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Measurements of current-voltage characteristics from ideal carbon nanotube (CNT) field electron emitters of a small apex radius have shown that these emitters can exhibit a linear Fowler-Nordheim plot [e.g., Dean and Chalamala, Appl. Phys. Lett. 76, 375 (2000)]. From such a plot, a constant (voltage-independent) characteristic field enhancement factor (FEF) can be deduced. Over 15 years later, this experimental result has not yet been convincingly retrieved from first-principles electronic-structure calculations or, more generally, from quantum mechanics (QMs). On the contrary, several QM calculations have deduced that the characteristic FEF should be a function of the macroscopic field applied to the CNT. This apparent contradiction between the experiment and the QM theory has been an unexplained feature of CNT emission science and has raised doubts about the ability of existing QM models to satisfactorily describe experimental CNT emission behavior. In this work, we demonstrate, by means of a density functional theory analysis of single-walled CNTs "floating" in an applied macroscopic field, the following significant result. This is that the agreement between the experiment, classical-conductor CNT models, and QM calculations can be achieved if the latter are used to calculate (from the "real" total-charge-density distributions initially obtained) the distributions of "induced" charge-density, induced local fields, and induced local FEFs. A similar result was previously obtained for so-called vertically aligned CNT/graphene structures. The present work confirms, more reliably and in significantly greater detail than in earlier work on a different system, that this finding applies to the common "post-on-a-conducting plane" situation of CNT field electron emission. This finding also brings out various further theoretical questions that need to be explored. [ABSTRACT FROM AUTHOR]

Experiments on field electron emission (FE) of single carbon nanotubes (CNTs) indicate that they exhibit a nearly linear Fowler–Nordheim plot, and the field enhancement factor (FEF) near the CNT apex is independent of the applied (macroscopic) field (F M) for small radii field emitters. Recent results, based on density functional theory calculations considering CNTs with small radii, retrieved the constancy of the FEF defined in terms of the corresponding induced electron density. As a consequence, it has been reported that the constancy of the FEF with F M could be connected with the linear response of the CNT. In this paper, we reinforce this connection, considering the problem of a floating (6,6) hybrid single-walled nanotube, whose cylindrical body is an insulating one and composed of alternating boron and nitrogen atoms end-capped with carbon atoms. Our results show that the constancy of the FEF is achieved when a linear dependence between the longitudinal component of the induced system dipole moment (μ i , z ) and F M is observed. Two regimes of constant polarizabilities have been found at sufficiently low and high F M -values. In the intermediate range 0.3 V / nm ≲ F M ≲ 5 V/nm, a crossover from insulating-to-conducting behavior, exhibiting a nonlinear dependence of μ i , z on F M , is found accompanied by an increase of the FEF with F M. This result reveals circumstances that could lead to dependence of the FEF on F M , being timely for interpretation of FE characteristics in the context of vacuum nanoelectronic devices. [ABSTRACT FROM AUTHOR]

COVID-19 is affecting healthcare resources worldwide, with lower and middle-income countries being particularly disadvantaged to mitigate the challenges imposed by the disease, including the availability of a sufficient number of infirmary/ICU hospital beds, ventilators, and medical supplies. Here, we use mathematical modelling to study the dynamics of COVID-19 in Bahia, a state in northeastern Brazil, considering the influences of asymptomatic/non-detected cases, hospitalizations, and mortality. The impacts of policies on the transmission rate were also examined. Our results underscore the difficulties in maintaining a fully operational health infrastructure amidst the pandemic. Lowering the transmission rate is paramount to this objective, but current local efforts, leading to a 36% decrease, remain insufficient to prevent systemic collapse at peak demand, which could be accomplished using periodic interventions. Non-detected cases contribute to a ∽55% increase in R0. Finally, we discuss our results in light of epidemiological data that became available after the initial analyses. Low-resource settings can face additional challenges in managing the COVID-19 pandemic. Here, the authors use mathematical modelling to investigate transmission in the state of Bahia, Brazil, and quantify control measures needed to prevent the hospital system becoming overwhelmed. [ABSTRACT FROM AUTHOR]

With a large-area field electron emitter (LAFE), the area of emission is expected to be dependent of the applied field. One possible explanation for this behavior is the statistical distribution of the local field enhancement factors (FEFs), as a consequence of an irregular surface's morphology of the LAFE. In this paper, the authors present a simple and more general theory for extracting the scaled barrier field, f, by considering the dependence of the formal area of emission of conducting LAFEs with an applied field. In our model, the local FEFs of LAFE sites are exponentially distributed, which is consistent with thin film electron emitters. As a byproduct of technological relevance, our results show that general effective f values extracted from linear Fowler-Nordheim plots are outside of the "experimentally reasonable" range of values for physically orthodox emission, when the area of emission varies significantly with the applied field. Thus, a more general criterion for detecting and interpreting nonorthodox field emission is proposed and can be applied to any distribution of local FEFs in conducting LAFEs. [ABSTRACT FROM AUTHOR]