Ismo T. Koponen

Morphology of ledge patterns during step flow growth of metal surfaces vicinal to fcc(001)

The morphological development of step edge patterns in the presence of meandering instability during step flow growth is studied by simulations and numerical integration of a continuum model. It is demonstrated that the kink Ehrlich-Schwoebel barrier responsible for the instability leads to an invariant shape of the step profiles. The step morphologies change with increasing coverage from a somewhat triangular shape to a more flat, invariant steady state form. The average pattern shape extracted from the simulations is shown to he in good agreement with that obtained from numerical integration of the continuum theory.

Instability and Wavelength Selection during Step Flow Growth of Metal Surfaces Vicinal to fcc(001)

We study the onset and development of ledge instabilities during growth of vicinal metal surfaces using kinetic Monte Carlo simulations. We observe the formation of periodic patterns at [110] close packed step edges on surfaces vicinal to fcc(001) under realistic molecular beam epitaxy conditions. The corresponding wavelength and its temperature dependence are studied in detail. Simulations suggest that the ledge instability on fcc(1,1,m) vicinal surfaces is controlled by the strong kink Ehrlich-Schwoebel barrier, with the wavelength determined by dimer nucleation at the step edge. Our results are in agreement with recent continuum theoretical predictions, and experiments on Cu(1,1,17) vicinal surfaces.

Modeling layer-by-layer growth in ion beam assisted deposition of thin films

Abstract A simple rate equation model for layer-by-layer growth in ion beam assisted deposition is developed. In terms of the rate coefficients, the model describes how adatom diffusion, island diffusion, detachment and breakup, coalescence of large islands and interlayer transitions of adatoms affect the growth. Results based on the model show that enhanced detachment of adatoms due to ion bombardment is required to promote layer-by-layer growth but without sufficiently fast downward transition path, the flux of adatoms to underlying layers may be too slow for the onset of layer-by-layer growth. In order to obtain optimum conditions, both requirements must be met.

The role of physics departments in developing student teachers' expertise in teaching physics

In physics teacher education the challenge is to promote the development of the expertise needed in physics teachers. To that end, close collaboration between physicists and physics education professionals is needed. This, however, poses many challenges which are not easily met. We describe here some guidelines based on our own experience from a pilot course for pre-service physics teacher education. We discuss how physics departments can meet these challenges by designing a special course and working models, which takes into account the aspect of expected expertise in physics teachers. The positive results and feedback received are presented.

Systemic view of learning scientific concepts: A description in terms of directed graph model

This study suggests a systemic view on concept learning, in which concepts are seen as complex, dynamically evolving structures with robust and persistent patterns emerging from loosely connected elemental pieces. The model is represented in the form of a directed graph, and it explains many empirically identified features of learning paths, including: (1) the stability of learned concepts, (2) changes in concept attributions (ontological shift) as guided by theoretical knowledge, and (3) the evolution of concepts as driven by model competition with the accumulation of evidence. The results thus show applicability of the systemic view and the directed graph model.

A rate-equation model for the growth of metallic thin films in ion beam assisted deposition

We describe a simple and fast rate-equation model for surface growth during ion beam assisted metal-on-metal deposition. In terms of the rate coefficients the model describes how island detachment and breakup, adatom diffusion and interlayer transitions of adatoms affect the growth. Results from our model can be directly compared to experimental data, also for coverages of several layers. We identify parameter values corresponding to different modes of growth and locate the phase boundary between them as a function of temperature. We also show that in ideal layer-by-layer growth island size distributions retain the regular scaling form. Our model shows that in order to obtain thin films of good quality, both efficient detachment of adatoms and sufficiently fast interlayer transitions are needed. The results validate the approximations made in an earlier, more coarse-grained model.

Generative Modelling in Physics and in Physics Education: From Aspects of Research Practices to Suggestions for Education

The extensive use of modelling in physics research has many implications on how it is used in physics education. An interesting case is the use of models in producing of new knowledge, which we here refer to as generative modelling. Generative modelling can serve as a cognitive tool bridging conceptual reality and real phenomena by mutually fitting of simulations and experiments. In this fitting process of fitting, pursuing partial mimetic similarity in simulations and experiments acquires a central epistemological role. At the core of generative modelling is the creative use of theoretical and empirical elements of modelling as well as the explorative manipulation of real conditions to fit the models. We argue here that such modelling is also identifiable as authentic by the modelling practitioners themselves and that such a modelling approach supports constructively oriented and creative teaching solutions.

Entropy and Energy in Characterizing the Organization of Concept Maps in Learning Science

Knowledge structures are often represented in the form of networks or maps of concepts. The coherence and connectivity of such knowledge representations is known to be closely related to knowledge production, acquisition and processing. In this study we use network theory in making the clustering and cohesion of concept maps measurable, and show how the distribution of these properties can be interpreted through the Maximum Entropy (MaxEnt) method. This approach allows to introduce new concepts of the “energy of cognitive load” and the “entropy of knowledge organization” to describe the organization of knowledge in the concept maps.

Meandering instability of curved step edges on growth of a crystalline cone

Abstract We study the meandering instability during growth of an isolated nanostructure, a crystalline cone, consisting of concentric circular steps. The onset of the instability is studied analytically within the framework of the standard Burton–Cabrera–Frank model, which is applied to describe step flow growth in circular geometry. We derive the correction to the most unstable wavelength and show that in general it depends on the curvature in a complicated way. Only in the asymptotic limit where the curvature approaches zero the results are shown to reduce to the rectangular case. The results obtained here are of importance in estimating growth regimes for stable nanostructures against step meandering.

Concept Networks in Learning and the Epistemic Support of Their Key Concepts

Concept maps are used in teaching and learning as representations of students’ understanding of conceptual knowledge. Concept maps are basically networks of interlinked web of concepts. A long-standing problem in educational research is identifying the key concepts of importance in such networks. Here we use network analysis to examine students’ representations of the relatedness of physics concepts in the form of concept maps, and suggest how key concepts and their epistemic support can be identified. The concept maps are analysed as directed and weighted networks, where nodes are concepts and links represent different types of connections between concepts. The notion of key concept is operationalised through the communicability, separately for out-going and in-coming weighted links. Using a collated concept network based on a sample of 12 original concept maps constructed by university students we show that the communicability is a simple and reliable way to identify the key concepts and examine their epistemic justification within the network.