Boundary conditions and Berry phase in magnetic nanostructures

The effect of micromagnetic boundary conditions on the Berry curvature and topological Hall effect in granular nanostructures is investi- gated by model calculations. Both free surfaces and grain boundaries between interacting particles or grains affect the spin structure. The Dzyaloshinskii-Moriya interactions yield corrections to the Erdmann-Weierstrass boundary conditions, but the Berry curvature remains an exclusive functional of the local spin structure, which greatly simplifies the treatment of nanostructures. An explicit example is a model nanostructure with cylindrical symmetry whose spin structure is described by Bessel function and which yields a mean-field-type Hall-effect contribution that can be related to magnetic-force-microscopy images

Near-thermal limit gating in heavily-doped III-V semiconductor nanowires using polymer electrolytes

Doping is a common route to reducing nanowire transistor on-resistance but has limits. High doping level gives significant loss in gate performance and ultimately complete gate failure. We show that electrolyte gating remains effective even when the Be doping in our GaAs nanowires is so high that traditional metal-oxide gates fail. In this regime we obtain a combination of sub-threshold swing and contact resistance that surpasses the best existing p-type nanowire MOSFETs. Our sub-threshold swing of 75 mV/dec is within 25% of the room-temperature thermal limit and comparable with n-InP and n-GaAs nanowire MOSFETs. Our results open a new path to extending the performance and application of nanowire transistors, and motivate further work on improved solid electrolytes for nanoscale device applications.Comment: 6 pages, 2 figures, supplementary available at journa

The influence of atmosphere on the performance of pure-phase WZ and ZB InAs nanowire transistors

We compare the characteristics of phase-pure MOCVD grown ZB and WZ InAs nanowire transistors in several atmospheres: air, dry pure N$_2$ and O$_2$, and N$_2$ bubbled through liquid H$_2$O and alcohols to identify whether phase-related structural/surface differences affect their response. Both WZ and ZB give poor gate characteristics in dry state. Adsorption of polar species reduces off-current by 2-3 orders of magnitude, increases on-off ratio and significantly reduces sub-threshold slope. The key difference is the greater sensitivity of WZ to low adsorbate level. We attribute this to facet structure and its influence on the separation between conduction electrons and surface adsorption sites. We highlight the important role adsorbed species play in nanowire device characterisation. WZ is commonly thought superior to ZB in InAs nanowire transistors. We show this is an artefact of the moderate humidity found in ambient laboratory conditions: WZ and ZB perform equally poorly in the dry gas limit yet equally well in the wet gas limit. We also highlight the vital role density-lowering disorder has in improving gate characteristics, be it stacking faults in mixed-phase WZ or surface adsorbates in pure-phase nanowires.Comment: Accepted for publication in Nanotechnolog

Towards low-dimensional hole systems in Be-doped GaAs nanowires

GaAs was central to the development of quantum devices but is rarely used for nanowire-based quantum devices with InAs, InSb and SiGe instead taking the leading role. p-type GaAs nanowires offer a path to studying strongly-confined 0D and 1D hole systems with strong spin-orbit effects, motivating our development of nanowire transistors featuring Be-doped p-type GaAs nanowires, AuBe alloy contacts and patterned local gate electrodes towards making nanowire-based quantum hole devices. We report on nanowire transistors with traditional substrate back-gates and EBL-defined metal/oxide top-gates produced using GaAs nanowires with three different Be-doping densities and various AuBe contact processing recipes. We show that contact annealing only brings small improvements for the moderately-doped devices under conditions of lower anneal temperature and short anneal time. We only obtain good transistor performance for moderate doping, with conduction freezing out at low temperature for lowly-doped nanowires and inability to reach a clear off-state under gating for the highly-doped nanowires. Our best devices give on-state conductivity 95 nS, off-state conductivity 2 pS, on-off ratio ~$10^{4}$, and sub-threshold slope 50 mV/dec at T = 4 K. Lastly, we made a device featuring a moderately-doped nanowire with annealed contacts and multiple top-gates. Top-gate sweeps show a plateau in the sub-threshold region that is reproducible in separate cool-downs and indicative of possible conductance quantization highlighting the potential for future quantum device studies in this material system

Assessment of groundwater contamination in an industrial city, Sialkot, Pakistan

Contamination of groundwater due to heavy metals is one of the most important concerns that have received attention at regional, local and global levels because of their toxicological importance in ecosystems and impact on public health. The present study was designed to assess the quality of groundwater in relation to heavy metal pollution and its implication on human health. The groundwater quality of Sialkot, an industrial city of Pakistan, was evaluated using water samples collected from 25 localities during October-November 2005. Twenty-two physiochemical parameters including pH, Electric Conductivity (EC), Total Dissolved Solids (TDS), Salinity, Temperature, Turbidity, Sulfate (SO4) Chloride (Cl), Total Hardness, Iodide, Fluoride, Ferric (Fe+3), Nitrate (NO3), Manganese (Mn), Total Chlorine, Alkalinity, Zinc (Zn), Lead (Pb), Iron (Fe), Copper (Cu), Nickel (Ni) and Chromium (Cr) were recorded. The results were compared with standard guidelines of World Health Organization (WHO) and Pakistan Standard Quality Control Authority (PSQCA) for groundwater quality. Cluster Analysis (CA) grouped all sites into four zones based on spatial similarities and dissimilarities of physiochemical properties. Zone 1 was highly ontaminated with high level of turbidity; EC, TDS, SO4, Cl, total hardiness, Zn, Pb and Fe concentrations were above the permissible levels of WHO and PSQCA. Cr+6 was detected in nineteen sampling sites and its concentration ranged between 0.01 - 0.30 mg/L. Factor Analysis (FA) and Discriminant Analysis (DA) revealed significant variables including pH, EC, TDS, SO4, NO3, Cl, Total Hardness, Iodide, Total Chlorine, Fluoride, alkalinity, Pb, Fe and Mn which are responsible for variations in groundwater quality and affect water chemistry. The results revealed that the groundwater of the study area cannot be considered of good quality as it is highly turbid (57% of total sites) with high level of Zn, Fe and Pb, which were above WHO and PSQCA permissible limits. The spatial distribution maps of water quality parameters were produced using Geographic Information System (GIS). The distribution maps served as important information to understand ecological status of the groundwater systems and for the identification of groundwater quality parameters withconcentration above the allowable limits of WHO and to fin d out potential areas where water treatment plants/technologies can be targeted in Sialkot

p-GaAs nanowire MESFETs with near-thermal limit gating

Difficulties in obtaining high-performance p-type transistors and gate insulator charge-trapping effects present two major challenges for III-V complementary metal-oxide semiconductor (CMOS) electronics. We report a p-GaAs nanowire metal-semiconductor field-effect transistor (MESFET) that eliminates the need for a gate insulator by exploiting the Schottky barrier at the metal-GaAs interface. Our device beats the best-performing p-GaSb nanowire metal-oxide-semiconductor field effect transistor (MOSFET), giving a typical sub-threshold swing of 62 mV/dec, within 4% of the thermal limit, on-off ratio $\sim 10^{5}$, on-resistance ~700 k$\Omega$, contact resistance ~30 k$\Omega$, peak transconductance 1.2 $\mu$S/$\mu$m and high-fidelity ac operation at frequencies up to 10 kHz. The device consists of a GaAs nanowire with an undoped core and heavily Be-doped shell. We carefully etch back the nanowire at the gate locations to obtain Schottky-barrier insulated gates whilst leaving the doped shell intact at the contacts to obtain low contact resistance. Our device opens a path to all-GaAs nanowire MESFET complementary circuits with simplified fabrication and improved performance

Phytochemical analysis of selected medicinal plants

Four medicinal plants including Ranunculus arvensis, Equisetum ravens, Carathamus lanatus and Fagonia critica were used for the study. All the plants were biologically active and were used for different types of ailments. Keeping in view their importance, this work was carried out to investigate the quantitative determination of their crude phytochemicals, vitamins and protein contents. The quantitative determination of crude phytochemicals (alkaloids, total phenols, flavonoids and saponins) vitamins (riboflavin, vitamin C, niacin and pectin) and protein were determined in the aforementioned herbs. The phytochemicals including alkaloids, total phenols, flavonoids and saponins were determined quantitatively using literature methods. Vitamins were measured using a UV/ visible spectrophotometer (UV- 1601 Shamidzu) and the protein was determined by the Micro KJeldahl’s method (Horwitz et al., 2000). The studied plants showed variable amounts of phytochemicals, vitamins and protein contents. The study is very important, in that it intended to show the contents of the studied medicinal herbs and also provide a scientific data base line which is of particular importance for the local practioners as well as for the local people using these herbs for a variety of body disorders.Key word: Phytochemical analysis, medicinal plants, Pakistan