Observation of the Kondo screening cloud.

Area of Science:

• Condensed Matter Physics
• Quantum Mechanics
• Materials Science

Background:

• The Kondo effect describes how conduction electrons form a spin cloud to screen magnetic impurities in metals.
• The existence and spatial extent of this spin cloud, crucial for understanding the Kondo effect, have remained experimentally unverified and controversial.
• Theoretical models predict the Kondo cloud can extend over micrometres, but direct detection has been elusive.

Purpose of the Study:

• To provide experimental evidence for the existence and spatial extent of the Kondo cloud.
• To investigate the physical manifestation of the Kondo cloud and its characteristic length scale.
• To develop a method for detecting and characterizing exotic spin-correlated systems.

Main Methods:

• Utilized a quantum dot to host a magnetic impurity coupled to a quasi-one-dimensional channel.
• Integrated a Fabry-Pérot interferometer with gate-defined lengths (L) exceeding one micrometre into the channel.
• Measured oscillations in Kondo temperature (T_K) as a function of interferometer length (L) by sweeping gate voltage.

Main Results:

• Observed oscillations in Kondo temperature (T_K) that directly correlate with the interferometer length (L), serving as a signature of the Kondo cloud.
• Demonstrated that the amplitude of T_K oscillations scales with L/ξ_K, where ξ_K is the Kondo length, and is significantly larger for L < ξ_K.
• Showed that T_K oscillations become much weaker when L > ξ_K, indicating the Kondo cloud's spatial extent is primarily governed by ξ_K.

Conclusions:

• The experimental results provide direct evidence for the physical existence of the Kondo cloud and confirm its spatial extent is characterized by the Kondo length (ξ_K).
• The study establishes ξ_K as the sole relevant length parameter for the Kondo effect, with the cloud predominantly residing within this length.
• The developed experimental technique offers a novel approach for probing the spatial distribution of non-Fermi liquids and other spin-correlated quantum phenomena.