VO2_2 as a natural optical metamaterial

VO$_2$ is a unique phase change material with strongly anisotropic electronic properties. Recently, samples have been prepared that present a co-existence of phases and thus form metal-insulator junctions of the same chemical compound. Using first principles calculations, the optical properties of metallic and semiconducting VO$_2$ are here discussed to design self-contained natural optical metamaterials, avoiding coupling with other dielectric media. The analysis of the optical properties complements the experiments in the description of the vast change in reflectance and metallicity for both disordered and planar compounds. The present results also predict the possibility to realize ordered VO$_2$ junctions operating as efficient hyperbolic metamaterials in the THz-visible range, by simply adjusting the ratio between metallic and insulating VO$_2$ content. The possibility to excite propagating {\em volume plasmom polariton} across the metamaterial is finally discussed.Comment: 10 pages, 8 figure

Theoretical investigations of a highly mismatched interface: the case of SiC/Si(001)

Using first principles, classical potentials, and elasticity theory, we investigated the structure of a semiconductor/semiconductor interface with a high lattice mismatch, SiC/Si(001). Among several tested possible configurations, a heterostructure with (i) a misfit dislocation network pinned at the interface and (ii) reconstructed dislocation cores with a carbon substoichiometry is found to be the most stable one. The importance of the slab approximation in first-principles calculations is discussed and estimated by combining classical potential techniques and elasticity theory. For the most stable configuration, an estimate of the interface energy is given. Finally, the electronic structure is investigated and discussed in relation with the dislocation array structure. Interface states, localized in the heterostructure gap and located on dislocation cores, are identified

Hydrogen induced surface metallization of β\beta-SiC(100)-(3×23\times 2) revisited by DFT calculations

Recent experiments on the silicon terminated $3\times 2$ SiC(100) surface indicated an unexpected metallic character upon hydrogen adsorption. This effect was attributed to the bonding of hydrogen to a row of Si atoms and to the stabilization of a neighboring dangling bond row. Here, on the basis of Density-Functional calculations, we show that multiple-layer adsorption of H at the reconstructed surface is compatible with a different geometry: besides saturating the topmost Si dangling bonds, H atoms are adsorbed at rather unusual sites, \textit{i.e.} stable bridge positions above third-layer Si dimers. The results thus suggest an alternative interpretation for the electronic structure of the metallic surfaceComment: 10 pages, 3 figures. Higher resolution figures may be obtained from the authors ([email protected]). Submitted to Physical Review Letters in September 2004, resubmitted after revision in February 200

Ab initio Study of Misfit Dislocations at the SiC/Si(001) Interface

The high lattice mismatched SiC/Si(001) interface was investigated by means of combined classical and ab initio molecular dynamics. Among the several configurations analyzed, a dislocation network pinned at the interface was found to be the most efficient mechanism for strain relief. A detailed description of the dislocation core is given, and the related electronic properties are discussed for the most stable geometry: we found interface states localized in the gap that may be a source of failure of electronic devices

Theoretical study of the (3x2) reconstruction of beta-SiC(001)

By means of ab initio molecular dynamics and band structure calculations, as well as using calculated STM images, we have singled out one structural model for the (3x2) reconstruction of the Si-terminated (001) surface of cubic SiC, amongst several proposed in the literature. This is an alternate dimer-row model, with an excess Si coverage of 1/3, yielding STM images in good accord with recent measurements [F.Semond et al. Phys. Rev. Lett. 77, 2013 (1996)].Comment: To be published in PRB Rapid. Com

Development of SSR markers and genetic diversity analysis in enset (Ensete ventricosum (Welw.) Cheesman), an orphan food security crop from Southern Ethiopia

Enset (Ensete ventricosum (Welw.) Cheesman; Musaceae) is a multipurpose drought-tolerant food security crop with high conservation and improvement concern in Ethiopia, where it supplements the human calorie requirements of around 20 million people. The crop also has an enormous potential in other regions of Sub-Saharan Africa, where it is known only as a wild plant. Despite its potential, genetic and genomic studies supporting breeding programs and conservation efforts are very limited. Molecular methods would substantially improve current conventional approaches. Here we report the development of the first set of SSR markers from enset, their cross-transferability to Musa spp., and their application in genetic diversity, relationship and structure assessments in wild and cultivated enset germplasm

Laparoscopic Treatment of Gastric Duplication in a Child

Introduction: Gastric duplication is a rare congenital anomaly with an incidence of 4-8% of all gastrointestinal duplications; enteric duplications are ectopic cystic or tubular structures with a mucous and muscular tunics and they can be in contiguity/continuity with the intestinal lumen.1 Gastric duplication is often an occasional finding, associated to aspecific sintomatology for which pre-operative diagnosis is not univoque; differential diagnosis with other retroperitoneal diseases or mesenteric cysts can be difficult.1 (Run time 8min). Material and Methods: We present a case of a one-year-old child with a pre-natal ultrasound (US) finding of endo-abdominal cystic lesion. After birth, US scans showed an anechogenic-cyst of 33x28mm in the left upper quadrant, between stomach, spleen and kidney. The magnetic resonance confirmed the presence of the lesion (40x34mm), imprinting the posterior gastric wall, the spleen and the anterior side of left kidney. An esophagous-stomach-duodenum contrast study was also performed, showing the imprinting cyst on the great curvature close to the gastric fundus without communication with gastric lumen. On follow-up, the child underwent to periodic US scans and no growth or ultrasonographic changes were described. At 13-months, the diagnosis was still unclear and the patient underwent explorative laparoscopy with esophagous-gastric-duodenoscopy (EGDS). The preliminary EGDS showed a 35mm convexity on the posterior wall of gastric fundus with no evidence of orifice. A 5mm trans-umbilical trocar was placed and 5mm trocar in the epigastric region and 10-12mm trocar in the left side were positioned. At the abdominal exploration the cyst resulted to be in continuity with the posterior gastric wall on the superior third of the great curvature. The lesion was isolated from other tissues, but the postero-medial wall of neoformation appeared to be not dissociable and in continuity with the stomach; a complete resection of the cyst, using 45mm linear stapler, was performed including a small portion of the great gastric curvature. At the following intra-operative endoscopic control no more evidence of irregularity of the gastric wall was seen and the suture was assured. Results: The operative time was 140-minutes. No complications occurred and the blood loss was minimal. The patient started oral intake on 5’ post-operative-day and was discharged on 6’ post-operative-day. The histological examination confirmed the gastric nature of cyst. At 1-year of follow-up no recurrences were diagnosed and the child presents in good health with a regular growth. Conclusion: We can assume that laparoscopic surgery is the correct procedure for gastric duplication cysts, to get both definitive diagnosis and treatment, and the radical surgical excision represents the treatment of choice in order to avoid neoplastic degeneration of internal lining mucosa.2 Furthermore surgical laparoscopy appears to be a feasible and safe technique

Duality in Supergravity Theories

We present a unified treatment in superspace of the two dual formulations of $D=10$, $N=1$ {\it pure} supergravity based on a strictly super-geometrical framework: the only fundamental objects are the super Riemann curvature and torsion, and the related Bianchi identities are sufficient to set the theory on shell; there is no need to introduce, from the beginning, closed three- or seven-superforms. This formulation extends also to {\it non minimal} models. Moreover, in this framework the algebraic analogy between pure super Yang--Mills theories and pure supergravity in $D=10$ is manifest. As an additional outcome in the present formulation the supersymmetric partner of the ABBJ-Lorentz anomaly in pure $D=10$ supergravity can be computed in complete analogy to the ABBJ-gauge anomaly in supersymmetric Yang--Mills theories in ten dimensions. In the same framework we attack the issue of duality in $N=1$, $D=11$ supergravity showing in detail that duality holds at the kinematical level in superspace while it is broken by the dynamics. We discuss also possible extensions of this theory which could be related to quantum corrections of the eleven dimensional membrane.Comment: 30 pages of tex, DFPD/93/TH/51 - (to appear in Nucl. Phys. B

Relaxation processes in thiophene-based random copolymers

The relaxation dynamics of soluble polyalkylthiophenes obtained by the random copolymerisation of 3,4-dibutylthiophene and 3-butylthiophene monomers is investigated. In these systems, the effective conjugation length, the optical gap and the non-radiative decay rate are controlled by varying the content of disubstituted monomers, the steric hindrance of which induces a twisting angle between thiophene rings. Several indications are reported in favour of spectral diffusion of the photoexcitations. Migration processes mainly occur within a few tens of picoseconds

Chiral Polyalkylthiophenes for Organic Light Emitting Diodes

Chiral polyalkylthiophenes are noncentrosymmetric organic materials which can be used both in second harmonic-generation devices and in polarized light emitting diodes. In this work we present the synthesis and the characterization of a polyalkylthiophene with a chiral center very close to the conjugated backbone: poly(3-[(S)-2-methylbutyl]thiophene) (PMBT). Circular dichroism (CD) measurements have been carried out to ascertain the chirality of these materials. The CD spectra show intense signals both in mixed solvents and in the solid state. The strong Cotton effect can be associated to a highly ordered aggregated phase whose nature is still under investigation. We also present the photo and electroluminescence characterization of single layer light emitting diode (LED) with the following configuration: ITO (Tin Indium Oxide)/PMBT/Al