Photonic crystals are some of the more spectacular realizations that periodic arrays can change the behavior of electromagnetic waves. In nature, so-called structural colors appear in insects and even plants. Some species create beautiful color patterns as part of biological behavior such as reproduction or defense mechanisms as a form of biomimetics. The interaction between light and matter occurs at the surface, producing diffraction, interference and reflectance, and light transmission is possible under suitable conditions. In particular, there are two Colombian butterflies, Morpho cypris and Greta oto, that exhibit iridescence phenomena on their wings, and in this work, we relate these phenomena to the photonic effect. The experimental and theoretical approaches of the optical response visible region were studied to understand the underlying mechanism behind the light-matter interaction on the wings of these Colombian butterflies. Our results can guide the design of novel devices that use iridescence as angular filters or even for cosmetic purposes. Nature is colored and attracts our attention due to its beauty and complex colors. Colors can be produced by pigmentation or by the arrangement of nanostructures. Recent studies showed that some species use this latter option to create beautiful color patterns as part of biological behavior such as reproduction or defense mechanisms as a form of mimicry 1-8 , and these tools have been perfected over millions of years in nature. In all cases, the interaction between light and matter occurs at the surface, producing diffraction, interference and reflectance phenomena. Additionally, light transmission is possible under suitable conditions. In the last decade, another optical phenomenon that has generated great interest among researchers is iridescence, which is related to the angular dependence of the observed color; there are many biological species and minerals that have this feature 9-11. In particular, there are two Colombian butterflies, Morpho cypris and Greta oto, that exhibit an iridescent effect on their wings. Iridescence has many feasible applications 12 , and a key advantage of structural colors over pigmented colors is the more intense coloration, especially under high light conditions. However, the physical interpretation of this phenomenon remains unclear and deserves more study. Iridescence is an effect that takes place in the visible spectrum 13,14 ; thus, the interaction of light with the surfaces of these systems implies the existence of nanostructures with sizes comparable to the wavelength in the visible region. On the other hand, there are man-made structures that exhibit iridescent effects, such as TiO 2 traces 15-17 , but total control over this property in man-made systems is not yet available. In this way, comprehension of the proper working conditions when light interacts with a butterfly's wings is a very important step toward man-made systems that will use this property. The Morpho cypris and Greta oto butterfly samples were provided, wit...

Quantum transport through single and multilayer icosahedral fullerenesElectron transport through single conjugated organic molecules: Basis set effects in ab initio calculationsIn this paper, we study the electronic transport properties through aromatic molecules connected to two semi-infinite leads. The molecules are in different geometrical configurations including arrays. Using a nearest neighbor tight-binding approach, the transport properties are analyzed into a Green's function technique within a real-space renormalization scheme. We calculate the transmission probability and the Current-Voltage characteristics as a function of a molecule-leads coupling parameter. Our results show different transport regimes for these systems, exhibiting metal-semiconductor-insulator transitions and the possibility to employ them in molecular devices. V C 2013 AIP Publishing LLC. [http://dx.FIG. 4. Normalized conductance G=G 0 for the aromatic biphenyl molecule (model 2), for different values intrasite coupling potential w ¼ V c cos 2 a. (Continuous black line) a ¼ 0, (dashed green line) a ¼ p=4, (dashed red line) a ¼ p=3, (dashed blue line) a ¼ p=2, and C ¼ V c . FIG. 5. (Contour plot) I-V characteristic I=I 0 as a function of bias voltage and aromatic molecule-leads coupling C (model 1).

In the last few years, the fascinating properties of graphene have been thoroughly investigated. The existence of Dirac cones is the most important characteristic of the electronic band-structure of graphene. In this theoretical paper, hexagonal monolayers of silicon (h-Si) and germanium (h-Ge) are examined using density functional theory, within the generalized gradient approximation. Our numerical results indicate that both h-Si and h-Ge are chemically stable. The lattice parameters, electronic dispersion relations and densities of states for these systems are reported. The electronic dispersion relations display Dirac cones with the symmetry of an equilateral triangle (the group D3) in the vicinity of the K points. Hence, the Fermi velocity depends on the wave vector direction around K points. Fermi velocities for holes and electrons are significantly different. The maximum and minimum Fermi velocities are also reported.

We discuss the electronic properties of a one-dimensional tight-binding model for binary alloys with correlated disorder. The correlation inhibits the bonds between atoms of one of the atomic species, leading to what is called repulsive binary alloys. As it is well known, the transmission probability of repulsive binary alloys shows resonances due to the delocalization of states in this peculiar disordered one-dimensional system. We show that this delocalization can be related to the memory of the band structures of different ordered chains, since short segments of these ordered chains constitute the repulsive binary alloy. This memory effect is revealed in the localization length as a function of energy. The same argument is used to show that competing correlations in the same linear chain destroy delocalization, as can be inferred from the properties of binary alloys with no correlations at all. We also show that, complementary to delocalized bands, smooth transmission gaps should also exist at specific energy ranges only for correlated disorder cases. ͓S0163-1829͑96͒10133-8͔

In this work we study the localization of vibrational modes in heuristic models for disordered DNA-like molecules. Within such approach, atomic groups are replaced by renormalized sites connected by effective springs. The oscillation amplitudes at each site are considered and the localization degree of the normal modes is analyzed by means of the participation ratio, as well as the relative fluctuation of an ensemble of disorder realizations for normal modes in different frequency ranges. The present results suggest that the dynamical properties at low frequencies are completely different for double-strand structures compared to single-strand ones. Irrespective to disorder, double-strand molecules show normal modes with macroscopic localization lengths at low frequencies, for a wide range of spring constants considered in the literature, in contrast to the strong localization in single strands.

Phonon spectra of CdTe and HgTe surfaces and interfaces are calculated within a Green function formalism. The dependence of phonon density of states spectn on the surface orientation is studied. Of the two compounds considered, HgTe shows the most drastic modification of its phonon stmchre as a surface is mated. ( 001) and ( 111) polar interfaces show very different vibrarional properties: while the former exhibits a two-mode or persistent behaviour with a vanishing phonon gap for any polariaion. the latter exhibits a one mode behaviour depending on the vibration polarization. For the (011) interface a two-mode behaviour is obtained. where bulk optical modes of one compound penetrate some distance into the other compound. The obtained phonon spectra show interface induced effects in a frequency range where Raman scattering data in CdTeMgTe superlattices were previously repoiied but not explained.

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Quantum confinement in one-dimensional disordered quantum wells is verified by means of resonant tunneling signatures in the transmission probability of an electron through barrier-well-barrier structures. The wells are simulated by short repulsive binary-alloy chains. These chains are interesting disordered systems, showing both localized and delocalized states. Bona fide confined states in the energy range where the bulk chain presents states with a localization length that exceeds at least three times the quantum-well width are found. These bona fide quantum-well states show clear symmetry properties for the envelope probability density in spite of disorder. In addition, the inadequacy of mean-field approximations to understand the quantization rules of disordered heterostructures is discussed. ͓S0163-1829͑98͒01123-0͔