Anomalous dimensions and phase transitions in superconductors

The anomalous scaling in the Ginzburg-Landau model for the superconducting phase transition is studied. It is argued that the negative sign of the $\eta$ exponent is a consequence of a special singular behavior in momentum space. The negative sign of $\eta$ comes from the divergence of the critical correlation function at finite distances. This behavior implies the existence of a Lifshitz point in the phase diagram. The anomalous scaling of the vector potential is also discussed. It is shown that the anomalous dimension of the vector potential $\eta_A=4-d$ has important consequences for the critical dynamics in superconductors. The frequency-dependent conductivity is shown to obey the scaling $\sigma(\omega)\sim\xi^{z-2}$. The prediction $z\approx 3.7$ is obtained from existing Monte Carlo data.Comment: RevTex, 20 pages, no figures; small changes; version accepted in PR

IFIRS: an Imaging Fourier Transform Spectrometer for NGST

To accomplish the scientific objectives of NGST, the observatory must be equipped with instruments suitable for panchromatic observations across the 1-15 mu m spectral region on the faintest detectable objects. A wide-field imaging spectrometer that is efficient in the confusion limit, which may occur in deep field images, will maximize the scientific return and opportunities for serendipity from NGST. An imaging Fourier transform spectrometer (IFTS) supports these requirements in a low-cost, efficient instrument package that functions as an electronically programmable infrared filter with both imaging and spectroscopic capability. The conceptual design of the Integral Field Infrared Spectrograph (IFIRS) is an imaging FTS configured as a 4-port Michelson interferometer. The added ports are obtained by the use of cube-corner retroreflectors. A 4-port design delivers complementary symmetric and antisymmetric interferograms to the primary and secondary focal plane assemblies (FPAs). In this design, the object field of the complementary input is also imaged and superimposed on each image of the primary input. In operation, when observing the sky in the primary input, the secondary input would be fed with a cold blackbody having negligible radiance. The final interferogram is constructed from the difference between the two outputs (which is therefore also immune to common mode noise) while the normalized ratio of the difference to the sum of the two outputs serves to compensate for temporal variations in the object radiance, and may reveal systematic variations due to telescope throughput or detector drifts. The interferometer aperture, field angle, beam waist control, beamsplitter/beamcombiner co-planar alignment, maximum optical frequency and maximum resolution have been traded at a conceptual level of detail. These tradeoffs suggest that a 12 cm beam splitter diameter is sufficient to accept the throughput of an 8 m primary over a 5.'3 x 5.'3 square field of view

Simulating the Performance of a Fourier Transform Imaging Spectrometer on NGST

Due to its simultaneous deep imaging and integral field spectroscopic capability, an Imaging Fourier Transform Spectrograph (IFTS) is ideally suited to the NGST mission, and offers opportunities for tremendous scientific return in many fields of astrophysical inquiry. We describe the operation and quantify the advantages of an IFTS for space applications. We present the expected signal-to-noise performance of an IFTS on NGST and show that its flexible imaging and spectroscopic capabilities can execute efficiently a substantial fraction of the design reference mission. We have built a high-fidelity model that simulates the interferometric data cubes produced by a space-borne IFTS. This artificial data is invaluable because it allows us to visualize directly the data products and performance of an IFTS for comparison with other instruments. We present simulations of deep ( =~ 10(5) s) NGST IFTS observations of rich star fields and the distant Universe. These simulations demonstrate that an IFTS on NGST will be able to spectroscopically classify stars in crowded fields approaching the confusion limit, find and classify distant supernovae on the basis of their spectral signatures alone in single-epoch images, and identify young, forming super star clusters out to redshifts of z =~ 12

First Observations with the LLNL Optical Imaging Fourier Transform Spectrometer

We present the results of the first observing run with an optical imaging Fourier transform spectrometer (FTS). We have designed and fabricated this FTS for low-background astronomical use as a testbed for a proposed imaging FTS for the Next Generation Space Telescope (NGST). The relatively low background in the optical allows us to mimic the long dwell, step-scan operation of the proposed infrared NGST FTS. In this first data set, we have demonstrated the operation of the system as a multi-band camera and as a medium-resolution 3D spectrometer. Our testbed FTS reflects our current design for the NGST FTS (IFIRS). It is a four-port (two input, two output) Michelson interferometer with two 45 degree, self-compensating beamsplitters and cube-corner retro-reflectors. This system was taken to the 1.5-m McMath-Pierce Solar Observatory (MPSO) in March 1999. MPSO provides a good facility for prototyping astronomical instruments with a horizontal focal plane projected onto a (de)rotating table. We collected data from one output port with an off-the-shelf PixelVision CCD camera with a 1024x1024, thinned SITe chip thermoelectrically cooled to 235K. Our final platescale was about 0.5 arcsec/pixel with an unvignetted field of about 4x4 arcmin. We collected imaging spectroscopy with resolutions of a few to 500 of well-known objects including globular clusters, open clusters, spiral galaxies, elliptical galaxies, and nebular regions. We describe our data reduction procedures with emphasis on the unique aspects of imaging FTS data. We present color-magnitude diagrams of star clusters to demonstrate the utility of the imaging FTS as a camera and compare the signal-to-noise performance with theoretical models and filter-based camera performance. Finally, we present datacubes demonstrating the ability of the imaging FTS to yield ``a spectrum for every pixel''