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Aluminum lumped-element kinetic inductance detectors (LEKIDs) sensitive tomillimeter-wave photons have been shown to exhibit high quality factors, makingthem highly sensitive and multiplexable. The superconducting gap of aluminumlimits aluminum LEKIDs to photon frequencies above 100 GHz. Manganese-dopedaluminum (Al-Mn) has a tunable critical temperature and could therefore be anattractive material for LEKIDs sensitive to frequencies below 100 GHz if theinternal quality factor remains sufficiently high when manganese is added tothe film. To investigate, we measured some of the key properties of Al-MnLEKIDs. A prototype eight-element LEKID array was fabricated using a 40 nmthick 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. Usingmeasurements of the forward scattering parameter $S_{21}$ at various bathtemperatures between 65 and 250 mK, we determined that the Al-Mn LEKIDs wefabricated have internal quality factors greater than $2 \times 10^5$, which ishigh enough for millimeter-wave astrophysical observations. In the darkconditions under which these devices were measured, the fractional frequencynoise spectrum shows a shallow slope that depends on bath temperature and probetone amplitude, which could be two-level system noise. The anticipated whitephoton noise should dominate this level of low-frequency noise when thedetectors are illuminated with millimeter-waves in future measurements. TheLEKIDs responded to light pulses from a 1550 nm light-emitting diode, and weused these light pulses to determine that the quasiparticle lifetime is 60{\mu}s.

High quality factor manganese-doped aluminum lumped-element kinetic inductance detectors sensitive to frequencies below 100 GHz