Millisecond electron-phonon relaxation in ultrathin disordered metal filmsat millikelvin temperatures

We have measured directly the thermal conductance between electrons and pho nons in ultrathin Hf and Ti films at millikelvin temperatures. The experime ntal data indicate that electron-phonon coupling in these films is signific antly suppressed by disorder. The electron cooling time tau (epsilon) follo ws the T-4 dependence with a record-long value tau (epsilon)=25 ms at T=0.0 4 K. The hot-electron detectors of far-infrared radiation, fabricated from such films, are expected to have a very high sensitivity. The noise-equival ent power of a detector with the area 1 mum(2) and the noise limited by flu ctuations of the temperature are expected to be (2-3)x10(-20) W/root Hz, wh ich is two orders of magnitude smaller than that of the state-of-the-art bo lometers. (C) 2001 American Institute of Physics.