Anisotropic linear and non-linear excitonic optical properties of buckled monolayer semiconductors

The optical properties of two-dimensional materials are exceptional in several respects. They are highly anisotropic and frequently dominated by excitonic effects. Dipole-allowed second order non-linear optical properties require broken inversion symmetry. Hence, several two-dimensional materials show strong in-plane (IP) non-linearity but negligible out-of-plane (OOP) response due to vertical symmetry. By considering buckled hexagonal monolayers, we analyze the critical role of broken vertical symmetry on their excitonic optical response. Both linear as well as second order shift current and second harmonic response are studied. We demonstrate that substantial OOP non-linear response can be obtained, in particular, through off-diagonal tensor elements coupling IP excitation to OOP response. Our findings are explained by excitonic selection rules for OOP response and the impact of dielectric screening on excitons is elucidated.Comment: 20 pages, 8 figure

Electrical transport properties of nanostructured ferromagnetic perovskite oxides La_0.67Ca_0.33MnO_3 and La_0.5Sr_0.5CoO_3 at low temperatures (5 K > T >0.3 K) and high magnetic field

We report a comprehensive study of the electrical and magneto-transport properties of nanocrystals of La_0.67Ca_0.33MnO_3 (LCMO) (with size down to 15 nm) and La_0.5Sr_0.5CoO_3 (LSCO) (with size down to 35 nm) in the temperature range 0.3 K to 5 K and magnetic fields upto 14 T. The transport, magnetotransport and non-linear conduction (I-V curves) were analysed using the concept of Spin Polarized Tunnelling in the presence of Coulomb blockade. The activation energy of transport, \Delta, was used to estimate the tunnelling distances and the inverse decay length of the tunnelling wave function (\chi) and the height of the tunnelling barrier (\Phi_B). The magnetotransport data were used to find out the magnetic field dependences of these tunnelling parameters. The data taken over a large magnetic field range allowed us to separate out the MR contributions at low temperatures arising from tunnelling into two distinct contributions. In LCMO, at low magnetic field, the transport and the MR are dominated by the spin polarization, while at higher magnetic field the MR arises from the lowering of the tunnel barrier by the magnetic field leading to an MR that does not saturate even at 14 T. In contrast, in LSCO, which does not have substantial spin polarization, the first contribution at low field is absent, while the second contribution related to the barrier height persists. The idea of inter-grain tunnelling has been validated by direct measurements of the non-linear I-V data in this temperature range and the I-V data was found to be strongly dependent on magnetic field. We made the important observation that a gap like feature (with magnitude ~ E_C, the Coulomb charging energy) shows up in the conductance g(V) at low bias for the systems with smallest nanocrystal size at lowest temperatures (T < 0.7 K). The gap closes as the magnetic field and the temperature are increased.Comment: 13 figure

Damage of Bemisia tabaci biotype B in transgenic common bean resistant to the Bean golden mosaic virus.

The whitefly (Bemisia tabaci biotype B) causes significant damage to common bean (Phaseolus vulgaris L.) mainly by virus transmission

Sustentabilidade na cafeicultura brasileira, uma análise em talhões de produção.

Informações de uma amostra de 90 propriedades, em regiões cafeeiras, permitiram inferir quanto a indicadores de eficiência, expressos em produtividades e em práticas de cultivo. Esses indicadores poderão ser utilizados como linha de base para análises longitudinais da produção integrada de café. Inferências quanto aos impactos de tecnologias de cultivo na propriedade como um todo foram também avaliadas em talhões de produção de café. Neste estudo, observou-se que existe considerável variabilidade entre talhões em relação a formas de condução da atividade cafeeira, idade e adensamento das plantas, tecnologias na implantação e cultivares. Conclui-se pela necessidade de não apenas analisar o conjunto das propriedades, mas as especificidades internas de cada talhão, permitindo comparações para melhor gestão das operações

Implementing IPM for bean golden mosaic virus in common bean in Brazil.

The GM crop with resistance to a disease will help to diversify the toolbox for Integrated Pest Management (IPM) in common bean. An integrated approach to pest management including this novel technology became a challenge and is considered essential to achieve both agricultural and environmental sustainability, in addition to contributing to food security and grower profitability.PS VI-

Astrocyte Ca2+-evoked ATP release regulates myelinated axon excitability and conduction speed

INTRODUCTION: Astrocytes support neuronal function throughout the central nervous system. In the gray matter, they regulate synapse number during development, remove synaptically released neurotransmitters to terminate their action and prevent excitotoxicity, control the extracellular potassium concentration to prevent hyperexcitability, regulate blood flow to ensure an adequate energy supply, provide lactate to neurons for energy, and respond to rises of intracellular calcium concentration ([Ca2+]i) by releasing adenosine triphosphate (ATP) and other gliotransmitters that act on neuronal receptors to modulate information processing. However, their role is unclear in the white matter, which transmits information rapidly between gray matter areas using axons wrapped with capacitance-reducing myelin (although they have been suggested to regulate myelination during development and during normal function). RATIONALE: Recently, it has been suggested that learning and memory may reflect not only changes in synaptic function in the gray matter, but also changes in white matter function. In particular, neural circuit function might be regulated by changes in the conduction speed of myelinated axons that result in an altered arrival time of action potentials at a distant neuron. These speed changes might be brought about by alterations of the properties of the passively conducting myelinated internodes or of the intervening excitable nodes of Ranvier, where the action potential is generated. We applied immunohistochemistry to assess how astrocytes interact with myelinated axons, neuronal stimulation and light-evoked calcium uncaging in astrocytes to evoke Ca2+-dependent release of gliotransmitters, and electrophysiology and pharmacology to characterize how astrocyte-released substances might affect the axon initial segment (AIS) and nodes of Ranvier of myelinated neurons. Measurements of conduction velocity and computer modeling allowed us to interpret the results. RESULTS: Astrocytes closely approach the axons of myelinated neurons in layer V of the cerebral cortex that enter the corpus callosum. Uncaging Ca2+ within astrocytes or stimulating spike trains in neurons evoked a rise of astrocyte [Ca2+]i that triggered the release of ATP-containing vesicles from these cells. This evoked an inward current in the AIS and nodes of Ranvier of the pyramidal neurons. Pharmacology showed that this was mediated by the activation of Gs-linked adenosine A2a receptors (A2aRs), implying that the released ATP was converted to adenosine by extracellular enzymes. The A2aRs raise the intracellular concentration of cyclic AMP, which activates hyperpolarization-activated cyclic nucleotide–gated (HCN) channels mediating the inward hyperpolarization-activated current (Ih) and thus depolarizes the cell. In the AIS, the activation of A2aRs alters excitability and hence action potential generation, whereas in the nodes of Ranvier, it decreases the conduction speed of the action potential along the axon. CONCLUSION: As in the gray matter, astrocyte [Ca2+]i regulates the release of ATP into the extracellular space in the white matter. After conversion to adenosine, this regulates the excitability and conduction speed of myelinated axons. The changes in excitability at the AIS will lead to changes in the relationship between the synaptic input and action potential output of the cell. The altered conduction speed of the myelinated axon may change neural circuit function by changing the action potential arrival time at the cell’s output synapses, thus altering the integration of signals in postsynaptic neurons. Variations in astrocyte-derived adenosine level can occur between wake and sleep states, and the extracellular adenosine concentration rises during energy deprivation conditions. These changes in adenosine level could thus control white matter information flow and neural circuit function

Resposta do arroz irrigado à infestação de Tibraca limbativentris (Heteroptera: pentatomidae).

O objetivo deste trabalho foi determinar o relacionamento entre a população de T. limbativentris e a redução na produção de arroz irrigado. A quantificação desta relação é essencial para a determinação do nível de dano econômico para esta praga

Damage of cowpea mild mottle virus and incidence of Bemisia tabaci biotype B in transgenic common bean lines resistant to bean golden mosaic virus.

The development of Embrapa 5.1 event of genetically modified (GM) common bean resistant to BGMV, and near isogenic lines of two commercial cultivars (Pérola and BRS Pontal) allows the evaluation of the damage caused only by CpMMV.This is because these transgenic isolines do not get infected by BGMV