Thermal coupling of silicon oscillators in cryogen-free dilution refrigerators

TL;DR

This study investigates the thermal coupling of silicon double paddle oscillators in cryogen-free dilution refrigerators, revealing the importance of shielding and environmental factors in maintaining low temperatures.

Contribution

It demonstrates the critical need for a hermetic shield around the DPO in cryogen-free setups to prevent heating and ensure accurate low-temperature measurements.

Findings

Shielding significantly improves thermal coupling.

Gaps in the shield reduce effectiveness.

Pulse tube vibrations are less impactful than shielding issues.

Abstract

Silicon double paddle osillators (DPO) have been successfully used for measuring the elastic properties of amorphous films down to 10 mK (see for example \cite{Fefferman16,Liu14}). Until now, our group has used a wet dilution refrigerator for the lowest temperature measurements. We present measurements carried out on a Bluefors cryogen-free dilution refrigerator that demonstrate an extreme sensitivity of the thermal coupling of the DPO to its environment. These measurements show that it is necessary to enclose the DPO in a shield at the mixing chamber (MXC) temperature. Any gaps in the shield limit its effectiveness, even if there is no line-of-sight path to the DPO. In the absence of a cryogenic hermetic shield surrounding the DPO, turning off the pulse tube while maintaining the MXC and still temperatures leads to heating of the DPO. This demonstrates that any heating of the sample due to pulse tube vibrations is a less important effect.