Atomic collapse, Lorentz boosts, Klein scattering, and other quantum-relativistic phenomena in graphene

Electrons in graphene, behaving as massless relativistic Dirac particles, provide a new perspective on the relation between condensed matter and high-energy physics. We discuss atomic collapse, a novel state of superheavy atoms stripped of their discrete energy levels, which are transformed into resonant states. Charge impurities in graphene provide a convenient condensed matter system in which this effect can be explored. Relativistic dynamics also manifests itself in another system, graphene p-n junctions. We show how the transport problem in the presence of magnetic field can be solved with the help of a Lorentz transformation, and use it to investigate magnetotransport in p-n junctions. Finally, we review recent proposal to use Fabry-Perot resonances in p-n-p structures as a vehicle to investigate Klein scattering, another hallmark phenomenon of relativistic dynamics.Comment: minireview, 9 pg

Percepções sobre a educação a distância : um estudo preliminar à implantação da EaD pela Escola de Contas Públicas do TCDF

Trabalho de conclusão de curso (especialização)—Universidade de Brasília, Faculdade de Economia, Administração e Contabilidade, Departamento de Administração, Núcleo de Estudos e Pesquisas em Inovação e Estratégia, Tribunal de Contas do Distrito Federal, 2017.O presente trabalho objetiva identificar as percepções dos servidores do Tribunal de Contas do Distrito Federal sobre o Ensino a Distância, para isso se deve: identificar o nível de domínio dos servidores do TCDF no uso de ferramentas relacionadas à tecnologia da informação; Identificar barreiras pessoais e institucionais que possam contribuir para a evasão dos servidores do TCDF dos cursos a distância; Fornecer à Escola de Contas do TCDF subsídio que possa auxiliá-la no processo de implantação do Ensino a Distância

Atomically thin boron nitride: a tunnelling barrier for graphene devices

We investigate the electronic properties of heterostructures based on ultrathin hexagonal boron nitride (h-BN) crystalline layers sandwiched between two layers of graphene as well as other conducting materials (graphite, gold). The tunnel conductance depends exponentially on the number of h-BN atomic layers, down to a monolayer thickness. Exponential behaviour of I-V characteristics for graphene/BN/graphene and graphite/BN/graphite devices is determined mainly by the changes in the density of states with bias voltage in the electrodes. Conductive atomic force microscopy scans across h-BN terraces of different thickness reveal a high level of uniformity in the tunnel current. Our results demonstrate that atomically thin h-BN acts as a defect-free dielectric with a high breakdown field; it offers great potential for applications in tunnel devices and in field-effect transistors with a high carrier density in the conducting channel.Comment: 7 pages, 5 figure

Observing imperfection in atomic interfaces for van der Waals heterostructures

Vertically stacked van der Waals heterostructures are a lucrative platform for exploring the rich electronic and optoelectronic phenomena in two-dimensional materials. Their performance will be strongly affected by impurities and defects at the interfaces. Here we present the firstsystematic study of interfaces in van der Waals heterostructure using cross sectional scanning transmission electron microscope (STEM) imaging. By measuring interlayer separations and comparing these to density functional theory (DFT) calculations we find that pristine interfaces exist between hBN and MoS2 or WS2 for stacks prepared by mechanical exfoliation in air. However, for two technologically important transition metal dichalcogenide (TMDC) systems, MoSe2 and WSe2 , our measurement of interlayer separations provide the first evidence for impurity species being trapped at buried interfaces with hBN: interfaces which are flat at the nanometer length scale. While decreasing the thickness of encapsulated WSe2 from bulk to monolayer we see a systematic increase in the interlayer separation. We attribute these differences to the thinnest TMDC flakes being flexible and hence able to deform mechanically around a sparse population of protruding interfacial impurities. We show that the air sensitive two dimensional (2D) crystal NbSe2 can be fabricated into heterostructures with pristine interfaces by processing in an inert-gas environment. Finally we find that adopting glove-box transfer significantly improves the quality of interfaces for WSe2 compared to processing in air

Quality of milk consumed in Samara region: An analysis of milk physical and chemical characteristics

Objectives an analysis of physical and chemical indicators of milk quality produced under the most popular brands in the Samara region. Material and methods. The study was selective and prospective, focused on the physical and chemical indices of milk quality. The sampling was based on the results of the assessment of the frequency of consumption of various brands of milk by the population of the Samara region. The analysis included five samples of milk of different brands, purchased in the retail food store chain, and assessed their various physical and chemical indicators. All measurements were performed in accordance with the regulatory documentation. Results. Only 1/3 part of respondents (32.6%) included milk in their daily diet. All five milk samples subjected to physical and chemical examination met the requirements of the current regulatory documentation. However, in assessing the nutritional value of the samples by mineral composition, it was found that the actual content of phosphorus and calcium is significantly lower than the values of the corresponding indicators set out in the handbook "Chemical composition and calorie content of Russian food products" that is used as a guide by nutrition specialists. Conclusion. Milk purchased in the food store chain met the safety requirements for physical and chemical indicators. The reduced level of milk consumption among the population of the Samara region requires management decisions. The actual chemical composition of this product, which has regional and seasonal characteristics, should be taken into account in the development of diets for defined population groups

Study of compliance to rational nutrition in various professional groups living in the Russian Federation and the Republic of Tajikistan

Objective to study the adherence to the principles of rational nutrition by representatives of various professional groups living in the Russian Federation and the Republic of Tajikistan. Material and methods.The study was conducted using a questionnaire-survey method among 543 mental workers of the Samara region and 158 students of the Avicenna Tajik State Medical University (Republic of Tajikistan), followed by statistical processing of the data. Results.Violations of the principles of rational nutrition are commonin various professional groups of the working-age population, including among students of a medical university. According to anthropometric studies, 61% of the surveyed workers were overweight and obese; in the group of students this indicator was only 8%, 29% of students were underweight. Violations of nutrition regimen were found among 31.1% of employees and 38% of students. Factor analysis of the actual nutrition of workers revealed 5 types of nutrition models characterized by a stereotype of eating behavior due to the consumption of certain foods and beverage. The regression analysis confirmed the relationship between the risk of obesity and adherence to types 2, 4 and 5 of nutritional models; an individual's adherence to nutritional model type 3 reduced this risk. In the group of the surveyed, the deterioration of the diet quality was revealed due to the excessive consumption of high-calorie foods, "fast food", sweet carbonated drinks, as well as insufficient consumption of vegetables, fruits, fish. The correlation analysis established the relationship between the body mass index and complaints presented by the survey participants concerning the cardiovascular, digestive, endocrine and musculoskeletal systems. The study identified the most common alimentary-dependent pathologies among students such as gastritis, chronic pancreatitis, and chronic cholecystitis. Conclusion.The revealed violations of the principles of rational nutrition, the nutritional status, form the risks of development of the gastrointestinal tract diseases, metabolic disorders and cardiovascular pathology. The results obtained indicate the need of preventive measures in relation to the adherence to the principles of rational nutrition, creating awareness in various professional groups of the population, including students of medical universities in different states

SRGeJ045359.9+622444: A 55-min Period Eclipsing AM CVn Discovered from a Joint SRG/eROSITA + ZTF Search

AM CVn systems are ultra-compact binaries where a white dwarf accretes from a helium-rich degenerate or semi-degenerate donor. Some AM CVn systems will be among the loudest sources of gravitational waves for the upcoming Laser Interferometer Space Antenna (LISA), yet the formation channel of AM CVns remains uncertain. We report the study and characterisation of a new eclipsing AM CVn, SRGeJ045359.9+622444 (hereafter SRGeJ0453), discovered from a joint SRG/eROSITA and ZTF program to identify cataclysmic variables (CVs). We obtained optical photometry to confirm the eclipse of SRGeJ0453 and determine the orbital period to be $P_\textrm{orb} = 55.0802 \pm 0.0003$ min. We constrain the binary parameters by modeling the high-speed photometry and radial velocity curves and find $M_\textrm{donor} = 0.044 \pm0.024 M_{\odot}$ and $R_\textrm{donor}=0.078 \pm 0.012 R_{\odot}$. The X-ray spectrum is approximated by a power-law model with an unusually flat photon index of $\Gamma\sim 1$ previously seen in magnetic CVs with SRG/eROSITA, but verifying the magnetic nature of SRGeJ0453 requires further investigation. Optical spectroscopy suggests that the donor star of SRGeJ0453 could have initially been a He star or a He white dwarf. SRGeJ0453 is the ninth eclipsing AM CVn system published to date, and its lack of optical outbursts have made it elusive in previous surveys. The discovery of SRGeJ0453 using joint X-ray and optical surveys highlights the potential for discovering similar systems in the near future.Comment: Submitted to Ap