Surface plasmons mediated energy transfer from a semiconductor quantum well to an organic overlayer

We consider the resonant energy transfer from a two-dimensional Wannier exciton (donor) to a Frenkel exciton of a molecular crystal overlayer (acceptor) when the active medias are separated by a metallic layer, possibly an electrode. We characterize the effect of the surface plasmon on this process. Using realistic values of material parameters, we show that it is possible to change the transfer rate within typically a factor of 5 (up to 44 according to geometrical configuration). We then take into account the quenching of the organic luminescence due to the proximity with the metal. This latter is significant and affect negatively the total internal efficiency that we discuss for different geometries.Comment: 8 pages, 9 figures. arXiv admin note: text overlap with arXiv:1111.515

Wrestling and wrangling with a worrisome wiki: an account of pedagogical change in the use of a Web 2.0 technology in a first year education course

The delivery of higher education in online and blended modes has implications across a range of contexts – economic, pedagogic, technical and social. This article explores the tensions and contradictions of teaching in a blended learning environment in terms of its pedagogic implications. It reports on how a specific Web 2.0 technology (a wiki) was used over a four-year period with and by students in an Education Course to enhance their learning outcomes during their first year of university study. Student feedback (qualitative and quantitative), and the personal reflections of the first author regarding her teaching approach, kept over a four-year period, provide the dataset for this article. Analysis of these data builds a story of how the wiki developed from an extraneous, inauthentic component of the course to an integral component of a successful teaching and learning experience for both the lead author and the students in the course. This story illustrates how an early career academic wrestled to develop appropriate approaches to adult education; wrangled with largely untested Web 2.0 technologies in higher education; and reaped the rewards of the use of such technologies in enhancing the educational experience of both the students and the lecturer. Although a highly personal account of wrestling, wrangling and reaping, the article provides valuable insights into the importance of establishing and maintaining authentic pedagogic relationships in increasing online educational environments. It cautions that the development of technical skills alone is insufficient to guarantee improved outcomes for students

VLT/X-shooter observations and the chemical composition of cool white dwarfs

We present a model atmosphere analysis of cool hydrogen-rich white dwarfs observed at the Very Large Telescope (VLT) with the X-shooter spectrograph. The intermediate-dispersion and high signal-to-noise ratio of the spectra allowed us to conduct a detailed analysis of hydrogen and heavy element line profiles. In particular, we tested various prescriptions for hydrogen Balmer line broadening parameters and determined the effective temperature and surface gravity of each star. Three objects (NLTT 1675, 6390 and 11393) show the presence of heavy elements (Mg, Al, Ca, or Fe). Our abundance analysis revealed a relatively high iron to calcium ratio in NLTT 1675 and NLTT 6390. We also present an analysis of spectropolarimetric data obtained at the VLT using the focal reducer and low dispersion spectrograph (FORS) and we established strict upper limits on the magnetic field strengths in three of the DAZ white dwarfs and determined the longitudinal magnetic field strength in the DAZ NLTT 10480. The class of DAZ white dwarfs comprises objects that are possibly accreting material from their immediate circumstellar environment and the present study contributes in establishing class properties.Comment: Accepted for publication in Astronomy and Astrophysic

A study of high proper-motion white dwarfs I. Spectropolarimetry of a cool hydrogen-rich sample

We conducted a spectropolarimetic survey of 58 high proper-motion white dwarfs which achieved uncertainties of >2 kG in the Halpha line and >5 kG in the upper Balmer line series. The survey aimed at detecting low magnetic fields (< 100 kG) and helped identify the new magnetic white dwarfs NLTT 2219, with a longitudinal field B_l = -97 kG, and NLTT 10480 (B_l=-212 kG). Also, we report the possible identification of a very low-field white dwarf with B_l = -4.6 kG. The observations show that ~5% of white dwarfs harbour low fields (~10 to ~10^2 kG) and that increased survey sensitivity may help uncover several new magnetic white dwarfs with fields below ~1 kG. A series of observations of the high field white dwarf NLTT 12758 revealed changes in polarity occurring within an hour possibly associated to an inclined, fast rotating dipole. Also, the relative strength of the pi and sigma components in NLTT 12758 possibly revealed the effect of a field concentration ("spot"), or, most likely, the presence of a non-magnetic white dwarf companion. Similar observations of NLTT 13015 also showed possible polarity variations, but without a clear indication of the timescale. The survey data also proved useful in constraining the chemical composition, age and kinematics of a sample of cool white dwarfs as well as in constraining the incidence of double degenerates.Comment: Accepted for publication in MNRA

A new extremely low-mass white dwarf in the NLTT catalogue

We report on the discovery of the extremely low-mass, hydrogen-rich white dwarf, NLTT 11748. Based on measurements of the effective temperature (8540+/-50 K) and surface gravity (log g = 6.20+/-0.15) obtained by fitting the observed Balmer line profiles with synthetic spectra, we derive a mass of 0.167+/-0.005 M_solar. This object is one of only a handful of white dwarfs with masses below 0.2 M_solar that are believed to be the product of close binary evolution with an episode of Roche lobe overflow onto a degenerate companion (neutron star or white dwarf). Assuming membership in the halo population, as suggested by the kinematics and adopting a cooling age of 4.0 - 6.3 Gyrs for the white dwarf, we infer a progenitor mass of 0.87 - 0.93 M_solar. The likely companion has yet to be identified, but a search for radial velocity variations may help constrain its nature.Comment: Accepted for publication in A&A Letter