Conductance of p-n-p graphene structures with 'air-bridge' top gates

We have fabricated graphene devices with a top gate separated from the graphene layer by an air gap--a design which does not decrease the mobility of charge carriers under the gate. This gate is used to realise p-n-p structures where the conducting properties of chiral carriers are studied. The band profile of the structures is calculated taking into account the specifics of the graphene density of states and is used to find the resistance of the p-n junctions expected for chiral carriers. We show that ballistic p-n junctions have larger resistance than diffusive ones. This is caused by suppressed transmission of chiral carriers at angles away from the normal to the junction.Comment: to be published in Nano Letter

Density of states and zero Landau level probed through capacitance of graphene

We report capacitors in which a finite electronic compressibility of graphene dominates the electrostatics, resulting in pronounced changes in capacitance as a function of magnetic field and carrier concentration. The capacitance measurements have allowed us to accurately map the density of states D, and compare it against theoretical predictions. Landau oscillations in D are robust and zero Landau level (LL) can easily be seen at room temperature in moderate fields. The broadening of LLs is strongly affected by charge inhomogeneity that leads to zero LL being broader than other levels

Andrew Moreton — Daniel Defoe’s Last Pseudonym

В данной работе рассматривается один из псевдонимов Даниэля Дефо — Эндрю Моретон. Автор приводится позиции западных исследователей Даниэля Дефо об образе Эндрю Моретона. Описаны проблемы, поднимаемые в работах Эндрю Моретона. Автор делает вывод касательно цели создания данного псевдонима.This article examines one of Daniel Defoe’s pseudonyms — Andrew Moreton. The author cites the positions of Western researcher of Daniel Defoe on the image of Andrew Moreton. The problems raised in the works of Andrew Moreton are described. The author draws a conclusion as to the purpose of the pseudonym

Giant Nonlocality near the Dirac Point in Graphene

Transport measurements have been a powerful tool for uncovering new electronic phenomena in graphene. We report nonlocal measurements performed in the Hall bar geometry with voltage probes far away from the classical path of charge flow. We observe a large nonlocal response near the Dirac point in fields as low as 0.1T, which persists up to room temperature. The nonlocality is consistent with the long-range flavor currents induced by lifting of spin/valley degeneracy. The effect is expected to contribute strongly to all magnetotransport phenomena near the neutrality point

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

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

Balancing environmental conservation and socioeconomic needs: the complexities of artisanal and small-scale coal mining in Africa

Artisanal and small-scale coal mining (ASM) in Africa presents a complex challenge where environmental conservation and socioeconomic development must be carefully balanced. This article explores the intricate dynamics surrounding ASM in the African context, highlighting the multifaceted impacts on the environment and the socioeconomic well-being of local communities. Artisanal and small-scale coal mining is often characterized by low capital investment, limited mechanization, and dependence on manual labor. Despite this, miners face numerous problems due to the unstable socioeconomic situation and weak labor legislation. The article describes the practice of ASM in Africa: provides an overview of the growing significance of ASM in Africa, and sets the stage for understanding the intricate trade-offs faced in managing this sector. The development of ASM in Africa is examined, considering its historical context, drivers, and patterns of growth, emphasizing the need for context-specific approaches to address its complexities. The article delves into the environmental challenges posed by ASM, focusing on deforestation, land degradation, water and air pollution, and the loss of biodiversity. It examines the specific manifestations of these challenges in selected African countries, shedding light on the varied ecological consequences and their ramifications for sustainable development. Additionally, the socioeconomic dimensions of ASM are explored, acknowledging its potential as a livelihood source for many communities while also recognizing the social and economic vulnerabilities associated with it. The article discusses the interplay between ASM and local economies, including job creation, income generation, and the broader implications for poverty alleviation and sustainable development