Using as a thermometer the temperature dependent magneto-transport of a two-dimensional electron gas, we find that effective temperature scales with current as $T_{ \rm e} \sim I^a$, where $a=0.4 \pm 2\%$ in the { \it Shubnikov de-Haas} regime, and $0.53 \pm 2\%$ in both the { \it integer and fractional} quantum Hall effect. This implies the phonon energy emission rate changes from the expected $P\sim T^5$ to $P\sim T^4$. We explain this, as well as the dramatic enhancement in phonon emission efficiency using a hydrodynamic model.
Categories: cond-mat