Hot-electron effects, energy transport and decoherence in nano-systems at low temperatures

Energy relaxation and phase relaxation processes are of interest both for better understanding of the physics of nano-scale systems, and for the development of applications based on these processes. In particular, investigation of the energy dissipation processes enables new designs of ultra-sensitive calorimeters; investigation of the dephasing processes in one dimensional metal wires helps us to understand the fundamental limits of phase coherence time at low temperatures.; This thesis consists of two inter-related projects. The first part describes the characterization of superconducting nanostructures used as a sensor for a hot-electron detector operating at sub-Kelvin temperatures. The main driver for this work is the moderate resolution spectroscopy on the future space telescopes with cryogenically cooled (∼ 4 K) mirrors. We fabricated superconducting Ti nanosensors with a volume of ∼ 3 x 10-3 mum 3 and measured the thermal conductance G between the sensor and the thermal bath. Scaling of G with the sample volume observed for large (10-cm-long) samples and sub-micron sensors suggests that the electron-phonon coupling remains the major energy relaxation mechanism in the sensors down to ∼ 0.1 K. A very low G ∼ 2.5 x 10-16 W/K, measured at 60 mK, is due to the weak electron-phonon coupling in the material and the thermal isolation provided by superconducting Nb contacts. This low G corresponds to NEP(60 mK) ∼ 7 x 10-21 W/ Hz . The hot-electron detector is expected to have a sufficient energy resolution for detecting individual photons with nu > 0.1 THz at 0.1 K.; The second part describes the effect of monochromatic microwave radiation on the weak localization corrections to the conductivity of quasi-one-dimensional silver wires. Due to the improved electron cooling in the wires, the MW-induced dephasing was observed without a concomitant overheating of electrons over wide ranges of the MW power PMW and frequency f. The observed dependences of the conductivity and MW-induced dephasing rate on PMW and f are in agreement with the theory by Altshuler, Aronov, and Khmelnitsky. Our results suggest that in the low-temperature experiments with 1D wires, saturation of the temperature dependence of the dephasing time can be caused by an MW electromagnetic noise with a sub-pW power.