New negative differential resistance device based on resonant interband tunneling

We propose and demonstrate a novel negative differential resistance device based on resonant interband tunneling. Electrons in the InAs/AlSb/GaSb/AlSb/InAs structure tunnel from the InAs conduction band into a quantized state in the GaSb valence band, giving rise to a peak in the current-voltage characteristic. This heterostructure design virtually eliminates many of the competing transport mechanisms which limit the performance of conventional double-barrier structures. Peak-to-valley current ratios as high as 20 and 88 are observed at room temperature and liquid-nitrogen temperature, respectively. These are the highest values reported for any tunnel structure

Observation of large peak-to-valley current ratios and large peak current densities in AlSb/InAs/AlSb double-barrier tunnel structures

We report improved peak-to-valley current ratios and peak current densities in InAs/AlSb double-barrier, negative differential resistance tunnel structures. Our peak-to-valley current ratios are 2.9 at room temperature and 10 at liquid-nitrogen temperatures. Furthermore, we have observed peak current densities of 1.7×10^5 A/cm^2. These figures of merit are substantially better than previously reported values. The improvements are obtained by adding spacer layers near the barriers, thinner well regions, and thinner barriers

Demonstration of large peak-to-valley current ratios in InAs/AlGaSb/InAs single-barrier heterostructures

We report large peak-to-valley current ratios in InAs/AlxGa1−xSb/InAs single-barrier tunnel structures. The mechanism for single-barrier negative differential resistance (NDR) has been proposed and demonstrated recently. A peak-to-valley current ratio of 3.4 (1.2) at 77 K (295 K), which is substantially larger than what has been previously reported, was observed in a 200-Å-thick Al0.42Ga0.58Sb barrier. A comparison with a calculated current-voltage curve yields good agreement in terms of peak current and the slope of the NDR region. The single-barrier structure is a candidate for high-speed devices because of expected short tunneling times and a wide NDR region

Remote sensing and on-farm experiments for determining in-season nitrogen rates in winter wheat – Options for implementation, model accuracy and remaining challenges

Optimised nitrogen (N) fertilisation can be used to increase farm profits, to realise the achievement of quality goals for produce, and to reduce environmental risks in the form of leaching and/or volatilisation of N compounds from the fields. This study examined options and challenges for remote sensing-based variable rate supplemental N fertilisation in winter wheat (Triticum aestivum L.). The models were based on data from ten field trials conducted in different regions across Sweden over three years. A two-step approach for modelling optimal N rates, suitable for practical implementation in precision agriculture, was developed and evaluated. The expected accuracies for new sites and years were assessed by leave-one-entire-trial-out cross-validation. In a first step, the average N rate was modelled from site-specific information, including data that can be obtained from on-farm experiments, i.e. N uptake in plots without N fertilisation (zero-plots) and N uptake in plots with non-limiting N supply (max-plots). In the second step, additions or subtractions from this average N rate was modelled based on vegetation indices (VIs) mapped by remote sensing. Mean absolute error of the best prediction was 14 kg N ha−1. In a practical application, however, there will be additional uncertainty from several sources, e.g. uncertainty in the assessment of yield potential. The best mean N rate model was based on geographical region, cultivar, N uptake in zero-plots and yield potential, while the best model of relative N rate within the field used a new multispectral index (d75r6), which was designed to give a standardized measure of the steepness of the red edge of reflectance of a crop canopy spectrum. Several other multispectral VIs also performed well but red-green-blue indices were less useful. We conclude that remote sensing (to capture within-field spatial variation patterns), on-farm experiments (to determine the field mean N rate), and the farmers’ experience and knowledge on local conditions (e.g. to assess the yield potential), is a useful combination of information sources in decision support systems for variable rate application of N. Options and remaining research needs for the setup of such a system are discussed

Spo0J and SMC are required for normal chromosome segregation in Staphylococcus aureus.

Bacterial chromosome segregation is an essential cellular process that is particularly elusive in spherical bacteria such as the opportunistic human pathogen Staphylococcus aureus. In this study, we examined the functional significance of a ParB homologue, Spo0J, in staphylococcal chromosome segregation and investigated the role of the structural maintenance of chromosomes (SMC) bacterial condensin in this process. We show that neither spo0J nor smc is essential in S. aureus; however, their absence causes abnormal chromosome segregation. We demonstrate that formation of complexes containing Spo0J and SMC is required for efficient S. aureus chromosome segregation and that SMC localization is dependent on Spo0J. Furthermore, we found that cell division and cell cycle progression are unaffected by the absence of spo0J or smc. Our results verify the role of Spo0J and SMC in ensuring accurate staphylococcal chromosome segregation and also imply functional redundancy or the involvement of additional mechanisms that might contribute to faithful chromosome inheritance

Growth and characterization of ZnTe films grown on GaAs, InAs, GaSb, and ZnTe

We report the successful growth of ZnTe on nearly lattice-matched III-V buffer layers of InAs (0.75%), GaSb (0.15%), and on GaAs and ZnTe by molecular beam epitaxy. In situ reflection high-energy electron diffraction measurements showed the characteristic streak patterns indicative of two-dimensional growth. Photoluminescence measurements on these films show strong and sharp features near the band edge with no detectable luminescence at longer wavelengths. The integrated photoluminescence intensity from the ZnTe layers increased with better lattice match to the buffer layer. The ZnTe epilayers grown on high-purity ZnTe substrates exhibited stronger luminescence than the substrates. We observe narrow luminescence linewidths (full width at half maximum ~ 1–2 Å) indicative of uniform high quality growth. Secondary-ion mass spectroscopy and electron microprobe measurements, however, reveal substantial outdiffusion of Ga and In for growths on the III-V buffer layers

Cell shape-independent FtsZ dynamics in synthetically remodeled bacterial cells.

FtsZ is the main regulator of bacterial cell division. It has been implicated in acting as a scaffolding protein for other division proteins, a force generator during constriction, and more recently, as an active regulator of septal cell wall production. FtsZ assembles into a heterogeneous structure coined the Z-ring due to its resemblance to a ring confined by the midcell geometry. Here, to establish a framework for examining geometrical influences on proper Z-ring assembly and dynamics, we sculpted Escherichia coli cells into unnatural shapes using division- and cell wall-specific inhibitors in a micro-fabrication scheme. This approach allowed us to examine FtsZ behavior in engineered Z-squares and Z-hearts. We use stimulated emission depletion (STED) nanoscopy to show that FtsZ clusters in sculpted cells maintain the same dimensions as their wild-type counterparts. Based on our results, we propose that the underlying membrane geometry is not a deciding factor for FtsZ cluster maintenance and dynamics in vivo

Experimental observation of negative differential resistance from an InAs/GaSb interface

We have observed negative differential resistance at room temperature from devices consisting of a single interface between n-type InAs and p-type GaSb. InAs and GaSb have a type II staggered band alignment; hence, the negative differential resistance arises from the same mechanism as in a p+-n+ tunnel diode. Room-temperature peak current densities of 8.2×10^4 A/cm^2 and 4.2×10^4 A/cm^2 were measured for structures with and without undoped spacer layers at the heterointerface, respectively

Upscaling proximal sensor N-uptake predictions in winter wheat (Triticum aestivum L.) with Sentinel-2 satellite data for use in a decision support system

Total nitrogen (N) content in aboveground biomass (N-uptake) in winter wheat (Triticum aestivum L.) as measured in a national monitoring programme was scaled up to full spatial coverage using Sentinel-2 satellite data and implemented in a decision support system (DSS) for precision agriculture. Weekly field measurements of N-uptake had been carried out using a proximal canopy reflectance sensor (handheld Yara N-Sensor) during 2017 and 2018. Sentinel-2 satellite data from two processing levels (top-of-atmosphere reflectance, L1C, and bottom-of-atmosphere reflectance, L2A) were extracted and related to the proximal sensor data (n = 251). The utility of five vegetation indices for estimation of N-uptake was compared. A linear model based on the red-edge chlorophyll index (CI) provided the best N-uptake prediction (L1C data: r(2) = 0.74, mean absolute error; MAE = 14 kg ha(-1)) when models were applied on independent sites and dates. Use of L2A data, rather than L1C, did not improve the prediction models. The CI-based prediction model was applied on all fields in an area with intensive winter wheat production. Statistics on N-uptake at the end of the stem elongation growth stage were calculated for 4169 winter wheat fields > 5 ha. Within-field variation in predicted N-uptake was > 30 kg N ha(-1) in 62% of these fields. Predicted N-uptake was compared against N-uptake maps derived from tractor-borne Yara N-Sensor measurements in 13 fields (1.7-30 ha in size). The model based on satellite data generated similar information as the tractor-borne sensing data (r(2) = 0.81; MAE = 7 kg ha(-1)), and can therefore be valuable in a DSS for variable-rate N application

Vertical transport and electroluminescence in InAs/GaSb/InAs structures: GaSb thickness and hydrostatic pressure studies

We have measured the current-voltage (I-V) of type II InAs/GaSb/InAs double heterojunctions (DHETs) with 'GaAs like' interface bonding and GaSb thickness between 0-1200 \AA. A negative differential resistance (NDR) is observed for all DHETs with GaSb thickness $>$ 60 \AA below which a dramatic change in the shape of the I-V and a marked hysteresis is observed. The temperature dependence of the I-V is found to be very strong below this critical GaSb thickness. The I-V characteristics of selected DHETs are also presented under hydrostatic pressures up to 11 kbar. Finally, a mid infra-red electroluminescence is observed at 1 bar with a threshold at the NDR valley bias. The band profile calculations presented in the analysis are markedly different to those given in the literature, and arise due to the positive charge that it is argued will build up in the GaSb layer under bias. We conclude that the dominant conduction mechanism in DHETs is most likely to arise out of an inelastic electron-heavy-hole interaction similar to that observed in single heterojunctions (SHETs) with 'GaAs like' interface bonding, and not out of resonant electron-light-hole tunnelling as proposed by Yu et al. A Zener tunnelling mechanism is shown to contribute to the background current beyond NDR.Comment: 8 pages 12 fig