Title: High quality factor manganese-doped aluminum lumped-element kinetic inductance detectors sensitive to frequencies below 100 GHz,
Abstract: Aluminum lumped-element kinetic inductance detectors (LEKIDs) sensitive to
millimeter-wave photons have been shown to exhibit high quality factors, making
them highly sensitive and multiplexable. The superconducting gap of aluminum
limits aluminum LEKIDs to photon frequencies above 100 GHz. Manganese-doped
aluminum (Al-Mn) has a tunable critical temperature and could therefore be an
attractive material for LEKIDs sensitive to frequencies below 100 GHz if the
internal quality factor remains sufficiently high when manganese is added to
the film. To investigate, we measured some of the key properties of Al-Mn
LEKIDs. A prototype eight-element LEKID array was fabricated using a 40 nm
thick film of Al-Mn deposited on a 500 {\mu}m thick high-resistivity,
float-zone silicon substrate. The manganese content was 900 ppm, the measured
$T_c = 694\pm1$ mK, and the resonance frequencies were near 150 MHz. Using
measurements of the forward scattering parameter $S_{21}$ at various bath
temperatures between 65 and 250 mK, we determined that the Al-Mn LEKIDs we
fabricated have internal quality factors greater than $2 \times 10^5$, which is
high enough for millimeter-wave astrophysical observations. In the dark
conditions under which these devices were measured, the fractional frequency
noise spectrum shows a shallow slope that depends on bath temperature and probe
tone amplitude, which could be two-level system noise. The anticipated white
photon noise should dominate this level of low-frequency noise when the
detectors are illuminated with millimeter-waves in future measurements. The
LEKIDs responded to light pulses from a 1550 nm light-emitting diode, and we
used these light pulses to determine that the quasiparticle lifetime is 60
{\mu}s.