Electron holography on HfO2/HfO2-x bilayer structures with multilevel resistive switching properties.

Unveiling the physical nature of the oxygen-deficient conductive filaments (CFs) that are responsible for the resistive switching of the HfO2-based resistive random access memory (RRAM) devices represents a challenging task due to the oxygen vacancy related defect nature and nanometer size of the CFs. As a first important step to this goal, we demonstrate in this work direct visualization and a study of physico-chemical properties of oxygen-deficient amorphous HfO2-x by carrying out transmission electron microscopy electron holography as well as energy dispersive x-ray spectroscopy on HfO2/HfO2-x bilayer heterostructures, which are realized by reactive molecular beam epitaxy. Furthermore, compared to single layer devices, Pt/HfO2/HfO2-x /TiN bilayer devices show enhanced resistive switching characteristics with multilevel behavior, indicating their potential as electronic synapses in future neuromorphic computing applications.

Revealing the quantum nature of the voltage-induced conductance changes in oxygen engineered yttrium oxide-based RRAM devices.

In this work, the quasi-analog to discrete transition occurring in the current-voltage characteristic of oxygen engineered yttrium oxide-based resistive random-access memory (RRAM) devices is investigated in detail. In particular, the focus of our research is not on the absolute conductance values of this characteristic but on the magnitude of its conductance changes occurring during the reset process of the device. It is found that the detected changes correspond to conductance values predominantly of the order of the quantum unit of conductance G0 = 2e2/h, where e is the electron charge and h the Planck constant. This feature is observed even at conductance levels far above G0, i.e. where electron transport is seemingly diffusive. It is also observed that such behavior is reproducible across devices comprising yttrium oxide layers with different oxygen concentrations and measured under different voltage sweep rates. While the oxygen deficiency affects the total number of quantized conductance states, the magnitude of the changes in conductance, close to 1 G0, is invariant to the oxygen content of the functional layer.

FIB based fabrication of an operative Pt/HfO2/TiN device for resistive switching inside a transmission electron microscope.

Recent advances in microelectromechanical systems (MEMS) based chips for in situ transmission electron microscopy are opening exciting new avenues in nanoscale research. The capability to perform current-voltage measurements while simultaneously analyzing the corresponding structural, chemical or even electronic structure changes during device operation would be a major breakthrough in the field of nanoelectronics. In this work we demonstrate for the first time how to electrically contact and operate a lamella cut from a resistive random access memory (RRAM) device based on a Pt/HfO2/TiN metal-insulator-metal (MIM) structure. The device was fabricated using a focused ion beam (FIB) instrument and an in situ lift-out system. The electrical switching characteristics of the electron-transparent lamella were comparable to a conventional reference device. The lamella structure was initially found to be in a low resistance state and could be reset progressively to higher resistance states by increasing the positive bias applied to the Pt anode. This could be followed up with unipolar set/reset operations where the current compliance during set was limited to 400 µA. FIB structures allowing to operate and at the same time characterize electronic devices will be an important tool to improve RRAM device performance based on a microstructural understanding of the switching mechanism.

Caracterização anatômica, histoquímica e de classes de metabólitos secundários de folhas de Guzmania lingulata (L. ) Mez / Anatomical, histochemical and class characterization of secondary metabolites in leaves of Guzmania lingulata (L. ) Mez

Na medicina natural pouco se sabe sobre a importância curativa da maior parte das espécies pertencentes à Bromeliaceae. Tal fato evidencia a necessidade da realização de investigações para a comprovação de possíveis potenciais fitoterápicos em espécies dessa família. Guzmania lingulata é caracterizada como sendo uma das espécies de bromélia mais comercializada e conhecida popularmente, a qual é amplamente propagada no Brasil. O presente trabalho objetivou a caracterização anatômica e a identificação de classes de metabólitos secundários presentes em G. lingulata, por meio de análises anatômicas, histoquímicas e químicas. As folhas foram fixadas em FAA 50 e transferidas para etanol 50%. Para o estudo anatômico, secções transversais da porção mediana das lâminas foliares foram obtidas à mão livre e submetidas ao processo de dupla coloração com safranina e azul de astra, sendo, logo após, montadas em gelatina glicerinada. Para a triagem das classes de metabólitos secundários foram coletadas 10 folhas frescas que passaram pelo processo de maceração em solução hidroalcoólica 70% por sete dias, à temperatura ambiente. O filtrado resultante da filtração a vácuo foi evaporado sob vácuo a temperatura de 50ºC até a remoção total do etanol, sendo obtido desta forma o extrato bruto hidroalcoólico a frio (EBHF). Os resultados das secções histológicas permitiram a caracterização anatômico-foliar baseada na qualidade das secções manuais obtidas. Os testes em relação às classes de metabólitos secundários revelaram a presença de antocianinas e taninos e ausência de flavonóides e saponinas. Esses resultados estimulam a realização de estudos mais aprofundados para identificar e quantificar os compostos dessas e de outras classes de metabólitos.

In natural medicine, little is known about the healing importance of most of the species belonging to the family Bromeliaceae. Such a fact points tothe need to perform investigations toprovethe possible phytotherapic potentials in species of the family. Guzmania lingulata is characterized as one of the most popular well-known and commercialized species of bromeliads, being widely spread in Brazil. This work aimed at the anatomic characterization and identification of classes of the secondary metabolites present in G. lingulata, by anatomic, histochemical and chemical analysis of the leaves. The leaves were fixed in FAA 50 and, afterwards, transferred to 50% ethanol. For the anatomic study, transverse sections of the middleproportion of the leafblades were obtained by hand and exposed to the process of double coloration with safranin and astra blue, being, right after that, mounted on glycerin jelly. For the sorting of the classes of secondary metabolites, 10 fresh leaves were collected and then put through the maceration process in 70% hydroalcoholic solution for 7 days, at room temperature. The resulting filtered solution from the vacuum filtration was evaporated under vacuum at 40ºC until the complete removal of the ethanol, obtaining, this way, the cold crude hydroalcoholic extract (EBHF). The results ofhistological sectionsallowed the anatomical characterization based onthe quality ofthe manual sectionsobtained. The testsin relationto the classes of secondary metabolitesrevealed the presence ofanthocyanins andtannins andabsence of flavonoids and saponins. These results encourage the completionoffurther studies to identify and quantify the compounds of these and other classes of metabolites.

A hidden pseudogap under the 'dome' of superconductivity in electron-doped high-temperature superconductors.

The ground state of superconductors is characterized by the long-range order of condensed Cooper pairs: this is the only order present in conventional superconductors. The high-transition-temperature (high-T(c)) superconductors, in contrast, exhibit more complex phase behaviour, which might indicate the presence of other competing ground states. For example, the pseudogap--a suppression of the accessible electronic states at the Fermi level in the normal state of high-T(c) superconductors-has been interpreted as either a precursor to superconductivity or as tracer of a nearby ground state that can be separated from the superconducting state by a quantum critical point. Here we report the existence of a second order parameter hidden within the superconducting phase of the underdoped (electron-doped) high-T(c) superconductor Pr2-xCe(x)CuO4-y and the newly synthesized electron-doped material La2-xCe(x)CuO4-y (ref. 8). The existence of a pseudogap when superconductivity is suppressed excludes precursor superconductivity as its origin. Our observation is consistent with the presence of a (quantum) phase transition at T = 0, which may be a key to understanding high-T(c) superconductivity. This supports the picture that the physics of high-T(c) superconductors is determined by the interplay between competing and coexisting ground states.