Browsing by Author "Havu, P."

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  • Conductance oscillations in metallic nanocontacts
    (2002) Havu, P.; Torsti, T.; Puska, Martti J.; Nieminen, Risto M.
     School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    We 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.
  • Effects of chemical functionalization on electronic transport in carbon nanobuds
    (2012) Havu, P.; Sillanpää, A.; Runeberg, N.; Tarus, J.; Seppälä, E. T.; Nieminen, Risto M.
     School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Carbon nanobuds form a class of hybrid structures consisting of carbon nanotubes onto which fullerene types of units are covalently grown. Due to higher electrophilicity and curvature of the fullerene moiety a carbon nanobud exhibits higher reactivity compared to a plain nanotube. In this paper we study how the electronic structure and transport properties of carbon nanobuds are affected by chemical modification. The studied model systems comprise carbon nanobuds that are chemically modified by attaching Li and F atoms as well as tetrathiafulvalene molecules. We use the density functional theory combined with Landauer-Büttiker electron transport formalism. According to the simulations, the attached units change the relative positions of the Fermi levels, creating a distinctive effect on the electronic transport properties along associated carbon nanotubes. In semiconducting nanotubes the change in the conductance is systematic and should be detectable in experiments. Hence, the carbon nanobuds are potential candidates for sensor applications.
  • Electron transport through quantum wires and point contacts
    (2004) Havu, P.; Puska, Martti J.; Nieminen, Risto M.; Havu, V.
     School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    We have studied quantum wires using the Green’s function technique within density-functional theory, calculating electronic structures and conductances for different wire lengths, temperatures, and bias voltages. For short wires, i.e., quantum point contacts, the zero-bias conductance shows as a function of the gate voltage and at a finite temperature a plateau at around 0.7G0. (G0=2e2/h is the quantum conductance.) The behavior, which is caused in our mean-field model by spontaneous spin polarization in the constriction, is reminiscent of the so-called 0.7 anomaly observed in experiments. In our model the temperature and the wire length affect the conductance–gate-voltage curves similarly as in experiments.
  • Finite-element implementation for electron transport in nanostructures
    (2006) Havu, P.; Havu, V.; Puska, Martti J.; Hakala, M. H.; Foster, Adam S.; Nieminen, Risto M.
     School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    We have modeled transport properties of nanostructures using Green’s-function method within the framework of the density-functional theory. The scheme is computationally demanding, so numerical methods have to be chosen carefully. A typical solution to the numerical burden is to use a special basis-function set, which is tailored to the problem in question, for example, the atomic-orbital basis. In this paper we present our solution to the problem. We have used the finite-element method with a hierarchical high-order polynomial basis, the so-called p elements. This method allows the discretation error to be controlled in a systematic way. The p elements work so efficiently that they can be used to solve interesting nanosystems described by nonlocal pseudopotentials. We demonstrate the potential of the implementation with two different systems. As a test system a simple Na-atom chain between two leads is modeled and the results are compared with several previous calculations. Secondly, we consider a thin hafnium dioxide (HfO2) layer on a silicon surface as a model for a gate structure of the next generation of microelectronics.
  • Interfacial oxide growth at silicon/high-k oxide interfaces: First principles modeling of the Si–HfO[sub 2] interface
    (2006) Hakala, M. H.; Foster, Adam S.; Gavartin, J. L.; Havu, P.; Puska, Martti J.; Nieminen, Risto M.
     School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    We have performed first principles calculations to investigate the structure and electronic properties of several different Si–HfOx interfaces. The atomic structure has been obtained by growing HfOx layer by layer on top of the Si(100) surface and repeatedly annealing the structure using ab initio molecular dynamics. The interfaces are characterized via their geometric and electronic properties, and also using electron transport calculations implementing a finite element based Green’s function method. We find that in all interfaces, oxygen diffuses towards the interface to form a silicon dioxide layer. This results in the formation of dangling Hf bonds in the oxide, which are saturated either by hafniumdiffusion or Hf–Si bonds. The generally poor performance of these interfaces suggests that it is important to stabilize the system with respect to lattice oxygen diffusio
  • Interfacial oxide growth in silicon/high-k oxide interfaces: First principles modeling of the Si-HfO2 interface
    (2006) Hakala, M. H.; Foster, A. S.; Gavartin, J.L.; Havu, P.; Puska, M. J.; Nieminen, R. M.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    We have performed first principles calculations to investigate the structure and electronic properties of several different Si–HfOx interfaces. The atomic structure has been obtained by growing HfOx layer by layer on top of the Si(100) surface and repeatedly annealing the structure using ab initio molecular dynamics. The interfaces are characterized via their geometric and electronic properties, and also using electron transport calculations implementing a finite element based Green’s function method. We find that in all interfaces, oxygen diffuses towards the interface to form a silicon dioxide layer. This results in the formation of dangling Hf bonds in the oxide, which are saturated either by hafnium diffusion or Hf–Si bonds. The generally poor performance of these interfaces suggests that it is important to stabilize the system with respect to lattice oxygen diffusion.
  • Nonequilibrium electron transport in two-dimensional nanostructures modeled using Green’s functions and the finite-element method
    (2004) Havu, P.; Havu, V.; Puska, Martti J.; Nieminen, Risto M.
     School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    We use the effective-mass approximation and the density-functional theory with the local-density approximation for modeling two-dimensional nanostructures connected phase coherently to two infinite leads. Using the nonequilibrium Green’s-function method the electron density and the current are calculated under a bias voltage. The problem of solving for the Green’s functions numerically is formulated using the finite-element method (FEM). The Green’s functions have nonreflecting open boundary conditions to take care of the infinite size of the system. We show how these boundary conditions are formulated in the FEM. The scheme is tested by calculating transmission probabilities for simple model potentials. The potential of the scheme is demonstrated by determining nonlinear current-voltage behaviors of resonant tunneling structures.
  • Nonequilibrium electron transport in two-dimensional nanostructures modeled using Green's functions and the finite-element method
    (2004-03-19) Havu, P.; Havu, V.; Puska, M. J.; Nieminen, R. M.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    We use the effective-mass approximation and the density-functional theory with the local-density approximation for modeling two-dimensional nanostructures connected phase coherently to two infinite leads. Using the nonequilibrium Green's-function method the electron density and the current are calculated under a bias voltage. The problem of solving for the Green's functions numerically is formulated using the finite-element method (FEM). The Green's functions have nonreflecting open boundary conditions to take care of the infinite size of the system. We show how these boundary conditions are formulated in the FEM. The scheme is tested by calculating transmission probabilities for simple model potentials. The potential of the scheme is demonstrated by determining nonlinear current-voltage behaviors of resonant tunneling structures.
  • Spin dependent electron transport through a magnetic resonant tunneling diode
    (2005-06-02) Havu, P.; Tuomisto, N.; Väänänen, R.; Puska, M.J.; Nieminen, R.M.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Electron-transport properties in nanostructures can be modeled, for example, by using the semiclassical Wigner formalism or the quantum-mechanical Green’s function formalism. We compare the performance and the results of these methods in the case of magnetic resonant-tunneling diodes. We have implemented the two methods within the self-consistent spin-density-functional theory. Our numerical implementation of the Wigner formalism is based on the finite-difference scheme whereas for the Green’s function formalism the finite-element method is used. As a specific application, we consider the device studied by Slobodskyy et al. [Phys. Rev. Lett. 90, 246601 (2003)] and analyze their experimental results. The Wigner and Green’s function formalisms give similar electron densities and potentials but, surprisingly, the former method requires much more computer resources in order to obtain numerically accurate results for currents. Both of the formalisms can be used to model magnetic resonant tunneling diode structures.
  • Spin-dependent electron transport through a magnetic resonant tunneling diode
    (2005) Havu, P.; Tuomisto, N.; Väänänen, R.; Puska, Martti J.; Nieminen, Risto M.
     School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Electron-transport properties in nanostructures can be modeled, for example, by using the semiclassical Wigner formalism or the quantum-mechanical Green’s function formalism. We compare the performance and the results of these methods in the case of magnetic resonant-tunneling diodes. We have implemented the two methods within the self-consistent spin-density-functional theory. Our numerical implementation of the Wigner formalism is based on the finite-difference scheme whereas for the Green’s function formalism the finite-element method is used. As a specific application, we consider the device studied by Slobodskyy et al. [Phys. Rev. Lett. 90, 246601 (2003)] and analyze their experimental results. The Wigner and Green’s function formalisms give similar electron densities and potentials but, surprisingly, the former method requires much more computer resources in order to obtain numerically accurate results for currents. Both of the formalisms can be used to model magnetic resonant tunneling diode structures.