Percolative conductivity in alkaline earth silicate melts and glasses

Ion conducting $(CaO)_x(SiO_2)_{1-x}$ glasses and melts show a threshold behaviour in dc conductivity near $x=x_t=0.50$, with conductivities increasing linearly at $x>x_t$. We show that the behaviour can be traced to a rigid ($x0.50$) elastic phase transition near $x=x_t$. In the floppy phase, conductivity enhancement is traced to increased mobility or diffusion of $Ca^{2+}$ carriers as the modified network elastically softens.Comment: 15 pages, 5 figures. Europhysics Letters (2003), in pres

Correlation between floppy to rigid transitions and non-Arrhenius conductivity in glasses

Non-Arrhenius behaviour and fast increase of the ionic conductivity is observed for a number of potassium silicate glasses $(1-x)SiO_2-xK_2O$ with potassium oxide concentration larger than a certain value $x=x_c=0.14$. Recovering of Arrhenius behaviour is provided by the annealing that enhances densification. Conductivity furthermore obeys a percolation law with the same critical concentration $x_c$. These various results are the manifestation of the floppy or rigid nature of the network and can be analyzed with constraint theory. They underscore the key role played by network rigidity for the understanding of conduction and saturation effects in glassy electrolytes.Comment: 4 pages, 4 EPS figure

The dynamics of income inequality in Africa: An empirical investigation on the role of macroeconomic and institutional forces

Reducing income inequality is a crucial goal of sustainable development as income inequality often viewed as harmful to economic growth. The main aim of this paper was to empirically assess the macroeconomic and institutional drivers of income inequality in Africa. We use a Kuznets curve framework, which emphasises the role of income per capita in explaining the time path of inequality. In contrast to much of the literature, we explicitly examine the possibility of the existence of multiple income steady states. Using the concept of clubs of convergence, we show that per capita income is divergent and identify four steady states to which groups of economies converge (i.e., high-income to low-income economies). Using panel data models and a data set encompassing 52 African countries spanning the years 1980–2017, we show that once these multiple steady states are accounted for, the Kuznets curve relationship becomes unstable. Our findings suggest that inequality may be increasing in high-income countries in Africa, while decreasing in low-income or the least developed economies. In addition, the role of macroeconomic and institutional factors in explaining income inequality is limited and differ across convergence clubs. Evidence suggests the importance of fiscal, employment and monetary policies and the rule of law to tackle inequality in high-income economies, while they have no statistically significant role in low-income economies’ income inequality

Delocalization of edge states in topological phases

The presence of a topologically non-trivial discrete invariants implies the existence of gapless modes in finite samples, but it does not necessarily imply their localization. The disappearance of the indirect energy gap in the bulk generically leads to the absence of localized edge states. We illustrate this behavior in two fundamental lattice models on the single-particle level. By tuning a hopping parameter the indirect gap is closed while maintaining the topological properties. The inverse participation ratio is used to measure the degree of localization.Comment: 5 pages including 6 figures and 4 pages supplement including 8 figur