Metallic Coulomb Blockade Thermometry down to 10 mK and below

We present an improved nuclear refrigerator reaching 0.3 mK, aimed at microkelvin nanoelectronicexperiments, and use it to investigate metallic Coulomb blockade thermometers (CBTs) with variousresistances R. The high-R devices cool to slightly lower T, consistent with better isolationfrom the noise environment, and exhibit electron-phonon cooling ∝T^5 and a residual heat-leak of40 aW. In contrast, the low-R CBTs display cooling with a clearly weaker T-dependence, deviatingfrom the electron-phonon mechanism. The CBTs agree excellently with the refrigerator temperatureabove 20 mK and reach a minimum-T of 7.5 ± 0.2 mK.