Size and shape of Mott regions for fermionic atoms in a two-dimensional optical lattice

We investigate the harmonic-trap control of size and shape of Mott regions in the Fermi Hubbard model on a square optical lattice. The use of Lanczos diagonalization on clusters with twisted boundary conditions, followed by an average over 50-80 samples, drastically reduce finite-size effects in some ground state properties; calculations in the grand canonical ensemble together with a local-density approximation (LDA) allow us to simulate the radial density distribution. We have found that as the trap closes, the atomic cloud goes from a metallic state, to a Mott core, and to a Mott ring; the coverage of Mott atoms reaches a maximum at the core-ring transition. A `phase diagram' in terms of an effective density and the on-site repulsion is proposed, as a guide to maximize the Mott coverage. We also predict that the usual experimentally accessible quantities, the global compressibility and the average double occupancy (rather, its density derivative) display detectable signatures of the core-ring transition. Some spin correlation functions are also calculated, and predict the existence N\'eel ordering within Mott cores and rings.Comment: 5 pages, 6 figure

Multiperiodic magnetic structures in Hubbard superlattices

We consider fermions in one-dimensional superlattices (SL's), modeled by site-dependent Hubbard-U couplings arranged in a repeated pattern of repulsive (i.e., U>0) and free (U=0) sites. Density Matrix Renormalization Group (DMRG) diagonalization of finite systems is used to calculate the local moment and the magnetic structure factor in the ground state. We have found four regimes for magnetic behavior: uniform local moments forming a spin-density wave (SDW), `floppy' local moments with short-ranged correlations, local moments on repulsive sites forming long-period SDW's superimposed with short-ranged correlations, and local moments on repulsive sites solely with long-period SDW's; the boundaries between these regimes depend on the range of electronic densities, rho, and on the SL aspect ratio. Above a critical electronic density, rho_{uparrow downarrow}, the SDW period oscillates both with rho and with the spacer thickness. The former oscillation allows one to reproduce all SDW wave vectors within a small range of electronic densities, unlike the homogeneous system. The latter oscillation is related to the exchange oscillation observed in magnetic multilayers. A crossover between regimes of `thin' to `thick' layers has also been observed.Comment: 9 two-column pages, 10 figure

Indução e análise bioquimica de calos obtidos de segmentos foliares de Coffea arabica L., cultivar Rubi.

Conduziu-se este trabalho com o objetivo de estudar o efeito de 2,4-D e BAP na indução da formação de calos originados de segmentos foliares de plântulas de Coffea arabica L., cv Rubi, obtidas in vitro, a partir da cultura de embriões, determinar sua curva de crescimento e analisar bioquimicamente, mediante determinação dos teores de proteínas totais, açúcares redutores e padrão proteíco. Segmentos foliares inoculados em meio desprovido de reguladores ou na presença de BAP em condições isoladas não apresentaram formação de calos. A produção máxima de calos é obtida utilizando-se 1,0 mg L-1 de 2,4-D. A curva de crescimento de calos apresenta crescimento tipo sigmóide, com três fases distintas: lag (entre 0 - 42 dias de inoculação), exponencial (42 ? 77 dias) e linear (77 ? 84 dias). Não foi observado acúmulo de proteínas em nenhuma fase de crescimento dos calos, sendo o teor máximo de açúcar redutor observado no dia de inoculação do explante. O padrão proteíco observado no período de desenvolvimento de calos apresenta diferenças quantitativas (intensidade das bandas) e qualitativas (presença ou ausência de bandas)