Coherent manipulation of the binary degrees of freedom is at the heart of modern quantum technologies. Graphene offers two binary degrees: the electron spin and the valley. Efficient spin control has been demonstrated in many solid-state systems, whereas exploitation of the valley has only recently been started, albeit without control at the single-electron level. Here, we show that van der Waals stacking of graphene onto hexagonal boron nitride offers a natural platform for valley control. We use a graphene quantum dot induced by the tip of a scanning tunnelling microscope and demonstrate valley splitting that is tunable from -5 to +10 meV (including valley inversion) by sub-10-nm displacements of the quantum dot position. This boosts the range of controlled valley splitting by about one order of magnitude. The tunable inversion of spin and valley states should enable coherent superposition of these degrees of freedom as a first step towards graphene-based qubits.

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In situ diagnostic techniques provide a means of understanding the internal workings of fuel cells so that improved designs and operating regimes can be identified. Here, a novel metrology approach is reported that combines current and temperature mapping with water visualisation using neutron radiography.The approach enables a hydro-electro-thermal performance map to be generated that is applied to an air-cooled, open-cathode polymer electrolyte fuel cell. This type of fuel cell exhibits a particularly interesting coupled relationship between water, current and heat, as the air supply has the due role of cooling the stack as well as providing the cathode reactant feed via a single source. It is found that water predominantly accumulates under the cooling channels (thickness of 70-100 µm under the cooling channels and 5-25 µm in the active channels at 0.5 A cm KeywordsAir-cooled open-cathode polymer electrolyte fuel cell; water mapping; neutron imaging; temperature mapping; current mapping.

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In situ diagnostic techniques provide a means of understanding the internal workings of fuel cells so that improved designs and operating regimes can be identified. Here, for the first time, a combined current density and temperature distributed measurement system is used to generate an electro-thermal performance map of an air-cooled, air-breathing polymer electrolyte fuel cell stack operating in an air / hydrogen cross-flow configuration. Analysis is performed in low-and high-current regimes and a complex relationship between localised current density, temperature and reactant supply is identified that describes the way in which the system enters limiting performance conditions. Spatiotemporal analysis was carried out to characterise transient operations in dead-ended anode / purge mode which revealed extensive current density and temperature gradients. KeywordsCurrent mapping; temperature mapping; air-breathing fuel cell; performance optimisation; spatiotemporal electro-thermal analysis. Current mapping was first introduced by Cleghorn and Derouin[1] and Brett et al. [2,3], using printed circuit board (PCB) technology to produce segmented current collector plates, along with Stumper et al.[4], using segmented membranes. This technique shows that significant current density gradients can be generated as a result of fuel and oxidant depletion. Other techniques have proven the value of localised current measurements, including segmented PCBs [1,3,5-9], segmented current collector plates [10-12], shunt resistors [13-15] and Hall effect sensors [16-21]. Measurements have also been performed at sub-millimetre resolution [22,23], investigating the gradients across the land and channel areas. Most studies highlighted uneven performance under high current density, uneven fuel consumptions [2,5,6,14], the crucial influence of the operating conditions [3,13,19], stoichiometric ratios [8,11,12,20], fuel orientations [6,13], water management [10], CO electro-oxidation [7] and CO poisoning [24,25] distribution.Temperature distribution has also been extensively studied, identifying areas of higher electrochemical activity, hot spot formation and fuel depletion. Thermocouples can provide a crude measure of temperature inside fuel cells [16,17,26,27] but have accuracy limited to ±1 o C and cannot provide high spatial resolution. Moreover, thermocouples need to be inserted inside the fuel cell, which often requires design modifications. In contrast, infrared thermal imaging can provide very high spatial and temperature resolution [28][29][30][31][32][33][34], yet it typically requires use of specially designed fuel cells with a window transparent to infrared radiation, or is otherwise confined to measurement of the outer surface of a cell or stack.(highlighted using neutron imaging [37,38]) and nitrogen accumulation (measured via off-gas analysis [37,42,43]).Meyer et al. [37] reported an increase of the overall temperature and ohmic resistance in dead-ended mode. Eventually, local current density could be much higher than the...

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A global crisis such as the COVID-19 pandemic that started in early 2020 poses significant challenges for how research is conducted and communicated. We present four case studies from the perspective of an interdisciplinary research institution that switched to “corona-mode” during the first two months of the crisis, focussing all its capacities on COVID-19-related issues, communicating to the public directly and via media, as well as actively advising the national government. The case studies highlight the challenges posed by the increased time pressure, high demand for transparency, and communication of complexity and uncertainty. The article gives insights into how these challenges were addressed in our research institution and how science communication in general can be managed during a crisis.