Direct observation of valley-hybridization and universal symmetry of graphene with mesoscopic conductance fluctuations

In graphene, the valleys represent spin-like quantities and can act as a physical resource in valley-based electronics to novel quantum computation schemes. Here we demonstrate a direct route to tune and read the valley quantum states of disordered graphene by measuring the mesoscopic conductance fluctuations. We show that the conductance fluctuations in graphene at low temperatures are reduced by a factor of four when valley triplet states are gapped in the presence of short range potential scatterers at high carrier densities. We also show that this implies a gate tunable universal symmetry class which outlines a fundamental feature arising from graphene's unique crystal structure.Comment: 5 pages, 5 figure

High contrast imaging and thickness determination of graphene with in-column secondary electron microscopy

We report a new method for quantitative estimation of graphene layer thicknesses using high contrast imaging of graphene films on insulating substrates with a scanning electron microscope. By detecting the attenuation of secondary electrons emitted from the substrate with an in-column low-energy electron detector, we have achieved very high thickness-dependent contrast that allows quantitative estimation of thickness up to several graphene layers. The nanometer scale spatial resolution of the electron micrographs also allows a simple structural characterization scheme for graphene, which has been applied to identify faults, wrinkles, voids, and patches of multilayer growth in large-area chemical vapor deposited graphene. We have discussed the factors, such as differential surface charging and electron beam induced current, that affect the contrast of graphene images in detail.Comment: 5 pages, 4 figure

O capital humano e o índice de desenvolvimento da educação básica : o caso da qualidade do ensino no sistema federal e estadual

Orientador: Angela WeltersMonografia(Graduação) - Universidade Federal do Paraná,Setor de Ciências Sociais Aplicadas, Curso de Ciências EconômicasResumo: A educação tem sido motivo de frequentes debates e estudos, sobretudo quando é vista como instrumento capaz de não somente alterar o quadro social de um país como alavanca-lo economicamente. Essa dinâmica somada a Teoria do Capital Humano permite construir uma perspectiva embasada em transformações sociais. Devido a pouca clareza na literatura sobre o indicador estatístico de qualidade do ensino relacionado com os fatores condicionantes, este trabalho teve como objetivo comparar o desempenho dos Colégios Militares com as melhores Escolas Estaduais mediante o Índice do Desenvolvimento da Educação Básica. A pesquisa qualitativa compilou dados referentes aos anos 2009, 2011 e 2013 e teve como base de dados INEP. Como resultados, observaram-se que as melhores condições de aprendizado, proficiência, infraestrutura, equipamentos, condições de trabalho do docente, indicadores socioeconômicos dos alunos e menores complexidade de gestão da escola concentram nos Colégios MilitaresAbstract: Education has been the subject of frequent debates and studies, especially when it is seen as an instrument to not only change the membership of a country as a lever it economically. This dynamic plus the Human Capital Theory allows you to build an informed perspective on social change. Due to lack of clarity in the literature on statistical indicator of education related to the conditioning factors, this study aimed to compare the performance of Military Colleges with the best state schools through the Basic Education Development Index. Qualitative research compiled data for the years 2009, 2011 and 2013 and had as INEP database. As a result, we observed that the best learning conditions, proficiency, infrastructure, equipment, the working conditions of teachers, students' socioeconomic indicators and lower complexity of school management focus in Military School

Probing interlayer interactions and commensurate-incommensurate transition in twisted bilayer graphene through Raman spectroscopy

Twisted 2D layered materials have garnered a lot of attention recently as a class of 2D materials whose interlayer interactions and electronic properties are dictated by the relative rotation / twist angle between the adjacent layers. In this work, we explore a prototype of such a twisted 2D system, artificially stacked twisted bilayer graphene (TBLG), where we probe the changes in the interlayer interactions and electron-phonon scattering pathways as the twist angle is varied from 0{\deg} to 30{\deg}, using Raman spectroscopy. The long range Moir\'e potential of the superlattice gives rise to additional intravalley and intervalley scattering of the electrons in TBLG which have been investigated through their Raman signatures. The density functional theory (DFT) calculations of the electronic band structure of the TBLG superlattices was found to be in agreement with the resonant Raman excitations across the van Hove singularities in the valence and conduction bands predicted for TBLG due to hybridization of bands from the two layers. We also observe that the relative rotation between the graphene layers has a marked influence on the second order overtone and combination Raman modes signalling a commensurate-incommensurate transition in TBLG as the twist angle increases. This serves as a convenient and rapid characterization tool to determine the degree of commensurability in TBLG systems

Highly Efficient Rubrene–Graphene Charge-Transfer Interfaces as Phototransistors in the Visible Regime

This is the final version of the article. Available from Wiley via the DOI in this record.Atomically thin materials such as graphene are uniquely responsive to charge transfer from adjacent materials, making them ideal charge-transport layers in phototransistor devices. Effective implementation of organic semiconductors as a photoactive layer would open up a multitude of applications in biomimetic circuitry and ultra-broadband imaging but polycrystalline and amorphous thin films have shown inferior performance compared to inorganic semiconductors. Here, the long-range order in rubrene single crystals is utilized to engineer organic-semiconductor–graphene phototransistors surpassing previously reported photogating efficiencies by one order of magnitude. Phototransistors based upon these interfaces are spectrally selective to visible wavelengths and, through photoconductive gain mechanisms, achieve responsivity as large as 10^7 A/W and a detectivity of 9 × 10^11 Jones at room temperature. These findings point toward implementing low-cost, flexible materials for amplified imaging at ultralow light levels.S.R. and M.F.C. acknowledge financial support from EPSRC (Grant 464 Nos. EP/J000396/1, EP/K017160/1, EP/K010050/1, EP/G036101/1, EP/M001024/1, and 465 EP/M002438/1), from Royal Society international Exchanges Scheme 2012/R3 and 466 2013/R2 and from European Commission (No. FP7-ICT-2013-613024-GRASP)

Electrochemical integration of graphene with light absorbing copper-based thin films

We present an electrochemical route for the integration of graphene with light sensitive copper-based alloys used in optoelectronic applications. Graphene grown using chemical vapor deposition (CVD) transferred to glass is found to be a robust substrate on which photoconductive Cu_{x}S films of 1-2 um thickness can be deposited. The effect of growth parameters on the morphology and photoconductivity of Cu_{x}S films is presented. Current-voltage characterization and photoconductivity decay experiments are performed with graphene as one contact and silver epoxy as the other

Study of Flux Pinning in Thick Film REBCO Coated Conductors Over a Wide Range of Magnetic Fields and Temperatures

RE-Ba-Cu-O (REBCO, RE = rare earth) coated conductors are approaching the large-scale electric power and magnetic applications over a wide range of temperatures and magnetic fields owing to their high critical current density (Jc), their high critical temperature (Tc) and their strong irreversibility field (Hirr). However, further enhancement of the engineering critical current (Je) is required to make REBCO more cost-effective. Improving Je can be achieved by combining two strategies: achieving a thickness independent Jc and enhancing the flux pinning landscape in REBCO through the incorporation of artificial pinning centers (APC). In this work, are studied REBCO tapes deposited using an advanced metal organic vapor deposition process (A-MOCVD) allowing the growth of up to 5 µm thick high performing REBCO films without a deterioration in Jc. BaMO3 (M: Zr, Hf, and Nb) self-assembled nanorods with different concentrations up to 15 mol% were incorporated into REBCO to enhance their Jc over a wide range of temperature (4.2—77 K) and applied magnetic field (0—31 T). The effect of the density, size, and continuity of the BMOs were systematically studied revealing a stronger contribution of the continuous and dense nanorods to the Jc at high magnetic fields and low temperatures. Additionally, BaZrO3 doped REBCO were subject to post-deposition tensile-creep-deformation at 580°C leading to an increase in the density of ab-plane stacking faults which correlated with up to 3 times higher critical current than a reference sample at 77 K and 1 T when the magnetic field is parallel to the ab-plane. Finally, Artificial Neural Networks (ANN) were trained to accurately predict lower temperature critical currents out of the 65 K critical currents. The ANN predictions showed an average error of 2.8% at 4.2 K and 13 T when applied on a validation dataset containing 100 samples.Mechanical Engineering, Department o

Universal conductance fluctuations as a direct probe to valley coherence and universality class of disordered graphene

We demonstrate that the Universal Conductance Fluctuations (UCF) can be used as a direct probe to study the valley quantum states in disordered graphene. The UCF magnitude in graphene is suppressed by a factor of four at high carrier densities where the short-range disorder essentially breaks the valley degeneracy of the K and K' valleys, leading to a density dependent crossover of symmetry class from symplectic near the Dirac point to orthogonal at high densities