Observing the universal screening of a Kondo impurity

TL;DR

This paper reports the direct thermodynamic observation of the Kondo impurity's screening process using a novel charge sensor in a tunable circuit, confirming universal behavior and linking microscopic parameters to the Kondo temperature.

Contribution

It introduces a new method combining a charge Kondo circuit with a charge sensor to observe the impurity state and its screening, advancing thermodynamic studies of Kondo systems.

Findings

Universal renormalization flow from free spin to singlet state

Reduction of impurity magnetization during screening

Correlation between Kondo temperature and microscopic parameters

Abstract

The Kondo effect, deriving from a local magnetic impurity mediating electron-electron interactions, constitutes a flourishing basis for understanding a large variety of intricate many-body problems. Its experimental implementation in tunable circuits has made possible important advances through well-controlled investigations. However, these have mostly concerned transport properties, whereas thermodynamic observations - notably the fundamental measurement of the spin of the Kondo impurity - remain elusive in test-bed circuits. Here, with a novel combination of a "charge" Kondo circuit with a charge sensor, we directly observe the state of the impurity and its progressive screening. We establish the universal renormalization flow from a single free spin to a screened singlet, the associated reduction in the magnetization, and the relationship between scaling Kondo temperature and microscopic parameters. In our device, a Kondo pseudospin is realized by two degenerate charge states of a metallic island, which we measure with a non-invasive, capacitively coupled charge sensor. Such pseudospin probe of an engineered Kondo system opens the way to the thermodynamic investigation of many exotic quantum states, including the clear observation of Majorana zero modes through their fractional entropy.