SINGLE-DEFECT THERMOMETER AS A PROBE OF ELECTRON HEATING IN BI

We have studied the-effect of Joule heating on the dynamics of a singl e bistable defect in a submicron Bi wire below 1 K. We interpret the r atio of the two defect transition rates as a local thermometer, via th e detailed balance relation. As the drive current increases, the defec t temperature approaches a power-law dependence with drive, independen t of the nominal lattice temperature. The data are consistent with a s imple model of electron heating, and strong thermal coupling between t he defect and the electron bath below 1 K. A second thermometer, based on the amplitude of the defect-induced resistance fluctuations, does not follow the simple heating model.