How close can one approach the Dirac point in graphene experimentally?

The above question is frequently asked by theorists who are interested in graphene as a model system, especially in context of relativistic quantum physics. We offer an experimental answer by describing electron transport in suspended devices with carrier mobilities of several 10^6 cm^2V^-1s^-1 and with the onset of Landau quantization occurring in fields below 5 mT. The observed charge inhomogeneity is as low as \approx10^8 cm^-2, allowing a neutral state with a few charge carriers per entire micron-scale device. Above liquid helium temperatures, the electronic properties of such devices are intrinsic, being governed by thermal excitations only. This yields that the Dirac point can be approached within 1 meV, a limit currently set by the remaining charge inhomogeneity. No sign of an insulating state is observed down to 1 K, which establishes the upper limit on a possible bandgap

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

EXPERIMENTAL AND THEORETICAL VALIDATION OF DOUBLE COLUMN INTERNAL FIXATION THEORY FOR DISTAL FEMORAL FRACTURES

Purpose of the study – to experimentally compare efficiency of lateral and bilateral (lateral and medial) plate fixation of distal femoral fractures and to validate the appropriateness of double column theory of distal femur anatomy for internal fixation for these fractures.Material and methods. The authors performed a biomechanical study in two series on polyurethane models of right femur corresponding in dimensions to natural femur. After simulating a 33 C2 fracture type it was fixed by one lateral plate or two (lateral and medial) plates. After fixation the models were tested in six load ranges with maximal load from 20 to 120 kgf in cyclic mode.Results. The authors obtained a significant difference in absolute values of fragments displacement amplitude depending on fixation method. In the first series of the experiment – one plate fixed on the lateral surface of the femur – under minimal load the displacement value was reported as0.3 mm and under maximal load —1.9 mm; in the second series of experiment – two plates fixed on the lateral and medial surfaces of the femur – displacement values were reported as0.35 mm and0.95 mm respectively. Conclusion. The presence or absence of medial support after internal fixation has a profound impact on ensuring stability in cases of comminuted fractures of distal femoral fractures. In this context the use of double column theory of distal femur anatomy for internal fixation can significantly improve the treatment outcomes for such patients. After trials of minimally invasive fixation method on anatomical specimen the described theory can be implemented into the clinical practice

Quantum electrodynamics with anisotropic scaling: Heisenberg-Euler action and Schwinger pair production in the bilayer graphene

We discuss quantum electrodynamics emerging in the vacua with anisotropic scaling. Systems with anisotropic scaling were suggested by Horava in relation to the quantum theory of gravity. In such vacua the space and time are not equivalent, and moreover they obey different scaling laws, called the anisotropic scaling. Such anisotropic scaling takes place for fermions in bilayer graphene, where if one neglects the trigonal warping effects the massless Dirac fermions have quadratic dispersion. This results in the anisotropic quantum electrodynamics, in which electric and magnetic fields obey different scaling laws. Here we discuss the Heisenberg-Euler action and Schwinger pair production in such anisotropic QEDComment: 5 pages, no figures, JETP Letters style, version accepted in JETP Letter

Analysis of Ground Level Enhancements (GLE) : Extreme solar energetic particle events have hard spectra

Nearly 70 Ground Level Enhancements (GLEs) of cosmic rays have been recorded by the worldwide neutron monitor network since the 1950s depicting a big variety of energy spectra of solar energetic particles (SEP). Here we studied a statistical relation between the event-integrated intensity of GLEs (calculated as count-rate relative excess, averaged over all available polar neutron monitors, and expressed in percent-hours) and the hardness of the solar particle energy spectra. For each event the integral omnidirectional event integrated fluences of particles with energy above 30 MeV (F-30) and above 200 MeV (F-200) were computed using the reconstructed spectra, and the ratio between the two fluences was considered as a simple index of the event's hardness. We also provided a justification of the spectrum estimate in the form of the Band-function, using direct PAMELA data for GLE 71 (17-May-2012). We found that, while there is no clear relation between the intensity and the hardness for weak events, all strong events with the intensity greater 100 % h are characterized by a very hard spectrum. This implies that a hard spectrum can be securely assumed for all extreme GLE events, e.g., those studied using cosmogenic isotope data in the past. (C) 2016 COSPAR. Published by Elsevier Ltd. All rights reserved.Peer reviewe

ТЕХНОЛОГИИ 3D-ПЕЧАТИ В ОБРАЗОВАТЕЛЬНОМ ПРОЦЕССЕ

The article touches upon the topic of using 3D-printing technologies in education. 3D-printing technologies can be used to teach technological skills in engineering and design as the main tool. They also help to reorient attention from the digital or virtual environment to the real world, because as a result of learning activity, not sketches and layouts, but real objects with specified characteristics act. 3D-printing technologies are fast-developing and promising technologies that can find their application in various fields, including in the field of education. These technologies due to the appearance of personal printing devices can facilitate the introduction of new forms of organization of the educational process, increase the motivation and formation of the necessary competencies of graduates and teachers.В статье затрагивается тема использования технологий 3D-печати в сфере образования. Технологии 3D-печати могут применяться для обучения технологическим навыкам в конструировании, машиностроении, проектировании в качестве основного инструмента. Также они способствуют переориентации внимания с цифровой или виртуальной среды на реальный мир, поскольку результатами учебной деятельности выступают не эскизы и макеты, а реальные объекты с заданными характеристиками. Технологии 3D-печати относятся к быстроразвивающимся и перспективным технологиям, которые могут найти свое применение в различных областях, в том числе и в сфере образования. Данные технологии благодаря появлению персональных печатающих устройств могут способствовать внедрению новых форм организации учебного процесса, повышению мотивации и формированию необходимых компетенций выпускников и преподавателей

Time dependence of the electron and positron components of the cosmic radiation measured by the PAMELA experiment between July 2006 and December 2015

Cosmic-ray electrons and positrons are a unique probe of the propagation of cosmic rays as well as of the nature and distribution of particle sources in our Galaxy. Recent measurements of these particles are challenging our basic understanding of the mechanisms of production, acceleration and propagation of cosmic rays. Particularly striking are the differences between the low energy results collected by the space-borne PAMELA and AMS-02 experiments and older measurements pointing to sign-charge dependence of the solar modulation of cosmic-ray spectra. The PAMELA experiment has been measuring the time variation of the positron and electron intensity at Earth from July 2006 to December 2015 covering the period for the minimum of solar cycle 23 (2006-2009) till the middle of the maximum of solar cycle 24, through the polarity reversal of the heliospheric magnetic field which took place between 2013 and 2014. The positron to electron ratio measured in this time period clearly shows a sign-charge dependence of the solar modulation introduced by particle drifts. These results provide the first clear and continuous observation of how drift effects on solar modulation have unfolded with time from solar minimum to solar maximum and their dependence on the particle rigidity and the cyclic polarity of the solar magnetic field.Comment: 11 pages, 2 figure