Microwave Loss Reduction in Cryogenically Cooled Conductors

TL;DR

This study measures microwave attenuation of various conductors at room temperature and 4.2 K, showing significant loss reductions upon cooling, especially for copper and gold, with implications for cryogenic receiver circuits.

Contribution

It provides empirical data on microwave loss reduction in conductors at cryogenic temperatures, highlighting the effects of material type and surface treatment.

Findings

Copper and gold show a threefold loss reduction when cooled to 4.2 K.

Passivation with benzotriazole does not affect copper's microwave loss.

Residual resistivity ratio indicates conduction in the anomalous skin effect regime at cryogenic temperatures.

Abstract

Measurements of microwave attenuation at room temperature and 4.2 K have been performed on some conductors commonly used in receiver input circuits. The reduction in loss on cooling is substantial, particularly for copper and plated gold, both of which showed a factor of 3 loss reduction. Copper passivated with benzotriazole shows the same loss as without passivation. The residual resistivity ratio between room temperature and 4.2 K, deduced from the measurements using the classical skin effect formula, was smaller than the measured DC value to a degree consistent with conduction in the extreme anomalous skin effect regime at cryogenic temperatures. The measurements were made in the 5-10 GHz range. The materials tested were: aluminum alloys 1100-T6 and 6061-O, C101 copper, benzotriazole treated C101 copper, and brass plated with electroformed copper, Pur-A-Gold 125-Au soft gold, and BDT200 bright gold.