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Strong Quasiparticle Trapping In A 6x6 Array Of Vanadium-Aluminum Superconducting Tunnel Junctions
A 6x6 array of symmetrical V/Al/AlOx/Al/V Superconducting Tunnel Junctions
(STJs) was fabricated. The base electrode is a high quality epitaxial film with
a residual resistance ratio (RRR) of ~30. The top film is polycrystalline with
an RRR of ~10. The leakage currents of the 25x25 mm^2 junctions are of the
order of 0.5 pA/mm^2 at a bias voltage of 100 mV, which corresponds to a
dynamical resistance of ~ 3 10^5 ohms. When the array was illuminated by 6 keV
X-ray photons from a 55Fe radioactive source the single photon charge output
was found to be low and strongly dependent on the temperature of the devices.
This temperature dependence at X-ray energies can be explained by the existence
of a very large number of quasiparticle (QP) traps in the Vanadium. QPs are
confined in these traps, having a lower energy gap than the surrounding
material, and are therefore not available for tunneling. The number of traps
can be derived from the energy dependence of the responsivity of the devices
(charge output per electron volt of photon input energy).Comment: 4 pages. presented at Low Temperature Detectors-
Searching for Dust around Hyper Metal-Poor Stars
We examine the mid-infrared fluxes and spectral energy distributions for
metal-poor stars with iron abundances [Fe/H] , as well as two
CEMP-no stars, to eliminate the possibility that their low metallicities are
related to the depletion of elements onto dust grains in the formation of a
debris disk. Six out of seven stars examined here show no mid-IR excess. These
non-detections rule out many types of circumstellar disks, e.g. a warm debris
disk ( K), or debris disks with inner radii AU, such as
those associated with the chemically peculiar post-AGB spectroscopic binaries
and RV Tau variables. However, we cannot rule out cooler debris disks, nor
those with lower flux ratios to their host stars due to, e.g. a smaller disk
mass, a larger inner disk radius, an absence of small grains, or even a
multicomponent structure, as often found with the chemically peculiar Lambda
Bootis stars. The only exception is HE0107-5240, for which a small mid-IR
excess near 10 microns is detected at the 2- level; if the excess is
real and associated with this star, it may indicate the presence of (recent)
dust-gas winnowing or a binary system.Comment: Accepted for publication in Ap
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