Browsing by Author "Achim, Cristian"

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  • Grain extraction and microstructural analysis method for two-dimensional poly and quasicrystalline solids
    (2018-10-16) Hirvonen, Petri; La Boissonière, Gabriel Martine; Fan, Zheyong; Achim, Cristian; Provatas, Nikolas; Elder, Ken R.; Ala-Nissilä, Tapio
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    While the microscopic structure of defected solid crystalline materials has significant impact on their physical properties, efficient and accurate determination of a given polycrystalline microstructure remains a challenge. In this paper, we present a highly generalizable and reliable variational method to achieve this goal for two-dimensional crystalline and quasicrystalline materials. The method is benchmarked and optimized successfully using a variety of large-scale systems of defected solids, including periodic structures and quasicrystalline symmetries to quantify their microstructural characteristics, e.g., grain size and lattice misorientation distributions. We find that many microstructural properties show universal features independent of the underlying symmetries.
  • The phase field crystal model with random pinning
    (2009) Lehikoinen, Joel
     Informaatio- ja luonnontieteiden tiedekunta | Bachelor's thesis
  • Studies of static and driven lattice systems with the phase field crystal model
    (2009) Achim, Cristian
     Doctoral dissertation (monograph)
    Study of static and dynamical properties of two dimensional lattice systems has become an important topic in nanoscience. Competition between the intrinsic ordering of adsorbed layers and the underlying substrate often leads to the appearance of spatially modulated structures. Important examples include spin density waves, charge density waves, vortex lattices in superconducting films with pinning centers and weakly adsorbed monolayers. In addition to static properties of such systems, their dynamics under external driving force is of great importance in tribology to understand the microscopic origins of friction. This Thesis deals with the static and dynamic properties of adsorbed layers under the influence of a driving force using an extended version of the Phase Field Crystal Model. In the case of periodic potentials, the various commensurate phases are described in detail and the complete phase diagram is mapped out as a function of pinning strength and lattice mismatch. For finite temperatures, Monte Carlo simulations are used to determine the phase diagram as a function of temperature. For one of the commensurate phases behavior is found consistent with the Ising universality class. The results concerning the effect of disordered pinning potential are also presented. The nonlinear response to a driving force on an initially pinned commensurate phase is then studied via overdamped dynamic equations of motion for different values of mismatch and pinning strengths. For large pinning strength the driven depinning transitions are continuous, and the sliding velocity varies with the force from the threshold with power-law exponents in agreement with analytical predictions. Transverse depinning transitions in the moving state are also found in two dimensions. Surprisingly, for sufficiently weak pinning potential we find a discontinuous depinning transition with hysteresis even in one dimension under overdamped dynamics. Also, structural changes of the system are characterized in some detail close to the depinning transitions.