Braids on surfaces and finite type invariants

We prove that there is no functorial universal finite type invariant for braids in Σ×I if the genus of Σ is positive. To cite this article: P. Bellingeri, L. Funar, C. R. Acad. Sci. Paris, Ser. I 338 (2004).     

Synthesis and properties of Hg-based high-T c superconductors

Il Nuovo Cimento D - The formation of Hg-based HTSC materials was investigated through the development of non-conventional techniques. The former was based on a singlestep synthesis at temperatures...     

Preparation of oriented thick HTSC films by cryoelectrophoretic technique

Il Nuovo Cimento D - A new technique, specifically designed for the preparation of preferentially oriented HTSC (YBCO and BSCCO) films on metallic substrates has been developed. The method, named...     

Investigation of FIB irradiation damage in La0.7Sr0.3MnO3 thin films

In this work we analyse systematically how morphological and magnetotransport properties of manganite thin films are affected by the damage induced by focused ion beam (FIB) irradiation. We irradiate different areas of the same sample with doses ranging from 5×10¹² to 3×10¹⁷ ions/cm² and we find that the film becomes swollen for doses up to 10¹⁶ ions/cm² and is eventually eroded by ion milling for further irradiation. On the other hand, transport properties are much more sensitive to FIB irradiation: the metal–insulator transition temperature is found to decrease monotonically with increasing doses up to 1.8×10¹³ ions/cm². At doses higher than 5.6×10¹³ ions/cm² the metallic state is completely suppressed and likely, also ferromagnetism.     

Light transmission behaviour as a function of the homogeneity in one dimensional photonic crystals

The average light transmission of one-dimensional photonic media has been studied as a function of the medium homogeneity, quantified by the Shannon–Wiener index. We have found a decrease in the average light transmission by increasing the Shannon–Wiener index up to minimum (corresponding to H′ = 0.9375): from this point, the transmission increases following the Shannon–Wiener index. The behaviour has been confirmed for different pairs of materials forming the photonic structure. Nevertheless, we have observed that the trend slope is proportional to the refractive index ratio between the two materials (n_hi/n_low).     

Inhomogeneous two-dimensional photonic media: A statistical study

Photonic media, in which disorder is introduced, are interesting materials for light management. In this paper, we have performed a statistical study of the average light transmission, over the range of wavelengths 450–1400 nm, for two-dimensional photonic structures with different homogeneity (quantified by the Shannon index). The photonic structure is a square lattice of circular pillars and the homogeneity is varied by clustering pillars in the crystal unit cells. We have calculated the light transmission for 50 different crystal realizations (permutating cluster position in the crystal) for each Shannon index value. Such Monte Carlo Markov Chain method produced the “a posteriori” distribution of the light transmission. We have observed a linear trend of the average transmission as a function of the crystal homogeneity. Furthermore, we have found a linear dependence of the average light transmission on the mean distance between pillars in the photonic structures.     

Surface framed braids

In this paper we introduce the framed pure braid group on n strands of an oriented surface, a topological generalisation of the pure braid group P _n . We give different equivalent definitions for framed pure braid groups and we study exact sequences relating these groups with other generalisations of P _n , usually called surface pure braid groups. The notion of surface framed braid groups is also introduced.     

Lower central series of Artin–Tits and surface braid groups

We consider algebraic and topological generalisations of braid groups and pure braid groups, namely Artin–Tits groups (of spherical type) and surface (pure) braid groups, and we determine their lower central series and related residual properties.