Non-equilibrium properties of the S=1/2 Heisenberg model in a time-dependent magnetic field

The time-dependent behavior of the Heisenberg model in contact with a phonon heat bath and in an external time-dependent magnetic field is studied by means of a path integral approach. The action of the phonon heat bath is taken into account up to the second order in the coupling to the heath bath. It is shown that there is a minimal value of the magnetic field below which the average magnetization of the system does not relax to equilibrium when the external magnetic field is flipped. This result is in qualitative agreement with the mean field results obtained within $\phi^{4}$-theory.Comment: To be published in Physica

Another analytic view about quantifying social forces

Montroll had considered a Verhulst evolution approach for introducing a notion he called "social force", to describe a jump in some economic output when a new technology or product outcompetes a previous one. In fact, Montroll's adaptation of Verhulst equation is more like an economic field description than a "social force". The empirical Verhulst logistic function and the Gompertz double exponential law are used here in order to present an alternative view, within a similar mechanistic physics framework. As an example, a "social force" modifying the rate in the number of temples constructed by a religious movement, the Antoinist community, between 1910 and 1940 in Belgium is found and quantified. Practically, two temple inauguration regimes are seen to exist over different time spans, separated by a gap attributed to a specific "constraint", a taxation system, but allowing for a different, smooth, evolution rather than a jump. The impulse force duration is also emphasized as being better taken into account within the Gompertz framework. Moreover, a "social force" can be as here, attributed to a change in the limited need/capacity of some population, coupled to some external field, in either Verhulst or Gompertz equation, rather than resulting from already existing but competing goods as imagined by Montroll.Comment: 4 figures, 29 refs., 15 pages; prepared for Advances in Complex System

Correlation amplitude and entanglement entropy in random spin chains

Using strong-disorder renormalization group, numerical exact diagonalization, and quantum Monte Carlo methods, we revisit the random antiferromagnetic XXZ spin-1/2 chain focusing on the long-length and ground-state behavior of the average time-independent spin-spin correlation function C(l)=\upsilon l^{-\eta}. In addition to the well-known universal (disorder-independent) power-law exponent \eta=2, we find interesting universal features displayed by the prefactor \upsilon=\upsilon_o/3, if l is odd, and \upsilon=\upsilon_e/3, otherwise. Although \upsilon_o and \upsilon_e are nonuniversal (disorder dependent) and distinct in magnitude, the combination \upsilon_o + \upsilon_e = -1/4 is universal if C is computed along the symmetric (longitudinal) axis. The origin of the nonuniversalities of the prefactors is discussed in the renormalization-group framework where a solvable toy model is considered. Moreover, we relate the average correlation function with the average entanglement entropy, whose amplitude has been recently shown to be universal. The nonuniversalities of the prefactors are shown to contribute only to surface terms of the entropy. Finally, we discuss the experimental relevance of our results by computing the structure factor whose scaling properties, interestingly, depend on the correlation prefactors.Comment: v1: 16 pages, 15 figures; v2: 17 pages, improved discussions and statistics, references added, published versio

The slimming effect of advection on black-hole accretion flows

At super-Eddington rates accretion flows onto black holes have been described as slim (aspect ratio $H/R \lesssim 1$) or thick (H/R >1) discs, also known as tori or (Polish) doughnuts. The relation between the two descriptions has never been established, but it was commonly believed that at sufficiently high accretion rates slim discs inflate, becoming thick. We wish to establish under what conditions slim accretion flows become thick. We use analytical equations, numerical 1+1 schemes, and numerical radiative MHD codes to describe and compare various accretion flow models at very high accretion rates.We find that the dominant effect of advection at high accretion rates precludes slim discs becoming thick. At super-Eddington rates accretion flows around black holes can always be considered slim rather than thick.Comment: 8 pages, 5 figures. Astronomy & Astrophysics, in pres

The Landless Voices Database: A Trajectory from Cultural Studies to Pedagogical Impact

This article initially addresses the conception of the web-enabled database Landless Voices (VIEIRA, 2003) as a contribution to Cultural Studies and, crucially, to the understanding of the relational workings of Brazil’s complex and regionally diverse culture of landlessness, and to the validation of the cultural self-expression of the sem-terra/Sem Terra. Secondly, it analyzes speculative data obtained from prospective primary teachers being trained at the Federal University of Paraná, Brazil, on the contribution of the database to the pedagogical context. The main findings are that the three predominant types of impact, using Meagher’s terminology (2013, p. 5) are conceptual (new knowledge), cultural (revising misconceptions) and instrumental (future development of pedagogic practices). It then moves to empirical research on social impact, understood as the contribution of academic research to non-academic users, more specifically to its presumably main beneficiaries – the Sem Terra learners themselves. The findings of these first exploratory workshops with learners in four rural schools in settlements in the states of Paraná and São Paulo, respectively in October and November 2013, are that the database broadens their educational resources and empowers a historically marginalized social segment. This article confirms, however, that impact is not a punctual activity (MEAGHER, 2013) and concludes on the need for continuous interaction with learners for the pedagogic impact of academic research to be generated[1].[1] This is part of Landless Voices Impact Enhancement Project, developed by Professor Else R. P. Vieira (Principal investigator) and co-researchers: Dr. Sônia Schwendler (Federal University of Paraná, UFPR) and Professor Bernardo Mançano Fernandes (São Paulo based UNESCO’s Chair in Territorial Development and Education for the Countryside; MA in Territorial Development in Latin American and Caribbean of the São Paulo State University – UNESP, and PhD Programme in Geography, Presidente Prudente Campus), both honorary researchers at Queen Mary’s Department of Iberian and Latin American Studies. This research on impact is financed by the School of Languages, Linguistics and Film (Queen Mary University of London) with the support of UFPR School of Education and of UNESP Centre for Agrarian Reform Studies, Research and Projects − NERA)