Observing the universal screening of a Kondo impurity.

Area of Science:

• Condensed Matter Physics
• Quantum Many-Body Physics

Background:

• The Kondo effect explains electron interactions with magnetic impurities.
• Experimental studies primarily focused on transport properties.
• Thermodynamic measurements, like impurity spin, were elusive.

Purpose of the Study:

• To directly observe the Kondo impurity state and its screening.
• To investigate thermodynamic properties in tunable circuits.
• To establish the renormalization flow and its relation to microscopic parameters.

Main Methods:

• Utilized a novel 'charge' Kondo circuit combined with a charge sensor.
• Employed a non-invasive, capacitively coupled charge sensor.
• Realized a Kondo pseudospin using degenerate charge states of a metallic island.

Main Results:

• Directly observed the Kondo impurity state and its progressive screening.
• Established the universal renormalization flow from a free spin to a screened singlet.
• Measured the reduction in magnetization and the Kondo temperature's scaling.

Conclusions:

• Engineered Kondo systems can be probed thermodynamically using pseudospins.
• This approach enables the study of exotic quantum states.
• Facilitates clear observation of Majorana zero modes via fractional entropy.