Plane Wave Basis Method Research Articles

Out-of-plane ionicity versus in-plane covalency interplay and electron–phonon coupling in MgB2 superconductor

Using the Density Functional Theory (DFT) within the Generalized Gradient Approximation (GGA) pseudopotential and plane wave basis method along with the frozen-phonon approach that starts from the ab initio evaluation of the total energy Etot of the solid with frozen-in atomic displacements, it is found that a superposition of A2u and the E2gvibrations modes is the key factor in the superconducting mechanism in MgB2 compound. Electron–Phonon coupling to these A2u and E2g phonon modes especially at the zone-boundary A point of the hexagonal Brilliouin zone leads to an interband hole charge transfer (and transfer back) between in-plane σ bond to the out-of-plane π bond along with an interatomic electron charge transfer (and transfer back) between the Magnesium s-states to the Boron out-of-plane pz-state. The direction of the electronic current is opposite to that of hole current so that it reinforces the polarization associated with these currents and may generate a large dynamical charge at a given critical temperature Tc that drives the compound into the superconducting state.

Effect of electric and magnetic field on impurity binding energy in InGaAsP/InP quantum ring

The ground state binding energy of hydrogenic donor impurity is calculated using the plane wave basis method in InGaAsP/InP quantum ring under applied electric and magnetic fields. The results show that the binding energy is nonlinear function of ring height and outer radius. The binding energy has a maximum near the center of radial or axial direction. The external electric field changes the probability distribution of electron, and then the difference of binding energies for impurity located at symmetrical positions increases. The binding energy increases with the increase of magnetic field and the effect of magnetic field is more obvious along the direction of magnetic field.

The impacts of electric and magnetic field on the binding energy of hydrogenic donor impurity in a InGaAsP/InP ring-shaped quantum well wire

The ground state binding energy of a hydrogenic donor impurity in a InGaAsP/InP ring-shaped quantum well wire is calculated via the plane wave basis method. It is shown that the donor binding energy is strongly dependent on the geometry, impurity position, axial electric field and transverse magnetic field. In particular, the impacts on the binding energy due to axial electric field and transverse magnetic field can mutually compensate.

Comparison of external electric and magnetic fields effect on binding energy of hydrogenic donor impurity in different shaped quantum wells

The effects of external electric and magnetic fields on the ground state binding energy of hydrogenic donor impurity are compared in square, V-shaped, and parabolic quantum wells. With the effective-mass envelope-function approximation theory, the ground state binding energies of hydrogenic donor impurity in InGaAsP/InP QWs are calculated through the plane wave basis method. The results indicate that as the quantum well width increases, the binding energy changes most fast in SQW. When the well width is fixed, the binding energy is the largest in VQW for the donor impurity located near the center of QWs. For the smaller and larger well width, the electric field effect on binding energy is the most significant in VQW and SQW, respectively. The magnetic field effect on binding energy is the most significant in VQW. The combined effects of electric and magnetic fields on the binding energy of hydrogenic donor impurity are qualitative consistent in different shaped QWs.

A parallel orbital-updating based plane-wave basis method for electronic structure calculations

Motivated by the recently proposed parallel orbital-updating approach in real space method [1], we propose a parallel orbital-updating based plane-wave basis method for electronic structure calculations, for solving the corresponding eigenvalue problems. In addition, we propose two new modified parallel orbital-updating methods. Compared to the traditional plane-wave methods, our methods allow for two-level parallelization, which is particularly interesting for large scale parallelization. Numerical experiments show that these new methods are more reliable and efficient for large scale calculations on modern supercomputers.

Electronic states in low-dimensional nano-structures: Comparison between the variational and plane wave basis method

A comparison is made between the plane wave basis and variational method. Within the framework of effective-mass approximation theory, the variational and plane wave basis method are used to calculate ground state energy and ground state binding energy in low-dimensional nano-structures under the external electric field. Comparing calculation results, the donor binding energies of ground state display the consistent trend, both of them are strongly dependent on the quantum size, impurity position and external electric field. However, the impurity ground state energy calculated using variational method may be larger than the real value and it results in the smaller binding energy for variational method. In addition, the binding energy is more sensitive to the external electric field for the variational method, which can be seen more clearly from Stark shift.

Effect of surface pressurization on the growth of α-Fe2O3nanostructures

By suitably pressurizing iron substrates under different conditions, the resulting α-Fe(2)O(3) nanostructures, formed by its direct thermal oxidation, can gradually change in succession from nanowires to nanoleaves and to micropillars as the pressure is increased. The inter-relation between the pressure conditions and the resulting nanostructure is studied by density functional calculations using ultrasoft pseudopotentials with a plane-wave basis method and with the generalized gradient approximation (GGA). It is shown that the shape of the formed nanostructures is primarily determined by the anisotropic activation energy and, as the latter is lowered, there is a shape change from wire to pillar. A simulation model of diffusion using the Monte Carlo method is applied in the 3-D (dimensional) case to show how the anisotropic activation energy influences the growth process of the α-Fe(2)O(3) nanostructure. The present study provides a way to control the shape of the nanostructures grown by the thermal-oxidation method.

General properties of confined hydrogenic impurities in spherical quantum dots

The effective-mass approximation using a plane wave basis method is applied to calculate the general properties of the ground state of the binding energy of hydrogenic impurities in spherical quantum dots. In particular, the effects of quantum-size, donor-position and electric field are investigated in detail. The present calculations were carried out for different potential shapes which correspond to four models of possible spherical quantum dots configurations. It is found that this binding energy is strongly linked to the electronic confinement and that its properties can be predicted in general from the knowledge of the electronic probability density and of the position of the impurity.

Relations of electronic energies and magnetic moments of tetra-3d metal (Mn, Fe, Co and Ni) nitrides calculated using a plane-wave basis method

Geometrical optimization of tetra-3d metal nitrides (Mn 4N, Fe 4N, Co 4N, and Ni 4N) has been performed and the relations of their energies ( E) and their total magnetic moments ( M) are obtained by plane-wave-basis density-functional calculations without any assumption of specific spin arrangement. The E vs. M relations obtained for Fe 4N and Mn 4N have a bimodal character. The ground state of Fe 4N is a high-spin state, which would correspond to the ferromagnetic character, while that of Mn 4N is a low-spin state, which would correspond to the observed ferrimagnetic character. Lattice constants and total magnetic moments of these tetra-3d metal nitrides are almost accurately predicted. From the spin-polarized densities of states curves, Co 4N would have the largest spin polarization ratio of 0.88, which suggests Co 4N can be a candidate material for ferromagnetic electrodes for spin-injection.

The Na-adsorbed Ge(0 0 1) surface: Structure and STM image simulations

Sodium adsorbed on the Ge(0 0 1) surface causes reconstruction of the surface with the type of reconstruction depending on the amount of the adsorbate. We present theoretical investigations of the structure and electronic properties of Na-adsorbed Ge(0 0 1) for the coverage of 0.5 monolayer using the combination of two methods: a plane-wave basis method and a local-orbital minimal-basis method. Two possible minimum-total-energy atomic configurations have been found, namely, the Na/Ge(0 0 1)-p(2 × 1) and Na/Ge(0 0 1)-p(4 × 1) reconstructions. The surface electronic structure for all calculated configurations occurs to be metallic. Our investigations are completed by a simulation of STM images for the obtained atomic structures.

Ab initio molecular dynamics study on the formation process of Al layers on Si(001) surface

To clarify the early stage of the Al formation process on the Si(001) surface, we conduct ab initio molecular dynamics calculations of the precipitation of Al atoms on to the Si surface based on the pseudopotential plane wave basis method. We examine two candidate structures of the first Al layer, where the Al dimers are between Si dimer rows: Al dimers perpendicular (case A) and parallel (case B) to Si dimers. In case A, the distance between the first and the second layers is shorter than that between the (111) planes in an Al crystal. Results show that the charge density of the Al–Al bonds is 0.03–0.04 au−3 and that of the Al–Si bonds is 0.06 au−3. The bond lengths are 0.27 nm, which are almost the same as those in the Al single crystal. Al atoms precipitated after the formation of the two layers can move onto the surface. In case B, the first layer of Al atoms are arranged on a groove between the Si dimer rows and form bonds with the Si dimers, where the charge population of the bonds is similar to that in case A. These results suggest that dense layers of Al can be formed on the surface.