120 research outputs found

    Variational neural network ansatz for steady states in open quantum systems

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    We present a general variational approach to determine the steady state of open quantum lattice systems via a neural network approach. The steady-state density matrix of the lattice system is constructed via a purified neural network ansatz in an extended Hilbert space with ancillary degrees of freedom. The variational minimization of cost functions associated to the master equation can be performed using a Markov chain Monte Carlo sampling. As a first application and proof-of-principle, we apply the method to the dissipative quantum transverse Ising model.Comment: 6 pages, 4 figures, 54 references, 5 pages of Supplemental Information

    Características estruturais e ultraestruturais da espermatogênese do cascudo-cinza Liposarcus anisitsi (Holmberg, 1893) (Teleostei, Siluriformes)

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    Liposarcus anisitsi √© um tele√≥steo de √°gua doce da ordem Siluriforme, com fertiliza√ß√£o externa, possui um processo de espematog√™nese c√≠stica com estrutura espermatogonial lobular irrestrita dos test√≠culos. Nesta esp√©cie, a espermatog√īnia prim√°ria intrac√≠stica ou isolada √© considerada como a "stem cell" da linhagem espermatogen√©tica. Ent√£o, como consequ√™ncia do processo da espermatog√™nese c√≠stica de Liposarcus anisitsi todas as tr√™s c√©lulas germinativas s√£o observadas no interior de cistos espermatogen√©ticos espec√≠ficos, desde as espermatog√īnias prim√°rias e secund√°rias at√© os espermat√≥citos e esperm√°tides jovens e maduras. A espermatog√īnia possui forma esf√©rica, com n√ļcleo central e grande quantidade de mitoc√īndrias redondas e ret√≠culo endoplasm√°tico, principalmente do tipo rugoso, em seu citoplasma. Os espermat√≥citos prim√°rios se inter-relacionam atrav√©s de pontes citoplasm√°ticas e s√£o claramente identificados pela presen√ßa de complexos sinapton√™micos no interior de seus n√ļcleos. Esperm√°tides jovens no in√≠cio da fase maturacional, tanto quanto as esperm√°tides maduras tendem a exibir n√ļcleos esf√©ricos, um modelo vari√°vel de progressiva condensa√ß√£o cromat√≠nica durante o est√°gio final da espermiog√™nese. Nestes est√°gios finais, as esperm√°tides maduras mostra uma fossa nuclear formada para a fixa√ß√£o do flagelo. O espermatoz√≥ide de Liposarcus anisitsi possui cabe√ßa redonda, n√ļcleo redondo, cromatina densa, aus√™ncia de acrossoma, curta pe√ßa intermedi√°ria e longa cauda (pe√ßas principal e terminal do flagelo).Liposarcus anisitsi is a freshwater teleost from Siluriform order with external fertilization that shows cystic spermatogenesis process with spermatogonial unrestricted lobular structure of the testes. In this species, isolated or intracystic primary spermatogonia were noted as the ¬®stem cells¬® of the spermatogenetic lineage. Thus, as consequence of the cystic spermatogenesis process of Liposarcus anisitsi all the three germinative cells are observed inside specific spermatogenetic cysts, since the primary and secondary spermatogonias until the spermatocytes and early and late spermatids. Spermatogonia show spherical shape with central nuclei and great amount of round mitochondria and ER profiles mainly of rough type in their cytoplasm. Primary spermatocytes have inter-relations through cytoplasmic bridges and are clearly identified by presence of synaptonemic complexes inside the nuclei. Early spermatids beginning maturational phases, as well as the late spermatids, tend to exhibit spherical nuclei, variable chromatin pattern with progressive condensation during the end steps of the spermiogenesis. In this end steps, the late spermatids show the nuclear fossa formed to the fixation of the flagella. The spermatozoon of Liposarcus anisitsi shows round head, round nuclei, very dense chromatin, lack of acrosoma, short middle piece and long tail (principal and end pieces of the flagellum)

    Reestablishment of Protium cordatum (Burseraceae) based on integrative taxonomy

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    Species delimitation remains a challenge worldwide, but especially in biodiversity hotspots such as the Amazon. Here, we use an integrative taxonomic approach that combines data from morphology, phylogenomics, and leaf spectroscopy to clarify the species limits within the Protium heptaphyllum species complex, which includes subsp. cordatum, subsp. heptaphyllum, and subsp. ulei. Molecular phylogeny indicates that populations of subsp. cordatum do not belong to the P. heptaphyllum clade, while morphology and near-infrared spectroscopy data provide additional support for the recognition of a separate taxon. Protium cordatum (Burseraceae) is reinstated at species rank and described in detail. As circumscribed here, P. cordatum is endemic to white-sand savannas located in the Faro and Tucuruí Districts, Pará State, Brazil, whereas P. heptaphyllum is a dominant and widespread plant lineage found in Amazonia, the Cerrado, and the Brazilian Atlantic Forest. We present an identification key to P. cordatum and closely related lineages and a detailed taxonomic description of P. cordatum, including habitat and distribution, a list and images of diagnostic features. This study demonstrates the importance of using multiple tools to characterize and distinguish plant species in highly diverse tropical regions. © 2019 International Association for Plant Taxonomy

    Species tree phylogeny and biogeography of the Neotropical genus Pradosia (Sapotaceae, Chrysophylloideae)

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    Recent phylogenetic studies in Sapotaceae have demonstrated that many genera need to be redefined to better correspond to natural groups. The Neotropical genus Pradosia is believed to be monophyletic and includes 26 recognized species. Here we reconstruct the generic phylogeny by a species-tree approach using *BEAST, 21 recognized species (36 accessions), sequence data from three nuclear markers (ITS, ETS, and RPB2), a relaxed lognormal clock model, and a fossil calibration. We explore the evolution of five selected morphological characters, reconstruct the evolution of habitat (white-sand vs. clayish soils) preference, as well as space and time by using a recently developed continuous diffusion model in biogeography. We find Pradosia to be monophyletic in its current circumscription and to have originated in the Amazon basin at ~47.5Ma. Selected morphological characters are useful to readily distinguish three clades. Preferences to white-sand and/or clay are somewhat important for the majority of species, but speciation has not been powered by habitat shifts. Pradosia brevipes is a relative young species (~1.3Ma) that has evolved a unique geoxylic life strategy within Pradosia and is restricted to savannahs. Molecular dating and phylogenetic pattern indicate that Pradosia reached the Brazilian Atlantic coast at least three times: at 34.4Ma (P. longipedicellata), at 11.7Ma (P. kuhlmannii), and at 3.9Ma (weakly supported node within the red-flowered clade). © 2015 Elsevier Inc
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