Artículos con la etiqueta ‘Other Condensed Matter (cond-mat.other)’

The education of Walter Kohn and the creation of density functional theory

Por • 26 mar, 2014 • Category: Leyes

The theoretical solid-state physicist Walter Kohn was awarded one-half of the 1998 Nobel Prize in Chemistry for his mid-1960’s creation of an approach to the many-particle problem in quantum mechanics called density functional theory (DFT). In its exact form, DFT establishes that the total charge density of any system of electrons and nuclei provides all the information needed for a complete description of that system. This was a breakthrough for the study of atoms, molecules, gases, liquids, and solids. Before DFT, it was thought that only the vastly more complicated many-electron wave function was needed for a complete description of such systems. Today, fifty years after its introduction, DFT (in one of its approximate forms) is the method of choice used by most scientists to calculate the physical properties of materials of all kinds. In this paper, I present a biographical essay of Kohn’s educational experiences and professional career up to and including the creation of DFT.

Gauge Symmetry, Spontaneous Breaking of Gauge Symmetry: Philosophical Approach

Por • 3 feb, 2014 • Category: Leyes

This paper deals with the ontology of the vector potential. When the state of the system has the full gauge symmetry of the Hamiltonian, the electromagnetic vector potential may be interpreted as a convenient tool of a mathematical formulation, with no ontological meaning. I argue that this interpretation is in difficulty because the vector potential becomes proportional to the supercurrent in the superfluid phases, which are spontaneously broken gauge symmetry phases, where particle number is not conserved. I suggest that when gauge symmetry is spontaneously broken, the vector potential becomes an emergent material object of nature.

The drastic outcomes from voting alliances in three-party bottom-up democratic voting (1990 $\rightarrow$ 2013)

Por • 27 abr, 2013 • Category: sociologia

The drastic effect of local alliances in three-party competition is investigated in democratic hierarchical bottom-up voting. The results are obtained analytically using a model which extends a sociophysics frame introduced in 1986 \cite{psy} and 1990 \cite{lebo} to study two-party systems and the spontaneous formation of democratic dictatorship. It is worth stressing that the 1990 paper was published in the Journal of Statistical Physics, the first paper of its kind in the journal. It was shown how a minority in power can preserve its leadership using bottom-up democratic elections. However such a bias holds only down to some critical value of minimum support. The results were used latter to explain the sudden collapse of European communist parties in the nineties. The extension to three-party competition reveals the mechanisms by which a very small minority party can get a substantial representation at higher levels of the hierarchy when the other two competing parties are big.