Page 7 of 9
"Electron-phonon Interaction in Nanostructures at sub-Kelvin Temperatures", Wednesday, November 2, 1:00pm, MH 629
Dragos Anghel, Horia Hulubei National Institute for Physics and Nuclear Engineering
ABSTRACT: Ultrasensitive nanoscopic detectors for electromagnetic radiation consist of thin metallic films deposited on dielectric membranes. The metallic films, of thickness d, of the order of 10 nm, form the thermal sensing element (TSE), which absorbs the incident radiation and measures its power flux or the photons’ energy. To achieve the sensitivity required for space born astronomical observations, the TSE works at temperatures of the order of 0.1 K. The dielectric membranes are used for the thermal insulation of the TSE and are of thickness of the order of 100 nm. In such conditions, the phonon gas in the detector assumes a quasi-two-dimensional distribution, whereas quantization of the electrons wavenumbers in the direction perpendicular to the film surfaces lead to the formation of quasi two-dimensional electronic sub-bands.
We analyze the heat power, P, between electrons and phonons at temperatures below 0.2 K, in detectors structures of such thicknesses. If we denote by Te the electron's temperature and by Tph the phonons temperature, we can write P ≡ P(0)(Te)-P(1)(Te,Tph); P(0) is the power “emitted” by the electron system to the phonons and P(1) is the power “absorbed” by the electrons from the phonons. Due to the quantization of the electronic states and the quasi-two-dimensional distribution of the phonon gas, P(d, Te) and P(d,Tph) show very strong oscillations with d, forming sharp crests almost parallel to the temperature axes. In the valleys between the crests, P ∝ T3.5e - T3.5ph. From valley to crest, P increases by more than one order of magnitude and on the crests P does not have a simple power law dependence on temperature.
The strong modulation of P with the thickness of the film may provide a way to control the electron-phonon heat power and the power dissipation in thin metallic films. Eventually the same mechanism may be used to detect small variations of d or surface contamination.