FLAMINGOS Near Infra-Red Survey of The Serpens Cloud Main Core

We obtained JHK images and low-resolution JH spectra in the embedded young cluster in the Serpens cloud Main core (also known as Serpens North). We determined spectral types for 15 previously identified cluster members (for 5 of them for the first time), 1 new candidate, and 11 stars that appear to be field interlopers. Extinction, for which we derived an analytical expression, was obtained by taking SpT and near-IR excess into account. The location on the Hertzsprung-Russell diagram indicates that we probed a low-mass population of the cloud (0.05 - 1.5 Msol), including 1 - 3 brown dwarfs. We used our individually determined photospheric parameters to analyze the ISO and the Spitzer determined spectral energy distribution classes. The latter were correlated with the age and location of the sources in the cloud. We find that most flat objects from our study (4 out of 5) have SEDs consistent with reddened classical T Tau stars; however, when comparing to the thick disk SEDs of lower mass M-type objects, we find that the flat ones show more excess, perhaps indicating an earlier evolutionary stage. We determined a median age for the cluster to be 1 Myr for distance of 380 pc, and 3 Myr for a less likely distance of 260 pc. The core of the cluster is on average younger than the rest of the cluster. We do not find objects with disks past 5 Myr. We do find diskless, X-ray bright objects younger than 1 Myr, as was also noted in the study of Winston et al (2009). We find two groups of young objects associated with dark filaments, indicating that star formation was not always confined to the core.Comment: 35 pages. A 4.7 Mb version with high resolution figures is available at http://ster.kuleuven.be/~nadya/serpens_highres.pd

Surface Resistance Imaging with a Scanning Near-Field Microwave Microscope

We describe near-field imaging of sample sheet resistance via frequency shifts in a resonant coaxial scanning microwave microscope. The frequency shifts are related to local sample properties, such as surface resistance and dielectric constant. We use a feedback circuit to track a given resonant frequency, allowing measurements with a sensitivity to frequency shifts as small as one parts in 50000 for a 30 ms sampling time. The frequency shifts can be converted to sheet resistance based on a simple model of the system.Comment: 6 pages, 3 figures; for color versions of figures see www.csr.umd.edu/research/hifreq/micr_microscopy.htm

Low Power Superconducting Microwave Applications and Microwave Microscopy

We briefly review some non-accelerator high-frequency applications of superconductors. These include the use of high-Tc superconductors in front-end band-pass filters in cellular telephone base stations, the High Temperature Superconductor Space Experiment, and high-speed digital electronics. We also present an overview of our work on a novel form of near-field scanning microscopy at microwave frequencies. This form of microscopy can be used to investigate the microwave properties of metals and dielectrics on length scales as small as 1 mm. With this microscope we have demonstrated quantitative imaging of sheet resistance and topography at microwave frequencies. An examination of the local microwave response of the surface of a heat-treated bulk Nb sample is also presented.Comment: 11 pages, including 6 figures. Presented at the Eight Workshop on RF Superconductivity. To appear in Particle Accelerator

Echo spectroscopy of bulk Bogoliubov excitations in trapped Bose-Einstein condensates

We propose and demonstrate an echo method to reduce the inhomogeneous linewidth of Bogoliubov excitations, in a harmonically-trapped Bose-Einstein condensate. Our proposal includes the transfer of excitations with momentum +q to -q using a double two photon Bragg process, in which a substantial reduction of the inhomogeneous broadening is calculated. Furthermore, we predict an enhancement in the method's efficiency for low momentum due to many-body effects. The echo can also be implemented by using a four photon process, as is demonstrated experimentally.Comment: 4 pages, 5 figure

Near-Field Scanning Microwave Microscopy: Measuring Local Microwave Properties and Electric Field Distributions

We describe the near-field microwave microscopy of microwave devices on a length scale much smaller than the wavelength used for imaging. Our microscope can be operated in two possible configurations, allowing a quantitative study of either material properties or local electric fields.Comment: 4 pages, 8 figures, minor corrections to text and 2 figure

Imaging of Microwave Permittivity, Tunability, and Damage Recovery in (Ba,Sr)TiO3 Thin Films

We describe the use of a near-field scanning microwave microscope to quantitatively image the dielectric permittivity and tunability of thin-film dielectric samples on a length scale of 1 micron. We demonstrate this technique with permittivity images and local hysteresis loops of a 370 nm thick barium strontium titanate thin film at 7.2 GHz. We also observe the role of annealing in the recovery of dielectric tunability in a damaged region of the thin film. We can measure changes in relative permittivity as small as 2 at 500, and changes in dielectric tunability as small as 0.03 V$^{-1}$.Comment: 5 pages, 2 figures. To be published in Applied Physics Letters, Nov. 15, 199

Near-Field Microwave Microscopy of Materials Properties

Near-field microwave microscopy has created the opportunity for a new class of electrodynamics experiments of materials. Freed from the constraints of traditional microwave optics, experiments can be carried out at high spatial resolution over a broad frequency range. In addition, the measurements can be done quantitatively so that images of microwave materials properties can be created. We review the five major types of near-field microwave microscopes and discuss our own form of microscopy in detail. Quantitative images of microwave sheet resistance, dielectric constant, and dielectric tunability are presented and discussed. Future prospects for near-field measurements of microwave electrodynamic properties are also presented.Comment: 31 pages, 9 figures, lecture given at the 1999 NATO ASI on Microwave Superconductivity Changes suggested by editor, including full reference

ERPs and task effects in the auditory processing of gender agreement and semantics in French

We investigated task effects on violation ERP responses to Noun-Adjective gender mismatches and lexical/conceptual semantic mismatches in a combined auditory/visual paradigm in French. Participants listened to sentences while viewing pictures of objects. This paradigm was designed to investigate language processing in special populations (e.g., children) who may not be able to read or to provide stable behavioral judgment data. Our main goal was to determine how ERP responses to our target violations might differ depending on whether participants performed a judgment task (Task) versus listening for comprehension (No-Task). Characterizing the influence of the presence versus absence of judgment tasks on violation ERP responses allows us to meaningfully interpret data obtained using this paradigm without a behavioral task and relate them to judgment-based paradigms in the ERP literature. We replicated previously observed ERP patterns for semantic and gender mismatches, and found that the task especially affected the later P600 component

Superconducting Material Diagnostics using a Scanning Near-Field Microwave Microscope

We have developed scanning near-field microwave microscopes which can image electrodynamic properties of superconducting materials on length scales down to about 2 $\mu$m. The microscopes are capable of quantitative imaging of sheet resistance of thin films, and surface topography. We demonstrate the utility of the microscopes through images of the sheet resistance of a YBa2Cu3O7-d thin film wafer, images of bulk Nb surfaces, and spatially resolved measurements of Tc of a YBa2Cu3O7-d thin film. We also discuss some of the limitations of the microscope and conclude with a summary of its present capabilities.Comment: 6 pages with 9 figures, Proceedings of the Applied Superconductivity Conference 199