Millisecond electron–phonon relaxation in ultrathin disordered metal films at millikelvin temperatures

We have measured directly the thermal conductance between electrons and phonons in ultrathin Hf and Ti films at millikelvin temperatures. The experimental data indicate that electron–phonon coupling in these films is significantly suppressed by disorder. The electron cooling time τe follows the T−4 dependence with a record-long value τe=25 ms at T=0.04 K. The hot-electron detectors of far-infrared radiation, fabricated from such films, are expected to have a very high sensitivity. The noise-equivalent power of a detector with the area 1 μm2 and the noise limited by fluctuations of the temperature are expected to be (2–3)×10−20 W/Hz, which is two orders of magnitude smaller than that of the state-of-the-art bolometers.