The inverse Mermin-Wagner theorem for classical spin models on graphs

In this letter we present the inversion of the Mermin-Wagner theorem on graphs, by proving the existence of spontaneous magnetization at finite temperature for classical spin models on transient on the average (TOA) graphs, i.e. graphs where a random walker returns to its starting point with an average probability $\bar F < 1$. This result, which is here proven for models with O(n) symmetry, includes as a particular case $n=1$, providing a very general condition for spontaneous symmetry breaking on inhomogeneous structures even for the Ising model.Comment: 4 Pages, to appear on PR

Topological Reduction of Tight-Binding Models on Complex Networks

Complex molecules and mesoscopic structures are naturally described by general networks of elementary building blocks and tight-binding is one of the simplest quantum model suitable for studying the physical properties arising from the network topology. Despite the simplicity of the model, topological complexity can make the evaluation of the spectrum of the tight-binding Hamiltonian a rather hard task, since the lack of translation invariance rules out such a powerful tool as Fourier transform. In this paper we introduce a rigorous analytical technique, based on topological methods, for the exact solution of this problem on branched structures. Besides its analytic power, this technique is also a promising engineering tool, helpful in the design of netwoks displaying the desired spectral features.Comment: 19 pages, 14 figure

The Type-problem on the Average for random walks on graphs

When averages over all starting points are considered, the Type Problem for the recurrence or transience of a simple random walk on an inhomogeneous network in general differs from the usual "local" Type Problem. This difference leads to a new classification of inhomogeneous discrete structures in terms of {\it recurrence} and {\it transience} {\it on the average}, describing their large scale topology from a "statistical" point of view. In this paper we analyze this classification and the properties connected to it, showing how the average behavior affects the thermodynamic properties of statistical models on graphs.Comment: 10 pages, 3 figures. to appear on EPJ

Phase-ordering kinetics on graphs

We study numerically the phase-ordering kinetics following a temperature quench of the Ising model with single spin flip dynamics on a class of graphs, including geometrical fractals and random fractals, such as the percolation cluster. For each structure we discuss the scaling properties and compute the dynamical exponents. We show that the exponent $a_\chi$ for the integrated response function, at variance with all the other exponents, is independent on temperature and on the presence of pinning. This universal character suggests a strict relation between $a_\chi$ and the topological properties of the networks, in analogy to what observed on regular lattices.Comment: 16 pages, 35 figure

Topological thermal instability and length of proteins

We present an analysis of the effects of global topology on the structural stability of folded proteins in thermal equilibrium with a heat bath. For a large class of single domain proteins, we computed the harmonic spectrum within the Gaussian Network Model (GNM) and determined the spectral dimension, a parameter describing the low frequency behaviour of the density of modes. We find a surprisingly strong correlation between the spectral dimension and the number of amino acids of the protein. Considering that larger spectral dimension value relate to more topologically compact folded state, our results indicate that for a given temperature and length of the protein, the folded structure corresponds to the less compact folding compatible with thermodynamic stability.Comment: 15 pages, 6 eps figures, 2 table

Autocatalytic reaction-diffusion processes in restricted geometries

We study the dynamics of a system made up of particles of two different species undergoing irreversible quadratic autocatalytic reactions: $A + B \to 2A$. We especially focus on the reaction velocity and on the average time at which the system achieves its inert state. By means of both analytical and numerical methods, we are also able to highlight the role of topology in the temporal evolution of the system

Fractal geometry of Ising magnetic patterns: signatures of criticality and diffusive dynamics

We investigate the geometric properties displayed by the magnetic patterns developing on a two-dimensional Ising system, when a diffusive thermal dynamics is adopted. Such a dynamics is generated by a random walker which diffuses throughout the sites of the lattice, updating the relevant spins. Since the walker is biased towards borders between clusters, the border-sites are more likely to be updated with respect to a non-diffusive dynamics and therefore, we expect the spin configurations to be affected. In particular, by means of the box-counting technique, we measure the fractal dimension of magnetic patterns emerging on the lattice, as the temperature is varied. Interestingly, our results provide a geometric signature of the phase transition and they also highlight some non-trivial, quantitative differences between the behaviors pertaining to the diffusive and non-diffusive dynamics

Universal features of information spreading efficiency on dd-dimensional lattices

A model for information spreading in a population of $N$ mobile agents is extended to $d$-dimensional regular lattices. This model, already studied on two-dimensional lattices, also takes into account the degeneration of information as it passes from one agent to the other. Here, we find that the structure of the underlying lattice strongly affects the time $\tau$ at which the whole population has been reached by information. By comparing numerical simulations with mean-field calculations, we show that dimension $d=2$ is marginal for this problem and mean-field calculations become exact for $d > 2$. Nevertheless, the striking nonmonotonic behavior exhibited by the final degree of information with respect to $N$ and the lattice size $L$ appears to be geometry independent.Comment: 8 pages, 9 figure

Holistic Learning: Seeking a Purposeful Life By Engaging Science and the Humanities

Scientific research begins as a focused inquiry that leads to answers and then progresses to further questions. Engaging in scientific research that is not cookbook science, or completely planned, provides an opportunity to explore a critical way of thinking and develops an acceptance of uncertainty and an appreciation for the mystery of life. Science has the potential to influence a person to be critical of their sources and have the confidence to challenge everything they learn, for the purpose of being an interactive learner, and bring their sources into conversation with one another. From discussion based science curriculum, students come to consider information learned as knowledge contributing to an always developing whole picture rather than a concrete final answer. Through a discussion engaging scientific experience and exposure to the humanities dialogue, the author witnessed the interconnectedness of the two different ways of thinking and considered the contribution of both essential in discerning purpose. Blending science and the humanities core curriculum enhances holistic learning because it encourages the metaphysical and material dialogue in seeking a purposeful life