Metallic Coulomb Blockade Thermometry down to 10 mK and below

Using a recently developed demagnetization refrigerator aimed at microkelvin nanoelectronic experiments, we investigate metallic Coulomb blockade thermometers (CBTs) with various tunnel-junction resistances Rj. The refrigerator cools as low as 0.3 mK while the CBTs saturate at ~10 mK, consistent with weak electron-phonon coupling for the high-Rj sensor and a residual heat leak of 40 aW. More efficient Wiedemann-Franz cooling contributes noticeably for lower-Rj junctions, though these CBTs appear to be more susceptible to environmental heating. Finally, we discuss possible improvements for cooling nanosamples well below 10 mK.