two‐dimensional system is explained theoretically by using the configuration coordinate model, where the adiabatic potential curve is shifted due to the exciton‐phonon interaction.

Skyrmion excitations are analyzed in imbalanced bilayer quantum Hall systems based on a microscopic theory. A skymion carries both the spin and pseudospin degrees of freedom. In the presence of the parallel magnetic field, the activation energy increases due to the spin component, while it decreases due to the pseudospoin component by deforming a skyrmion into a bimeron (a pair of merons). We propose an approximate formula for the activation energy in imbalanced systems.

We report on the first experimental observation of neutral modes in particle‐hole conjugate fractional quantum Hall states using shot noise measurements. The presence of the neutral modes, in addition to producing excess noise in a quantum point contact (QPC), was seen to affect strongly the charge of the tunneling quasiparticles of the charge mode in the QPC and to increase their apparent temperature. The observation of an upstream neutral mode in the 5/2 state may constitute an added indication of its non‐abelian nature.

We have investigated the exciton spin relaxation in InGaAs/AlAsSb quantum wells by time‐resolved spin‐dependent pump and probe reflectance measurements. The spin relaxation time of 1.38 μm‐electron‐heavy‐hole excitons at 150 K is obtained to be 34–43 ps at an excitation power of 50–80 mW. The observed carrier density dependence and temperature independence of the spin relaxation time indicate that the spin relaxation mechanism is dominated by the Bir‐Aronov‐Pikus process.

Spin‐relaxation and spin diffusion of conduction electrons were studied in silicon enriched by 28Si and 29Si isotopes in order to determine the contribution of hyperfine interaction of conduction electrons with magnetic nuclei of 29Si isotope. The g‐factors dependencies on temperature, donor concentration and atomic number of donors were analyzed to study the impurity and lattice spin‐orbit interaction involvement to the relaxation processes for conduction electrons.

We theoretically investigated the electrical manipulation of hole g‐factor by switching the band alignments of a hybrid quantum well, made out of compressive layer and tensile strained layer. Since heavy‐hole and light‐hole are mainly confined to the compressively strained layer and tensile strained layer, respectively, we can switch the major band character and g‐factor of upper hole states by external electric fields. And we showed that the signs of the g‐factors can also be manipulated by controlling the hole mixing with magnetic fields.

We report experimental results of electrical transport properties on superconducting quantum interference device (SQUID) coupled with InAs self‐assembled quantum dot (SAQD) which has a unique structural zero‐dimensionality. We successfully fabricated SAQD‐SQUID with two side gates by means of AFM and e‐beam lithography. The side‐gate controlled supercurrent and π junction transition were demonstrated and such results may provide valuable information for the development of quantum information device by employing InAs SAQD.

The results of a group theoretical analysis of the excitonic fine structure are presented and compared with spectroscopic data on single quantum dots. The spectral features reveal the signatures of a symmetry higher than the crystal symmetry (C-3v). A consistent picture of the fine structure patterns for various exciton complexes is obtained with group theory and the concepts of symmetry elevation and symmetry breaking.