Transmission Phase in the Kondo Regime Revealed in a Two-Path Interferometer

S. Takada, C. Bäuerle, M. Yamamoto, S. Hermelin, T. Meunier, A. Alex, A. Weichselbaum, J. von Delft, A. D. Wieck, S. Tarucha

Published 2013-11-27, updated 2014-09-16Version 2

We report on the direct observation of the transmission phase shift through a Kondo correlated quantum dot by employing a new type of two-path interferometer. We observed a clear $\pi/2$-phase shift, which persists up to the Kondo temperature $T_{\rm K}$. Above this temperature, the phase shifts by more than $\pi/2$ at each Coulomb peak, approaching the behavior observed for the standard Coulomb blockade regime. These observations are in remarkable agreement with 2-level numerical renormalization group calculations. The unique combination of experimental and theoretical results presented here fully elucidates the phase evolution in the Kondo regime.