The first observation of the phase shift of an electron wave function scattered off a Kondo state ｰ Proof of a prediction made 40 years ago ｰ

Researchers at the University of Tokyo have experimentally confirmed that the phase of an electron wave function scattered off a Kondo state is shifted by 90 degrees using a newly developed two-path interferometer.

The Kondo effect first proposed theoretically by Jun Kondo in 1964 is one of the best known many-body spin related electronic correlations. It arises due to the interaction between a localized spin and many surrounding conducting electrons, resulting in screening of the localized spin. Electronic properties of the Kondo state are described by scattering of an electron incident on the Kondo state. An electron scattered off the Kondo state preserves its spin because the localized spin is screened, but it acquires a π/2 phase shift in the wave function, which can be viewed as a fingerprint of the spin screening. This π/2 phase shift predicted about 40 years ago is known as one of the hallmarks of the Kondo effect. Although numerous experimental attempts have been made to observe it all over the world, it remained elusive due to technical difficulties. 

 A group at Department of Applied Physics at the University of Tokyo, consisting of Dr. Shintaro Takada, Dr. Michihisa Yamamoto (lecturer) and Prof. Dr. Seigo Tarucha, succeeded in observing the Kondo π/2 phase shift for the first time, embedding a quantum dot into a unique two-path interferometer and tuning it into the Kondo regime. The obtained result is clear and of historic importance in the field of electronic properties of solids.

 The key ingredient of this work is use of a newly developed interferometer, which enables detection of the scattering phase with great precision. The group also proved the usefulness of such a two-path interferometer; it works as a new type of electronic device utilizing the phase as an information resource and can therefore be used in quantum information technology.  

 This result was obtained in collaboration with groups at RIKEN, Neel Institute, France, Ludwig-Maximilians University and Bochum Ruhr University, Germany.