Exchange interaction and its tuning in magnetic binary chalcogenides

Using a first-principles Green's function approach we study magnetic properties of the magnetic binary chalcogenides Bi2Te3, Bi2Se3, and Sb2Te3. The magnetic coupling between transition-metal impurities is long-range, extends beyond a quintuple layer, and decreases with increasing number of d electrons per 3d atom. We find two main mechanisms for the magnetic interaction in these materials: the indirect exchange interaction mediated by free carriers and the indirect interaction between magnetic moments via chalcogen atoms. The calculated Curie temperatures of these systems are in good agreement with available experimental data. Our results provide deep insight into magnetic interactions in magnetic binary chalcogenides and open a way to design new materials for promising applications

Probing the Fermi surface by positron annihilation and Compton scattering

Positron annihilation and Compton scattering are important probes of the Fermi surface. Relying on conservation of energy and momentum, being bulk sensitive and not limited by short electronic mean-free-paths, they can provide unique information in circumstances when other methods fail. Using a variety of examples, their contribution to knowledge about the electronic structure of a wide range of materials is demonstrated

Elastic anomalies and long/short-range ordering effects: A first-principles investigation of the AgcPd1-c solid solution

We investigate the elastic properties of the binary alloy Ag-Pd. The lattice constant of this system shows significant deviations from the linear behavior anticipated by the semi-empirical Vegard's rule. This effect was formerly studied by assuming total substitutional disorder, and described by the coherent potential approximation (CPA). Theoretical phase diagram investigations have however suggested three ordered phases at low temperatures, and we extend our first-principles investigation to include such scenarios through the adoption of an extended unit cell representation and a recently developed multisublattice generalization of the original CPA. This allows us to explore equilibrium lattice constant and bulk modulus within a unified approach even in the presence of partial long-range order. We obtain significant variations of the bulk modulus in comparison to the totally disordered picture, and again very rich deviations from more intuitive predictions of a simple linear behavior. We follow former suggestions to analyze the different regimes in connection with topological transitions of the Fermi surface, examined through Bloch spectral function calculations

Mario Bunge and the Current Revival of Causal Realism

Mario Bunge’s Causality and Modern Science is arguably one of the best treatments of the causal realist tradition ever to have been written, one that defends the place of causality as a category in the conceptual framework of modern science. And yet in the current revival of causal realism in contemporary metaphysics, there is very little awareness of Bunge’s work. This paper seeks to remedy this, by highlighting one particular criticism Bunge levels at the Aristotelian view of causation and illustrating its relevance for contemporary powers-based accounts. Roughly, the Aristotelian view depicts interactions between objects as involving a unidirectional exertion of influence of one object upon another. This idea of unidirectional action is central to the Aristotelian distinction between active and passive powers, and its corresponding distinction between active and passive objects. As Bunge points out, modern physics does not recognise the existence of any unidirectional actions at all; all influence comes in the form of reciprocal action, or interaction. If this is right, all notions deriving from or influenced by the idea of unidirectional actions—such as the concept of mutual manifestation and reciprocal disposition partners—risk being false by the same measure. Bunge drew the conclusion that the Aristotelian view is ontologically inadequate, but still advocated its use as the most useful approximation available in science. He considered, but ultimately rejected the possibility of a modified view of causation built on reciprocal action, because, in his view, it couldn’t account for the productivity of causation. Bunge’s critique of this particular aspect of the Aristotelian view cannot be overlooked in contemporary metaphysics, but it is possible to construe a modified view of causation that takes the reciprocity of interactions seriously without loss of productivity.Peer reviewe

Using density functional theory to describe slowly varying fluctuations at finite temperatures : local magnetic moments in Gd and the ‘not so local’ moments of Ni

We briefly describe the density functional theory (DFT)-based 'disordered local moment' (DLM) picture for magnetism at finite temperatures. It shows how relatively slowly fluctuating local moments can emerge from the interacting electrons of many materials. Such entities have rigid magnitudes and fluctuate their orientations from atomic site to atomic site on a timescale long compared to other electronic times. We illustrate this theory with calculations of the magnetocaloric effect in Gd where we find excellent agreement with experiments. Fluctuating moments do not appear to establish naturally over such small regions for some other materials. We show how the DFT-DLM theory can be extended to these materials with the use of the Korringa–Kohn–Rostoker nonlocal coherent potential approximation (KKR-NLCPA) to allow for more extensive, slow magnetic fluctuations. We present the first application of this approach by revisiting the description of the magnetic fluctuations prevalent in the paramagnetic state of nickel. We find that local moments can emerge above Tc and that these form coherently over small clumps of atomic site

Do powers need powers to make them powerful?: from pandispositionalism to Aristotle

The nature of powers is vigorously debated in the literature between, broadly, the dispositionalist camp and the categoricalist camp.1 The main difference that divides the two camps is whether there are powers that do not depend ontologically on categorical properties-pure powers;2 a secondary issue is whether, if there are pure powers in the world, there need to be categorical properties at all in the ontology.</p

Gregory of Nyssa on the creation of the world

Written by a group of leading scholars, this unique collection of essays investigates the views of both pagan and Christian philosophers on causation and the creation of the cosmos. Structured in two parts and..

The extended mind in ontological entanglements

The human mind often relies on external mechanisms for carrying out its cognitive processes, for example by using a calculator. Vehicle externalism and in particular the Extended Mind theory (Clark and Chalmers 1998) explore whether such external mechanisms, under specified conditions, are more than mere instruments for the mind; the claim is that they are parts of the mind and thus extend it beyond its outer boundaries of skin and skull. This chapter argues that high degrees of interdependence between the mind and its extensions give rise to ontological entanglements, with respect to which there is at present no theory of individuation which can tell us how many entities there are in the entanglement and what type of entity the entanglement is. The chapter argues that the Extended Mind theory sheds light on the problem of the incarnation by motivating, in a more intuitively compelling way, the need for new ways of thinking about oneness and distinctness. The problems we encounter with the metaphysics of the incarnation are not peculiar to it; the solutions might not be either.</p

What's dynamic about causal powers? A black box!

Modern science cannot do without Aristotelian powers – thus have argued Cartwright and Pemberton (2013) among many others. Aristotelian powers are essentially dynamic entities, which account for causal phenomena, and thus explain how change comes about in the world. In this chapter I argue that explaining causation in terms of interacting causal powers places causation … beyond the reach of our understanding(!) – because causal interaction shows us what powers do, and not what powers are. Metaphysicians by and large agree that the intrinsic nature of powers is to be dynamic entities. I contend here that their dynamism is irreducible, and crucially, unknowable, rendering what powers are ‘black boxes’ to us, despite multiple attempts of defining them in the literature. The sciences discover only how powers behave, and classify them teleologically to tell us what they do. Powers, however, are mysterious and unexplorable black boxes to us, even though they are indispensable in our scientific explanations of change in the world