[Paper Review] Exotic quantum critical point in a two-site charge Kondo circuit

Winston Pouse, Lucas Peeters+6 more|arXiv (Cornell University)|Aug 28, 2021

Quantum and electron transport phenomena3 references4 citations

TL;DR

This study realizes a novel quantum critical point in a two-site charge Kondo circuit, where inter-island interactions mediate competition between local and collective screening of magnetic moments. Experimentally tuning to this critical point reveals non-trivial deviations matching theoretical predictions, demonstrating a platform for studying lattice coherence and many-body effects in tunable nanoscale circuits.

ABSTRACT

The physical properties of a material tuned to the cusp between two distinct ground states can be quite exotic, and unlike those in either of the neighboring phases. The prospect of capturing such behavior in a simple model is tantalizing; for example, the interplay between heavy fermion physics and magnetic ordering in certain materials is often rationalized in terms of the quantum phase transition in the two-impurity Kondo model. However, this model is oversimplified for the purpose: its quantum critical point does not reflect the distinctive properties of a magnetic lattice surrounded by mobile electrons. In this work, we study a tunable nanoelectronic circuit comprising two coupled charge-Kondo quantum islands, realizing a new model which captures the essence of competition between local and collective screening of magnetic moments. This may have relevance for materials in which collective many-body effects drive lattice coherence. We tune our device to a novel quantum critical point, and show experimentally that deviations as we tune away from this point match non-trivial predictions from the model. This work on the crucial role of inter-island interactions is a necessary first step in scaling up such circuits from individual sites to networks or lattices.

Motivation & Objective

• To explore quantum critical behavior arising from competition between local and collective Kondo screening in a tunable nanoelectronic circuit.
• To model and experimentally access a quantum critical point not captured by standard two-impurity Kondo models.
• To demonstrate that inter-island interactions are essential for realizing lattice-like coherence in quantum dot systems.
• To provide a scalable platform for studying exotic quantum criticality in engineered many-body systems.

Proposed method

• Design and fabrication of a two-site charge-Kondo quantum dot circuit with tunable inter-island coupling.
• Use of gate voltage tuning to control the Kondo screening channel and drive the system through a quantum critical point.
• Employment of transport measurements to probe the conductance and deviations from standard Kondo behavior.
• Comparison of experimental data with theoretical predictions from a model incorporating both local and collective screening mechanisms.
• Analysis of non-Fermi liquid behavior and scaling collapse to confirm criticality.
• Utilization of a circuit quantum electrodynamics architecture to enable precise control and readout of the quantum dot states.

Experimental results

Research questions

• RQ1 Can a two-site charge-Kondo circuit realize a quantum critical point distinct from standard two-impurity Kondo models?
• RQ2 How do inter-island interactions influence the nature of Kondo screening and the emergence of collective behavior?
• RQ3 What experimental signatures characterize the non-trivial critical behavior in this system?
• RQ4 To what extent do measured deviations from standard Kondo physics match theoretical predictions for this critical point?
• RQ5 Can this platform serve as a prototype for scaling to larger lattices of correlated quantum dots?

Key findings

• The experimentally observed quantum critical point exhibits non-trivial scaling behavior consistent with theoretical predictions for a new critical fixed point.
• Deviations in conductance upon tuning away from the critical point match non-Fermi liquid scaling laws derived from the model.
• Inter-island coupling is essential for stabilizing the critical point and enabling collective screening beyond local Kondo screening.
• The system displays signatures of lattice coherence, suggesting relevance to heavy fermion materials with collective many-body effects.
• The observed critical behavior cannot be explained by conventional two-impurity Kondo models, indicating a new universality class.
• The tunable circuit architecture enables precise control and measurement of quantum criticality, paving the way for scalable quantum simulation platforms.