Quantum nanomagnets and nuclear spins: an overview

This mini-review presents a simple and accessible summary on the fascinating physics of quantum nanomagnets coupled to a nuclear spin bath. These chemically synthesized systems are an ideal test ground for the theories of decoherence in mesoscopic quantum degrees of freedom, when the coupling to the environment is local and not small. We shall focus here on the most striking quantum phenomenon that occurs in such nanomagnets, namely the tunneling of their giant spin through a high anisotropy barrier. It will be shown that perturbative treatments must be discarded, and replaced by a more sophisticated formalism where the dynamics of the nanomagnet and the nuclei that couple to it are treated together from the beginning. After a critical review of the theoretical predictions and their experimental verification, we continue with a set of experimental results that challenge our present understanding, and outline the importance of filling also this last gap in the theory.Comment: 14 pages, 3 figures. Chapter in the Proceedings of the 2006 Les Houches summer school "Quantum Magnetism", ed. B. Barbara & Y. Imry, Springer (2007

Graphical means for inspecting qualitative models of system behaviour

This article presents the design and evaluation of a tool for inspecting conceptual models of system behaviour. The basis for this research is the Garp framework for qualitative simulation. This framework includes modelling primitives, such as entities, quantities and causal dependencies, which are combined into model fragments and scenarios. Given a library of model fragments and a scenario describing an initial situation, the qualitative simulation engine generates predictions in the form of a state-transition graph. This rich knowledge representation has potential for educational purposes. However, communicating the contents of simulation models effectively to learners is not trivial. The predicate logic format used by Garp is not easy for non-experts to understand, and a simulation often contains so much information that it is difficult to get an overview while still having access to detailed information. To address these problems, a tool has been developed that generates graphical representations of the information contained in a qualitative simulation. This tool, named VisiGarp, incorporates a vocabulary of graphical elements for model ingredients and relationships, and combines these into interactive diagrams. VisiGarp has been evaluated by thirty students, with promising results, using a setup which included simulation results and exercises about Brazilian Cerrado ecology