ASC’98 DRAFT 10:36 AM 8/24/98 Optimal Choice of Material for HEB Superconducting Mixers

Abstract-We demonstrate that a potential distinction in ultimate performance of phonon-cooled and diffusion-cooled HEB mixers is not due to the cooling mechanisms but rather due to the different properties of available superconductors. The only available material for a phonon-cooled mixer with sufficiently large IF bandwidth (-4 GHz) is NbN, whereas a variety of clean materials (e.g., Nb, NbC, AI) are suitable for a diffusion-cooled mixer. For a readily achievable device length of 0.1 km for example, the IF bandwidth can be 2 10 GHz. The requirement of low local oscillator (LO) power can also be more easily met in diffusion-cooled devices by selection of a material with lower critical temperature and low density o f electron states. In contrast, the parameters in the NbN-based mixer cannot be widely varied because of the high resistivity and high transition temperature of the material and the necessity of using ultrathin f lms. Given the limited availability of LO power from compact solid-state sources at frequencies above lTHz, a diffusion-cooled mixer based on aluminum is a very attractive choice for low-background radioastronomy applications. I