102 research outputs found
The absence of finite-temperature phase transitions in low-dimensional many-body models: a survey and new results
After a brief discussion of the Bogoliubov inequality and possible
generalizations thereof, we present a complete review of results concerning the
Mermin-Wagner theorem for various many-body systems, geometries and order
parameters. We extend the method to cover magnetic phase transitions in the
periodic Anderson Model as well as certain superconducting pairing mechanisms
for Hubbard films. The relevance of the Mermin-Wagner theorem to approximations
in many-body physics is discussed on a conceptual level.Comment: 33 pages; accepted for publication as a Topical Review in Journal of
Physics: Condensed Matte
Ferromagnetic Kondo-Lattice Model
We present a many-body approach to the electronic and magnetic properties of
the (multiband) Kondo-lattice model with ferromagnetic interband exchange. The
coupling between itinerant conduction electrons and localized magnetic moments
leads, on the one hand, to a distinct temperature-dependence of the electronic
quasiparticle spectrum and, on the other hand, to magnetic properties, as
e.~g.the Curie temperature T_C or the magnon dispersion, which are strongly
influenced by the band electron selfenergy and therewith in particular by the
carrier density. We present results for the single-band Kondo-lattice model in
terms of quasiparticle densities of states and quasiparticle band structures
and demonstrate the density-dependence of the self-consistently derived Curie
temperature. The transition from weak-coupling (RKKY) to strong-coupling
(double exchange) behaviour is worked out.
The multiband model is combined with a tight-binding-LMTO bandstructure
calculation to describe real magnetic materials. As an example we present
results for the archetypal ferromagnetic local-moment systems EuO and EuS. The
proposed method avoids the double counting of relevant interactions and takes
into account the correct symmetry of atomic orbitals.Comment: 15 pages, 10 figure
Exploring modularity in biological networks
Network theoretical approaches have shaped our understanding of many different kinds of biological modularity. This essay makes the case that to capture these contributions, it is useful to think about the role of network models in exploratory research. The overall point is that it is possible to provide a systematic analysis of the exploratory functions of network models in bioscientific research. Using two examples from molecular and developmental biology, I argue that often the same modelling approach can perform one or more exploratory functions, such as introducing new directions of research, offering a complementary set of concepts, methods and algorithms for individuating important features of natural phenomena, generating proofs of principle demonstrations and potential explanations for phenomena of interest and enlarging the scope of certain research agendas. This article is part of the theme issue 'Unifying the essential concepts of biological networks: biological insights and philosophical foundations'
(Non)Invariance of dynamical quantities for orbit equivalent flows
We study how dynamical quantities such as Lyapunov exponents, metric entropy,
topological pressure, recurrence rates, and dimension-like characteristics
change under a time reparameterization of a dynamical system. These quantities
are shown to either remain invariant, transform according to a multiplicative
factor or transform through a convoluted dependence that may take the form of
an integral over the initial local values. We discuss the significance of these
results for the apparent non-invariance of chaos in general relativity and
explore applications to the synchronization of equilibrium states and the
elimination of expansions
Kondo-lattice model: Application to the temperature-dependent electronic structure of EuO(100) films
We present calculations for the temperature-dependent electronic structure
and magnetic properties of thin ferromagnetic EuO films. The treatment is based
on a combination of a multiband-Kondo lattice model with first-principles
TB-LMTO band structure calculations. The method avoids the problem of
double-counting of relevant interactions and takes into account the correct
symmetry of the atomic orbitals. We discuss the temperature-dependent
electronic structures of EuO(100) films in terms of quasiparticle densities of
states and quasiparticle band structures. The Curie temperature T_C of the EuO
films turns out to be strongly thickness-dependent, starting from a very low
value = 15K for the monolayer and reaching the bulk value at about 25 layers
Thermodynamic formalism for contracting Lorenz flows
We study the expansion properties of the contracting Lorenz flow introduced
by Rovella via thermodynamic formalism. Specifically, we prove the existence of
an equilibrium state for the natural potential for the contracting Lorenz flow and for in an interval
containing . We also analyse the Lyapunov spectrum of the flow in terms
of the pressure
Physics of Neutron Star Crusts
The physics of neutron star crusts is vast, involving many different research
fields, from nuclear and condensed matter physics to general relativity. This
review summarizes the progress, which has been achieved over the last few
years, in modeling neutron star crusts, both at the microscopic and macroscopic
levels. The confrontation of these theoretical models with observations is also
briefly discussed.Comment: 182 pages, published version available at
<http://www.livingreviews.org/lrr-2008-10
The Philosopher's Medicine of the Mind: Kant's Account of Mental Illness and the Normativity of Thinking
Kant’s conception of mental illness is unlikely to satisfy contemporary readers. His classifications of mental illness are often fluid and ambiguous, and he seems to attribute to human beings at least some responsibility for preventing mental illness. In spite of these apparent disadvantages, I argue that Kant’s account of mental illness can be illuminating to his views about the normative dimensions of human cognition. In contrast to current understandings of mental illness, Kant’s account is what I refer to as “non-pathological.” That is, most mental illnesses are for Kant continuous with normally functioning cognition. Someone with a healthy reason can easily fall into mental illness and someone with mental illness can (perhaps not as easily) re-establish healthy reason. By accepting a non-pathological definition of mental illness, it follows for Kant that humans have more agency and responsibility regarding their mental health than current views allow, which explains why several of his writings aim to prescribe a “diet of the mind” (2:271). Contrary to popular readings of Kant as a champion of reason’s power, Kant’s conception of mental illness shows that he recognizes how fragile human reason can be
Why so serious? Theorising playful model-driven group decision support with situated affectivity
This is the author accepted manuscript. The final version is available from Springer via the DOI in this record.An integrative approach to theorising behavioural, affective and cognitive processes in modeldriven
group decision support (GDS) interventions is needed to gain insight into the (micro-)processes
by which outcomes are accomplished. This paper proposes that the theoretical lens of situated
affectivity, grounded in recent extensions of scaffolded mind models, is suitable to understand the
performativity of affective micro-processes in model-driven GDS interventions. An illustrative vignette
of a humorous micro-moment in a group decision workshop is presented to reveal the performativity of
extended affective scaffolding processes for group decision development. The lens of situated
affectivity constitutes a novel approach for the study of interventionist practice in the context of group
decision making (and negotiation). An outlook with opportunities for future research is offered to
facilitate an integrated approach to the study of cognitive-affective and behavioural micro-processes in
model-driven GDS interventions.This work was supported in part by the EU FP7-ENERGY- SMARTCITIES-2012
(314277) project STEEP (Systems Thinking for Comprehensive City Efficient Energy Planning
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