Charge Carriers at milliKelvin Temperatures

We describe an experimental cell which enables the cooling of electrons in a solid state host to milliKelvin temperatures. In order to achieve a good thermal contact as well as electrical isolation we use sintered Silver sample heatsinks which are immersed into liquid He-3. We have evaluated our experimental cell by measuring the resistance of a carbon composition thermometer connected to these sample heatsinks. While we cannot directly measure electronic temperatures, we find that the resistance of the carbon thermometer is a strong function of the temperature down to 5 mK. We interpret these results as evidence of electron temperatures in the mK range.