Aluminum lumped-element kinetic inductance detectors (LEKIDs) sensitive to millimeter-wave photons haveudbeen shown to exhibit high quality factors, making them highly sensitive and multiplexable. The superconductingudgap of aluminum limits aluminum LEKIDs to photon frequencies above 100 GHz. Manganese-dopedudaluminum (Al-Mn) has a tunable critical temperature and could therefore be an attractive material forudLEKIDs sensitive to frequencies below 100 GHz if the internal quality factor remains sufficiently high whenudmanganese is added to the film. To investigate, we measured some of the key properties of Al-Mn LEKIDs.udA prototype eight-element LEKID array was fabricated using a 40 nm thick film of Al-Mn deposited on aud500 µm thick high-resistivity, float-zone silicon substrate. The manganese content was 900 ppm, the measuredudTc = 694 ± 1mK, and the resonance frequencies were near 150 MHz. Using measurements of the forwardudscattering parameter S21 at various bath temperatures between 65 and 250 mK, we determined that theudAl-Mn LEKIDs we fabricated have internal quality factors greater than 2 × 105ud, which is high enough forudmillimeter-wave astrophysical observations. In the dark conditions under which these devices were measured,udthe fractional frequency noise spectrum shows a shallow slope that depends on bath temperature and probeudtone amplitude, which could be two-level system noise. The anticipated white photon noise should dominateudthis level of low-frequency noise when the detectors are illuminated with millimeter-waves in future measurements.udThe LEKIDs responded to light pulses from a 1550 nm light-emitting diode, and we used these lightudpulses to determine that the quasiparticle lifetime is 60 µs.
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