Wireless local area network planning: an overview

When planning a wireless local area network, there are design issues that need to be considered. In this paper, the fundamentals of planning a wireless local area network are introduced and discussed to highlight the requirements involved. Network constraints, as their relevance to wireless network design is investigated. The paper concludes with an overview of wireless network planning solutions including commercial and free software, and an introduction to the author’s research

Age, Growth, Life History, and Fisheries of the Sand Sole, Psettichthys melanostictus

Sand sole, Psettichthys melanostictus, is a small but important part of the west coast groundfish fishery. It has never been assessed and there is a limited amount of biological data for the species. We provide the first estimates of age and growth for California populations and compare them with studies from other areas. We found that sand sole is a rapidly growing species which may show a strong latitudinal gradient in growth rate. We also found evidence of a recent, strong cohortrelated shift in the sex ratio of the population towards fewer females. In addition we examined data from the Washington, Oregon, and California commercial fishery to make an initial determination of population status. We found that catch per unit of effort in commercial trawls experienced a decline over time but has rebounded in recent years, except central California (the southern part of its commercial range), where the decline has not reversed

Probing short-range magnetic order in a geometrically frustrated magnet by spin Seebeck effect

Competing magnetic interactions in geometrically frustrated magnets give rise to new forms of correlated matter, such as spin liquids and spin ices. Characterizing the magnetic structure of these states has been difficult due to the absence of long-range order. Here, we demonstrate that the spin Seebeck effect (SSE) is a sensitive probe of magnetic short-range order (SRO) in geometrically frustrated magnets. In low temperature (2 - 5 K) SSE measurements on a model frustrated magnet \mathrm{Gd_{3}Ga_{5}O_{12}}, we observe modulations in the spin current on top of a smooth background. By comparing to existing neutron diffraction data, we find that these modulations arise from field-induced magnetic ordering that is short-range in nature. The observed SRO is anisotropic with the direction of applied field, which is verified by theoretical calculation.Comment: 5 pages, 4 figure

A study of brown dwarf and star formation in NGC 2264

Brown dwarfs are substellar objects intermediate between stars and planets that are not massive enough to sustain stable hydrogen fusion (<75M[sub](Jup)). They are a natural outcome of the processes that lead to the formation of stars and planets, and we find them in abundance in every environment we have searched with sufficient depth. Brown dwarfs are not rare; we find a brown dwarf for every 2 -- 5 stars, equating to tens of billions across the Milky Way. However, despite their prevalence, the inherent faintness of brown dwarfs makes studying them challenging. Many fundamental questions about these objects therefore remain unanswered, key among them: how do brown dwarfs form? The overall aim of this thesis has been to work towards this question and better understand the formation of brown dwarfs. To achieve this goal, my research has focused on brown dwarf and star formation in the young cluster, NGC 2264. The aim of studying this cluster was to build the large and well characterised sample of young brown dwarfs needed to answer the outstanding questions in the field of brown dwarf formation. Although NGC 2264 is more distant than the nearest star forming regions, I have shown that it is still accessible for detailed spectroscopic observations of young substellar objects, and crucially has the much larger population that is needed to achieve a robust statistical analysis of the key tracers of brown dwarf formation. I have identified 448 brown dwarf candidates in NGC 2264, based on a combination of NIR colours and additional signatures of youth. I have utilised follow up spectroscopy to show this selection method significantly improves selection efficiency. I find that the star-to-brown dwarf ratio and the slope of the substellar mass function for NGC 2264 are consistent with other young clusters, which points to a uniform IMF across a diverse range of star forming environments. I find no evidence for a variation in the star-to-brown dwarf ratio due to stellar surface density or the presence of OB stars. This rules out brown dwarf formation scenarios that predict strong variations in the relative frequency of brown dwarfs due to environmental conditions. Using Gaia EDR3 kinematics and photometry, I have selected a clean sample of 664 NGC 2264 stellar cluster members and divided these members into north and south sub-clusters using a K-means clustering algorithm. Through comparison with isochrones I have shown that the average age of the northern sub-cluster is ~1 Myr younger than the southern sub-cluster. Using MIR Spitzer data I have classified the disc morphology of the cluster members, based on the slope of their infrared excess. I have shown that despite the younger age of the northern sub-cluster the disc fraction is approximately the same for both the north and south (~ 40%). I attribute this dearth of northern discs to photoevaporation from S Mon, which contains a massive O7 type star - the only O type star in NGC 2264. I have shown that there is a reduction in the local disc fraction up to 1.5 pc away from S Mon, implying that O type stars have a significant and far reaching impact on the local disc lifetime

Antiferromagnetic spin Seebeck Effect

We report on the observation of the spin Seebeck effect in antiferromagnetic MnF$_2$. A device scale on-chip heater is deposited on a bilayer of Pt (4 nm)/MnF$_2$ (110) (30 nm) grown by molecular beam epitaxy on a MgF$_2$ (110) substrate. Using Pt as a spin detector layer it is possible to measure thermally generated spin current from MnF$_2$ through the inverse spin Hall effect. The low temperature (2 - 80 K) and high magnetic field (up to 140 kOe) regime is explored. A clear spin flop transition corresponding to the sudden rotation of antiferromagnetic spins out of the easy axis is observed in the spin Seebeck signal when large magnetic fields (>9 T) are applied parallel the easy axis of the MnF$_2$ thin film. When magnetic field is applied perpendicular to the easy axis, the spin flop transition is absent, as expected

Charge pair dissociation and recombination dynamics in a P3HT–PC60BM bulk heterojunction

The authors thank the EPSRC, SUPA, the Condensed Matter Doctoral Training Centre and the European Union Seventh Framework programme (grant agreement 321305) for financial support.The mechanism by which Coulombically bound charge pairs dissociate into free carriers in photovoltaic donor–acceptor blends is of great interest. Here, we use polarization-sensitive transient absorption (TA) to study the diffusion of photogenerated holes in a polythiophene (P3HT)–fullerene (PC60BM) blend. We observe an initial anisotropy value of 0.4 for the absorption of photogenerated holes, indicating that holes generated on a 100 fs time scale are localized on the same polymer chain as their precursor excitons. Depolarization dynamics indicate fast initial hole motion on a 0.3 ps time scale and slower migration up to 100 ps. Charge pair recombination is found to occur on a much longer time scale of 10–1000 ns via a purely bimolecular process independent of excess energy. Our results show that nearly all charge pairs get separated by at least 7 nm in the absence of an external field and indicate that high charge mobility is crucial for charge separation.PostprintPeer reviewe

The first spectroscopically confirmed brown dwarfs in NGC 2264

We present spectroscopic follow-up observations of 68 red, faint candidates from our multi-epoch, multiwavelength, previously published survey of NGC 2264. Using near-infrared spectra from VLT/KMOS, we measure spectral types and extinction for 32 young low-mass sources. We confirm 13 as brown dwarfs in NGC 2264, with spectral types between M6 and M8, corresponding to masses between 0.02 and 0.08 M⊙. These are the first spectroscopically confirmed brown dwarfs in this benchmark cluster. 19 more objects are found to be young M-type stars of NGC 2264 with masses of 0.08-0.3 M⊙. 7 of the confirmed brown dwarfs as well as 15 of the M-stars have IR excess caused by a disc. Comparing with isochrones, the typical age of the confirmed brown dwarfs is &lt;0.5-5 Myr. More than half of the newly identified brown dwarfs and very low-mass stars have ages &lt;0.5 Myr, significantly younger than the bulk of the known cluster population. Based on the success rate of our spectroscopic follow-up, we estimate that NGC 2264 hosts 200-600 brown dwarfs in total (in the given mass range). This would correspond to a star-to-brown dwarf ratio between 2.5:1 and 7.5:1. We determine the slope of the substellar mass function as $\alpha = 0.43^{+0.41}_{-0.56}$; these values are consistent with those measured for other young clusters. This points to a uniform substellar mass function across all star-forming environments.</p