### Browsing by Author "Torsti, T."

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Item Calculation of positron states and annihilation in solids: A density-gradient-correction scheme(American Physical Society (APS), 1996) Barbiellini, B.; Puska, Martti J.; Korhonen, T.; Harju, A.; Torsti, T.; Nieminen, Risto M.; Department of Applied Physics; Teknillisen fysiikan laitos; Perustieteiden korkeakoulu; School of ScienceThe generalized gradient correction method for positron-electron correlation effects in solids [B. Barbiellini et al., Phys. Rev. B 51, 7341 (1995)] is applied in several test cases. The positron lifetime, energetics, and momentum distribution of the annihilating electron-positron pairs are considered. The comparison with experiments shows systematic improvement in the predictive power of the theory compared to the local-density approximation results for positron states and annihilation characteristics.Item Calculation of positron states and annihilation in solids: A density-gradient-correction scheme(1996-06-15) Barbiellini, B.; Puska, M.J.; Korhonen, T.; Harju, A.; Torsti, T.; Nieminen, R.M.; Department of Applied PhysicsThe generalized gradient correction method for positron-electron correlation effects in solids [B. Barbiellini et al., Phys. Rev. B 51, 7341 (1995)] is applied in several test cases. The positron lifetime, energetics, and momentum distribution of the annihilating electron-positron pairs are considered. The comparison with experiments shows systematic improvement in the predictive power of the theory compared to the local-density approximation results for positron states and annihilation characteristics.Item Conductance oscillations in metallic nanocontacts(American Physical Society (APS), 2002) Havu, P.; Torsti, T.; Puska, Martti J.; Nieminen, Risto M.; Department of Applied Physics; Teknillisen fysiikan laitos; Perustieteiden korkeakoulu; School of ScienceWe examine the conductance properties of a chain of Na atoms between two metallic leads in the limit of low bias. Resonant states corresponding to the conductance channel and the local charge neutrality condition cause conductance oscillations as a function of the number of atoms in the chain. Moreover, the geometrical shape of the contact leads influences the conductivity by giving rise to additional oscillations as a function of the lead opening angle.Item Conductance oscillations in metallic nanocontacts(2002-08-01) Havu, Paula; Torsti, T.; Puska, M.J.; Nieminen, R.M.; Department of Applied PhysicsWe examine the conductance properties of a chain of Na atoms between two metallic leads in the limit of low bias. Resonant states corresponding to the conductance channel and the local charge neutrality condition cause conductance oscillations as a function of the number of atoms in the chain. Moreover, the geometrical shape of the contact leads influences the conductivity by giving rise to additional oscillations as a function of the lead opening angle.Item Electronic resonance states in metallic nanowires during the breaking process simulated with the ultimate jellium model(American Physical Society (APS), 2003) Ogando, E.; Torsti, T.; Zabala, N.; Puska, Martti J.; Teknillisen fysiikan laitos; Department of Applied Physics; Perustieteiden korkeakoulu; School of ScienceWe investigate the elongation and breaking process of metallic nanowires using the ultimate jellium model in self-consistent density-functional calculations of the electronic structure. In this model the positive background charge deforms to follow the electron density and the energy minimization determines the shape of the system. However, we restrict the shape of the wires by assuming rotational invariance about the wire axis. First, we study the stability of infinite wires and show that the quantum-mechanical shell structure stabilizes the uniform cylindrical geometry at the given magic radii. Next, we focus on finite nanowires supported by leads modeled by freezing the shape of a uniform wire outside the constriction volume. We calculate the conductance during the elongation process using the adiabatic approximation and the WKB transmission formula. We also observe the correlated oscillations of the elongation force. In different stages of the elongation process two kinds of electronic structures appear: one with extended states throughout the wire and one with an atom-cluster-like unit in the constriction and with well-localized states. We discuss the origin of these structures.Item Gradient correction for positron states in solids(American Physical Society (APS), 1995) Barbiellini, B.; Puska, Martti J.; Torsti, T.; Nieminen, Risto M.; Department of Applied Physics; Teknillisen fysiikan laitos; Perustieteiden korkeakoulu; School of ScienceFirst-principles calculations of positron-annihilation characteristics in solids are usually based on the local-density approximation (LDA) for positron-electron correlation. The LDA systematically overestimates the annihilation rate. As a remedy we introduce a generalized gradient approximation (GGA). Our results for several metals and semiconductors show that the GGA systematically improves the predictive power of positron lifetime calculations over those based on the LDA. We compare also the resulting positron energy levels in solids with data from slow-positron experiments.Item Model study of adsorbed metallic quantum dots: Na on Cu(111(2002-12-30) Torsti, T.; Lindberg, V.; Puska, M.J.; Hellsing, B.; Department of Applied PhysicsWe model electronic properties of the second-monolayer Na adatom islands (quantum dots) on the Cu(111) surface covered homogeneously by the first Na monolayer. An axially symmetric three-dimensional jellium model, taking into account the effects due to the first Na monolayer and the Cu substrate, has been developed. The electronic structure is solved within the local-density approximation of the density-functional theory using a real-space multigrid method. The model enables the study of systems consisting of thousands of Na atoms. The results for the local density of states are compared with differential conductance (dI/dV) spectra and constant current topographs from scanning tunneling microscopy.Item Model study of adsorbed metallic quantum dots: Na on Cu(111)(American Physical Society (APS), 2002) Torsti, T.; Lindberg, V.; Puska, Martti J.; Hellsing, B.; Teknillisen fysiikan laitos; Department of Applied Physics; Perustieteiden korkeakoulu; School of ScienceWe model electronic properties of the second-monolayer Na adatom islands (quantum dots) on the Cu(111) surface covered homogeneously by the first Na monolayer. An axially symmetric three-dimensional jellium model, taking into account the effects due to the first Na monolayer and the Cu substrate, has been developed. The electronic structure is solved within the local-density approximation of the density-functional theory using a real-space multigrid method. The model enables the study of systems consisting of thousands of Na atoms. The results for the local density of states are compared with differential conductance (dI/dV) spectra and constant current topographs from scanning tunneling microscopy.Item Multigrid method for electronic structure calculations(American Physical Society (APS), 2001) Heiskanen, M.; Torsti, T.; Puska, Martti J.; Nieminen, Risto M.; Department of Applied Physics; Teknillisen fysiikan laitos; Perustieteiden korkeakoulu; School of ScienceA general real-space multigrid algorithm for the self-consistent solution of the Kohn-Sham equations appearing in the state-of-the-art electronic-structure calculations is described. The most important part of the method is the multigrid solver for the Schrödinger equation. Our choice is the Rayleigh quotient multigrid method (RQMG), which applies directly to the minimization of the Rayleigh quotient on the finest level. Very coarse correction grids can be used, because there is, in principle, no need to represent the states on the coarse levels. The RQMG method is generalized for the simultaneous solution of all the states of the system using a penalty functional to keep the states orthogonal. The performance of the scheme is demonstrated by applying it in a few molecular and solid-state systems described by nonlocal norm-conserving pseudopotentials.