In-situ growth of superconducting MgB2 thin films by molecular beam epitaxy

The in-situ growth of superconducting MgB2 thin films was examined from various perspectives. The paper discusses (1) growth temperature, (2) the effect of excess Mg, (3) the effect of residual gas during growth, (4) the effect of in-situ annealing, (5) thickness dependence and (6) the effect of substrates. Our results provide a guide to the preparation of high-quality superconducting MgB2 films for potential electronics applications.Comment: 26 pages, 16 figures, submitted to J. Appl. Phy

Detection of topological phase transitions through entropy measurements: the case of germanene

We propose a characterization tool for studies of the band structure of new materials promising for the observation of topological phase transitions. We show that a specific resonant feature in the entropy per electron dependence on the chemical potential may be considered as a fingerprint of the transition between topological and trivial insulator phases. The entropy per electron in a honeycomb two-dimensional crystal of germanene subjected to the external electric field is obtained from the first principle calculation of the density of electronic states and the Maxwell relation. We demonstrate that, in agreement to the recent prediction of the analytical model, strong spikes in the entropy per particle dependence on the chemical potential appear at low temperatures. They are observed at the values of the applied bias both below and above the critical value that corresponds to the transition between the topological insulator and trivial insulator phases, while the giant resonant feature in the vicinity of zero chemical potential is strongly suppressed at the topological transition point, in the low temperature limit. In a wide energy range, the van Hove singularities in the electronic density of states manifest themselves as zeros in the entropy per particle dependence on the chemical potential.Comment: 8 pages, 5 figures; final version published in PR

In-situ Magnesium Diboride Superconducting Thin Films grown by Pulsed Laser Deposition

Superconducting thin films of MgB2 were deposited by Pulsed Laser Deposition on magnesium oxide and sapphire substrates. Samples grown at 450C in an argon buffer pressure of about 10-2 mbar by using a magnesium enriched target resulted to be superconducting with a transition temperature of about 25 K. Film deposited from a MgB2 sintered pellet target in ultra high vacuum conditions showed poor metallic or weak semiconducting behavior and they became superconducting only after an ex-situ annealing in Mg vapor atmosphere. Up to now, no difference in the superconducting properties of the films obtained by these two procedures has been evidenced.Comment: 10 pages, 4 figure

High quality MgB2 thin films in-situ grown by dc magnetron sputtering

Thin films of the recently discovered magnesium diboride (MgB2) intermetalic superconducting compound have been grown using a magnetron sputtering deposition technique followed by in-situ annealing at 830 C. High quality films were obtained on both sapphire and MgO substrates. The best films showed maximum Tc = 35 K (onset), a transition width of 0.5 K, a residual resistivity ratio up to 1.6, a low temperature critical current density Jc > 1 MA/cm2 and anisotropic critical field with gamma = 2.5 close to the values obtained for single crystals. The preparation technique can be easily scaled to produce large area in-situ films.Comment: 7 pages, 4 figure

Percurso artístico coletivo: o caso da revista Guapa

During covid-19 pandemic outbreak (2020-2022), people from different regions of Brazil gathered remotely to dialogue, reflect, and produce artist books for the online course “Dobras de si” (“folds of oneself”). Its first version was so stimulating that, even after the end of the classes, some of the students decided to continue together to research and produce artist books. Thus, Guapa group was born and, to maintain the vivacity of the creative production, the group founded an artist magazine with semesterly periodicity. In a friendly feast of ideas, the artists offer a productive collective dialogue that enhances individual perceptions. For the development of this article, we accessed documents about the manufacturing processes of the magazine pages that the artists share with the group. Therefore, we corroborate Salles’ assertion about the importance of contemplating the process as part of the work of art history, even if it is transitory. We also consulted other authors who develop studies about creative processes, such as Ostrower. The final discussion is about the result of a collective path that preserves individual artistic production, although above all the creative process carried out together and the dialogue taken over the group become fundamental for the realization of the magazine.Durante o período pandêmico decorrente da Covid-19 (2020-2022), pessoas de distintos lugares do Brasil puderam se reunir de maneira remota para dialogar, refletir e produzir livros de artista no curso on-line “Dobras de si”. A primeira versão do curso foi tão estimulante que, após o término das aulas, alguns participantes resolveram continuar juntos para pesquisar e produzir livros de artista. Surgia o grupo Guapa, que, para manter a vivacidade da produção criativa, criou uma revista de artista com periodicidade semestral. Em um amistoso banquete de ideias, os artistas oferecem um diálogo produtivo e coletivo que potencializa as percepções individuais. Para o desenvolvimento deste artigo, acessaram-se os documentos de registro do processo criativo para a manufatura das páginas da revista que os artistas compartilham entre o grupo, com o propósito de agregar informação à metodologia de produção. Assim, corrobora-se o pensamento de Salles de que o processo faz parte da história da obra e, por isso, deve ser contemplado, mesmo que seja transitório. Textos de outros estudiosos do processo criativo, como Ostrower, também foram consultados. A discussão final é sobre o resultado de um percurso coletivo que preserva a produção artística individual, mas, sobretudo, o aproveitamento de um processo criativo realizado em conjunto e em diálogo, elementos fundamentais para a concretização da revista.

Work function, deformation potential, and collapse of Landau levels in strained graphene and silicene

We perform a systematic {\it ab initio} study of the work function and its uniform strain dependence for graphene and silicene for both tensile and compressive strains. The Poisson ratios associated with armchair and zigzag strains are also computed. Based on these results, we obtain the deformation potential, crucial for straintronics, as a function of the applied strain. Further, we propose a particular experimental setup with a special strain configuration that generates only the electric field, while the pseudomagnetic field is absent. Then, applying a real magnetic field, one should be able to realize experimentally the spectacular phenomenon of the collapse of Landau levels in graphene or related two-dimensional materials.Comment: 9 pages, 7 figures; final version published in PR

2N+4-rule and an atlas of bulk optical resonances of zigzag graphene nanoribbons

Development of on-chip integrated carbon-based optoelectronic nanocircuits requires fast and non-invasive structural characterization of their building blocks. Recent advances in synthesis of single wall carbon nanotubes and graphene nanoribbons allow for their use as atomically precise building blocks. However, while cataloged experimental data are available for the structural characterization of carbon nanotubes, such an atlas is absent for graphene nanoribbons. Here we theoretically investigate the optical absorption resonances of armchair carbon nanotubes and zigzag graphene nanoribbons continuously spanning the tube (ribbon) transverse sizes from 0.5(0.4) nm to 8.1(12.8) nm. We show that the linear mapping is guaranteed between the tube and ribbon bulk resonance when the number of atoms in the tube unit cell is 2 N+ 4 , where N is the number of atoms in the ribbon unit cell. Thus, an atlas of carbon nanotubes optical transitions can be mapped to an atlas of zigzag graphene nanoribbons