Continuous Wave THz System Based on an Electrically Tunable Monolithic Dual Wavelength Y-Branch DBR Diode Laser

We analyse the use of a tunable dual wavelength Y-branch DBR laser diode for THz applications. The laser generates electrically tunable THz difference frequencies in the range between 100 and 300 GHz. The optical beats are tuned via current injection into a micro-resistor heater integrated on top of one of the distributed Bragg reflector (DBR) section of the diode. The laser is integrated in a homodyne THz system employing fiber coupled ion-implanted LT-GaAs log spiral antennas. The applicability of the developed system in THz spectroscopy is demonstrated by evaluating the spectral resonances of a THz filter as well as in THz metrology in thickness determination of a polyethylene sample

Absence of superconductivity in ultra-thin layers of FeSe synthesized on a topological insulator

The structural and electronic properties of FeSe ultra-thin layers on Bi$_{2}$Se$_{3}$ have been investigated with a combination of scanning tunneling microscopy and spectroscopy and angle-resolved photoemission spectroscopy. The FeSe multi-layers, which are predominantly 3-5 monolayers (ML) thick, exhibit a hole pocket-like electron band at \bar{\Gamma} and a dumbbell-like feature at \bar{M}, similar to multi-layers of FeSe on SrTiO$_{3}$. Moreover, the topological state of the Bi2Se3 is preserved beneath the FeSe layer, as indicated by a heavily \it{n}-doped Dirac cone. Low temperature STS does not exhibit a superconducting gap for any investigated thickness down to a temperature of 5 K

Graphene-passivated nickel as an efficient hole-injecting electrode for large area organic semiconductor devices

Efficient injection of charge from metal electrodes into semiconductors is of paramount importance to obtain high performance optoelectronic devices. The quality of the interface between the electrode and the semiconductor must, therefore, be carefully controlled. The case of organic semiconductors presents specific problems: ambient deposition techniques, such as solution processing, restrict the choice of electrodes to those not prone to oxidation, limiting potential applications. Additionally, damage to the semiconductor in sputter coating or high temperature thermal evaporation poses an obstacle to the use of many device-relevant metals as top electrodes in vertical metal–semiconductor–metal structures, making it preferable to use them as bottom electrodes. Here, we propose a possible solution to these problems by implementing graphene-passivated nickel as an air stable bottom electrode in vertical devices comprising organic semiconductors. We use these passivated layers as hole-injecting bottom electrodes, and we show that efficient charge injection can be achieved into standard organic semiconducting polymers, owing to an oxide free nickel/graphene/polymer interface. Crucially, we fabricate our electrodes with low roughness, which, in turn, allows us to produce large area devices (of the order of millimeter squares) without electrical shorts occurring. Our results make these graphene-passivated ferromagnetic electrodes a promising approach for large area organic optoelectronic and spintronic devices.We acknowledge funding from EPSRC (EP/P005152/1, EP/M005143/1). R.M. and K.N. acknowledges funding from the EPSRC Cambridge NanoDTC (Grant No. EP/G037221/1). J.A.-W. acknowledges the support of his Research Fellowship from the Royal Commission for the Exhibition of 1851, and Royal Society Dorothy Hodgkin Research Fellowship. R. S. W. acknowledges support from a CAMS-UK fellowship

Dietary niche partitioning between sympatric brown hares and rabbits

Coexistence of ecologically similar species is sustained by niche partitioning, a fundamental element of which is diet. Overlapping of resource requirements between sympatric species can create interspecific competitive or facilitative effects on the foraging behaviour of herbivores. Brown hares (Lepus europaeus) and European rabbits (Oryctolagus cuniculus) are similar in size, morphology, feeding type and occupy the same habitats, but direct evidence of competition for resources between them is lacking. Both species are widespread and simultaneously pests and species of conservation concern in different parts of their range. We investigated dietary overlap of brown hares and European rabbits in pastures in relation to pasture management and hare and rabbit abundance. Grasses were the predominant component in both hare and rabbit diets with high overlap of plant species. Both rabbits and hares showed some selectivity for particular plants with evidence of consistent selection for Phleum spp. and relative avoidance of Poa spp. However, differences in the smaller components of hare and rabbit diet resulted in significant differences in diet overall. There was no evidence that higher relative density of one species led to dietary shifts but pasture management affected the diet of both species. Nutritional composition of diets of both species also differed between cattle and sheep pastures with higher fibre, ash and fat in the former. Our data provide no evidence of competitive exclusion between rabbits and hares on the basis of diet, but suggest that the effects of livestock on their respective diets may influence indirect competition in favour of rabbits over hares

Quantum Noise and Polarization Fluctuations in Vertical Cavity Surface Emitting Lasers

We investigate the polarization fluctuations caused by quantum noise in quantum well vertical cavity surface emitting lasers (VCSELs). Langevin equations are derived on the basis of a generalized rate equation model in which the influence of competing gain-loss and frequency anisotropies is included. This reveals how the anisotropies and the quantum well confinement effects shape the correlations and the magnitude of fluctuations in ellipticity and in polarization direction. According to our results all parameters used in the rate equations may be obtained experimentally from precise time resolved measurements of the intensity and polarization fluctuations in the emitted laser light. To clarify the effects of anisotropies and of quantum well confinement on the laser process in VCSELs we therefore propose time resolved measurements of the polarization fluctuations in the laser light. In particular, such measurements allow to distinguish the effects of frequency anisotropy and of gain-loss anisotropy and would provide data on the spin relaxation rate in the quantum well structure during cw operation as well as representing a new way of experimentally determinig the linewidth enhancement factor alpha.Comment: 16 pages and 3 Figures, RevTex, to be published in Phys. Rev.

The UV Scattering Halo of the Central Source Associated with Eta Carinae

We have made an extensive study of the UV spectrum of Eta Carinae, and find that we do not directly observe the star and its wind in the UV. Because of dust along our line of sight, the UV light that we observe arises from bound-bound scattering at large impact parameters. We obtain a reasonable fit to the UV spectrum by using only the flux that originates outside 0.033". This explains why we can still observe the primary star in the UV despite the large optical extinction -- it is due to the presence of an intrinsic coronagraph in the Eta Carinae system, and to the extension of the UV emitting region. It is not due to peculiar dust properties alone. We have computed the spectrum of the purported companion star, and show that it could only be directly detected in the UV spectrum preferentially in the Far Ultraviolet Spectroscopic Explorer (FUSE) spectral region (912-1175 Ang.). However, we find no direct evidence for a companion star, with the properties indicated by X-ray studies and studies of the Weigelt blobs, in UV spectra. This might be due to reprocessing of the companion's light by the dense stellar wind of the primary. Broad FeII and [FeII] emission lines, which form in the stellar wind, are detected in spectra taken in the SE lobe, 0.2" from the central star. The wind spectrum shows some similarities to the spectra of the B & D Weigelt blobs, but also shows some marked differences in that high excitation lines, and lines pumped by Ly-alpha, are not seen. The detection of the broad lines lends support to our interpretation of the UV spectrum, and to our model for Eta Carinae.Comment: To appear in ApJ. 57 pages with 18 figure

Performance Analysis of Mobile Agents for Filtering Data Streams on Wireless Networks

Wireless networks are an ideal environment for mobile agents, since their mobility allows them to move across an unreliable link to reside on a wired host, next to or closer to the resources that they need to use. Furthermore, client-specific data transformations can be moved across the wireless link and run on a wired gateway server, reducing bandwidth demands. In this paper we examine the tradeoffs faced when deciding whether to use mobile agents in a data-filtering application where numerous wireless clients filter information from a large data stream arriving across the wired network. We develop an analytical model and use parameters from filtering experiments conducted during a U.S. Navy Fleet Battle Experiment (FBE) to explore the model\u27s implications