16 research outputs found
Kilohertz laser ablation for doping helium nanodroplets
A new setup for doping helium nanodroplets by means of laser ablation at
kilohertz repetition rate is presented. The doping process is characterized and
two distinct regimes of laser ablation are identified. The setup is shown to be
efficient and stable enough to be used for spectroscopy, as demonstrated on
beam-depletion spectra of lithium atoms attached to helium nanodroplets. For
the first time, helium droplets are doped with high temperature refractory
materials such as titanium and tantalum. Doping with the non-volatile DNA basis
Guanine is found to be efficient and a number of oligomers are detected
Critical State Flux Penetration and Linear Microwave Vortex Response in YBa_2Cu_3O_{7-x} Films
The vortex contribution to the dc field (H) dependent microwave surface
impedance Z_s = R_s+iX_s of YBa_2Cu_3O_{7-x} thin films was measured using
suspended patterned resonators. Z_s(H) is shown to be a direct measure of the
flux density B(H) enabling a very precise test of models of flux penetration.
Three regimes of field-dependent behavior were observed: (1) Initial flux
penetration occurs on very low field scales H_i(4.2K) 100Oe, (2) At moderate
fields the flux penetration into the virgin state is in excellent agreement
with calculations based upon the field-induced Bean critical state for thin
film geometry, parametrized by a field scale H_s(4.2K) J_c*d 0.5T, (3) for very
high fields H >>H_s, the flux density is uniform and the measurements enable
direct determination of vortex parameters such as pinning force constants
\alpha_p and vortex viscosity \eta. However hysteresis loops are in
disagreement with the thin film Bean model, and instead are governed by the low
field scale H_i, rather than by H_s. Geometric barriers are insufficient to
account for the observed results.Comment: 20 pages, LaTeX type, Uses REVTeX style files, Submitted to Physical
Review B, 600 dpi PostScript file with high resolution figures available at
http://sagar.physics.neu.edu/preprints.htm
Wide Field Infrared Survey Telescope (WFIRST) Observatory Overview
NASA's Wide Field Infrared Survey Telescope (WFIRST) is being designed to deliver unprecedented capability in dark energy and exoplanet science, and to host a technology demonstration coronagraph for exoplanet imaging and spectroscopy. The observatory design has matured since 2013; we present a comprehensive description of the observatory configuration as refined during the WFIRST Phase-A study. The observatory is based on an existing, repurposed 2.4 meter space telescope coupled with a 288 megapixel near-infrared (0.6 to 2 microns) HgCdTe focal plane array with multiple imaging and spectrographic modes. Together they deliver a 0.28 square degree field of view, which is approximately 100 times larger than the Hubble Space Telescope, and a sensitivity that enables rapid science surveys. In addition, the coronagraph technology demonstration will prove the feasibility of new techniques for exoplanet discovery, imaging, and spectral analysis. A composite truss structure meters both instruments to the telescope assembly, and the instruments and the spacecraft are flight serviceable. We present configuration changes since 2013 that improved interfaces, improved testability, and reduced technical risk. We provide an overview of our Integrated Modeling results, performed at an unprecedented level for a phase-A study, to illustrate performance margins with respect to static wavefront error, jitter, and thermal drift
The Flux-Line Lattice in Superconductors
Magnetic flux can penetrate a type-II superconductor in form of Abrikosov
vortices. These tend to arrange in a triangular flux-line lattice (FLL) which
is more or less perturbed by material inhomogeneities that pin the flux lines,
and in high- supercon- ductors (HTSC's) also by thermal fluctuations. Many
properties of the FLL are well described by the phenomenological
Ginzburg-Landau theory or by the electromagnetic London theory, which treats
the vortex core as a singularity. In Nb alloys and HTSC's the FLL is very soft
mainly because of the large magnetic penetration depth: The shear modulus of
the FLL is thus small and the tilt modulus is dispersive and becomes very small
for short distortion wavelength. This softness of the FLL is enhanced further
by the pronounced anisotropy and layered structure of HTSC's, which strongly
increases the penetration depth for currents along the c-axis of these uniaxial
crystals and may even cause a decoupling of two-dimensional vortex lattices in
the Cu-O layers. Thermal fluctuations and softening may melt the FLL and cause
thermally activated depinning of the flux lines or of the 2D pancake vortices
in the layers. Various phase transitions are predicted for the FLL in layered
HTSC's. The linear and nonlinear magnetic response of HTSC's gives rise to
interesting effects which strongly depend on the geometry of the experiment.Comment: Review paper for Rep.Prog.Phys., 124 narrow pages. The 30 figures do
not exist as postscript file