Observation of enhanced thermal noise due to multiple Andreev reflection in ballistic InGaAs-based superconducting weak links

We have experimentally obtained clear evidence of enhanced thermal noise in a ballistic superconductor-normal metal-superconductor junction with an InGaAs-based two-dimensional electron gas (2DEG). The thermal noise was estimated from a comparison of measured current-voltage characteristics with those obtained with an extended Ambegaokar and Halperin theory. As a consequence, we have observed enhanced thermal noise that is much larger than that expected with normal reservoirs. This enhanced thermal noise can be explained by a theory that considers both the ballistic transport of the 2DEG and the thermal fluctuation in the coherent multiple Andreev reflection regime.