Temperature-dependent sputtering of metals and insulators

The temperature dependence of the sputter yield and the energy spectrum of sputtered atoms have been investigated on the basis of a standard model for thermal spikes. A high-temperature and a low-temperature regime have been identified in the temperature spectrum making up the evaporation yield. The high-temperature component of the yield as well as the associated energy spectrum are only very weakly dependent on ambient target temperature. The relative variation is the less pronounced the higher the spike temperature. The low-temperature component is associated with the long-time behavior of the spike, and measurable evaporation takes place over time intervals where spikes overlap. The importance of time constants for macroscopic heat transport is pointed out. The results are shown to provide a framework within which experimental results on the temperature dependence of the sputter yield of metals can be explained. The results are also consistent with measured temperature dependences in the sputter yield of insulators.On leave from Instytut Fizyki, Uniwersytet Jagiellonski, PL-30-059 Krakow, Poland     

Self-interaction errors in density-functional calculations of electronic transport

All density-functional calculations of single-molecule transport to date have used continuous exchange-correlation approximations. The lack of derivative discontinuity in such calculations leads to the erroneous prediction of metallic transport for insulating molecules. A simple and computationally undemanding atomic self-interaction correction (SIC) opens conduction gaps in I-V characteristics that otherwise are predicted metallic, as in the case of the prototype Au/ditholated-benzene/Au junction.     

Band offsets at semiconductor-oxide interfaces from hybrid density-functional calculations

Band offsets at semiconductor-oxide interfaces are determined through a scheme based on hybrid density functionals, which incorporate a fraction alpha of Hartree-Fock exchange. For each bulk component, the fraction alpha is tuned to reproduce the experimental band gap, and the conduction and valence band edges are then located with respect to a reference level. The lineup of the bulk reference levels is determined through an interface calculation, and shown to be almost independent of the fraction alpha. Application of this scheme to the Si-SiO2, SiC-SiO2, and Si-HfO2 interfaces yields excellent agreement with experiment.     

Stopping power in insulators and metals without charge exchange

The slowing-down process of pointlike charged particles in matter has been investigated by measuring the stopping power for antiprotons in materials of qualitatively very different nature. Whereas the velocity-proportional stopping power observed for metal-like targets such as aluminum over a wide energy range of 1-50 keV is in agreement with expectations, it is surprising that the same velocity dependence is seen for a large band-gap insulator such as LiF. The validity of these observations is supported by several measurements with protons and several checks of the target properties. The observations call for both a qualitative explanation and a quantitative theoretical model.     

On variational thermodynamic calculations for liquid metals

It is demonstrated that a recently described ab initio variational thermodynamic calculation for pure liquid metals can be significantly improved by using the hard-sphere Yukawa model instead of the usual hard-sphere model as the reference system.     

First-principles density-functional calculations on the field emission properties of BN nanocones

The first-principles density-functional theory is used to study the geometrical structures and field emission properties of different boron nitride nanocones with 240 disclination. It is found that the nanocones can be stable under applied electric field and the emission current is sensitively dependent on the tips of nanocones. The nanocones with homonuclear bonds at the tip can introduce additional energy states near Fermi level, which can reduce the ionization potential and increase the emission current of these boron nitride nanocones. This investigation indicates that the boron nitride nanocone can be a promising candidate as a field emission electron source.     

Hamiltonian of the V15 spin system from first-principles density-functional calculations

We report first-principles all-electron density-functional-based studies of the electronic structure, magnetic ordering, and anisotropy for the V15 molecular magnet. From these calculations, we determine a Heisenberg Hamiltonian with five antiferromagnetic and one ferromagnetic exchange couplings. We perform direct diagonalization to determine the temperature dependence of the susceptibility. This Hamiltonian reproduces the experimentally observed spin S = 1/2 ground state and low-lying S = 3/2 excited state. A small anisotropy term is necessary to account for the temperature independent part of the magnetization curve.     

Exact ground state density-functional theory for impurity models coupled to external reservoirs and transport calculations

A method is presented employing the density matrix renormalization group to construct exact ground state (GS) exchange correlation functionals for models of correlated electrons coupled to leads. We apply it to show that conductance calculations with Kohn-Sham GS density-functional theory can yield quantitative results in the Coulomb blockade regime. Our study is relevant for "molecular electronics" since it strongly suggests that the well documented discrepancies between theoretical and experimental transport coefficients originate (mainly) from approximations in GS functionals.     

Electronic structure and physical properties of ScN in pressure: density-functional theory calculations

Local density functional is investigated by using the full-potential linearized augmented plane wave （FP-LAPW） method for ScN in the hexagonal structure and the rocksalt structure and for hexagonal structures linking a layered hexagonal phase with wurtzite structure along a homogeneous strain transition path. It is found that the wurtzite ScN is unstable and the layered hexagonal phase, labelled as ho, in which atoms are approximately fivefold coordinated, is metastable, and the rocksalt ScN is stable. The electronic structure, the physical properties of the intermediate structures and the energy band structure along the transition are presented. It is found that the band gaps change from 4.0 to 1.0 eV continuously when c/a value varies from 1.68 to 1.26. It is noticeable that the study of ScN provides an opportunity to apply this kind of material （in wurtzite[h]-derived phase）.     

Electronic structure of ScN and YN： density-functional theory LDA and GW approximation calculations

The desirable physical properties of hardness, high temperature stability, and conductivity make the early transition metal nitrides important materials for various technological applications. To learn more about the nature of these materials, the local-density approximation(LDA) and GW approximation i.e. combination of the Green function G and the screened Coulomb interaction W, have been performed. This paper investigates the bulk electronic and physical properties of early transition metal mononitrides, ScN and YN in the rocksalt structure. In this paper, the semicore electrons are regarded as valance electrons. ScN appears to be a semimetal, and YN is semiconductor with band gap of 0.142eV within the LDA, but are in fact semiconductors with indirect band gaps of 1.244 and 0.544\,eV respectively, as revealed by calculations performed using GW approximation.     

Polarization fluctuations in insulators and metals: new and old theories merge

The ground-state fluctuation of polarization P is finite in insulators and divergent in metals, owing to the SWM sum rule [I. Souza, T. Wilkens, and R. M. Martin, Phys. Rev. B 62, 1666 (2000)]. This is a virtue of periodic (i.e., transverse) boundary conditions. I show that within any other boundary conditions the P fluctuation is finite even in metals, and a generalized sum rule applies. The boundary-condition dependence is a pure correlation effect, not present at the independent-particle level. In the longitudinal case inverted triangle x P = -rho, and one equivalently addresses charge fluctuations: the generalized sum rule reduces then to a well-known result of the many-body theory.     

Spatial uniformity of laser-accelerated ultrahigh-current MeV electron propagation in metals and insulators

The evolution of laser-generated MeV, MA electron beams propagating through conductors and insulators has been studied by comparing measurement and modeling of the distribution of MeV protons that are sheath accelerated by the propagated electrons. We find that electron flow through metals is uniform and can be laser imprinted, whereas propagation through insulators induces spatial disruption of the fast electrons. Agreement is found with material dependent modeling.     

Employment of curvilinear coordinates in ab initio calculations of insulators using pseudopotentials

The standard ab initio scheme for calculating the structure of crystals using nonlocal pseudopotentials is modified for use in curvilinear coordinates. A method for solving the Poisson equation for the Coulomb potential in a curved space in the k representation is found. It is shown in the example of calculations for crystals of insulators having an NaCl structure that the employment of a curved space permits a very significant decrease in the required size of the basis set. Fiz. Tverd. Tela (St. Petersburg) 41, 241–246 (February 1999)