Tunable Kondo Effect in Bilayer Graphene Quantum Channel Demonstrates 0.7 Anomaly and Enhanced Kondo Physics

The interplay between electron flow and localized atomic magnetism governs many electronic properties of materials, and researchers are particularly interested in systems exhibiting both spin and valley degeneracy, leading to complex quantum behaviour. Josep Ingla-Aynés, Serhii Volosheniuk, and Talieh S. Ghiasi, working at Delft University of Technology with colleagues including Angelika Knothe from Universität Regensburg and Kenji Watanabe and Takashi Taniguchi from the National Institute for Materials Science, now demonstrate a tunable Kondo effect in bilayer graphene, offering new insights into these fundamental interactions. The team observed a distinct subband within a graphene quantum point contact exhibiting signatures of Kondo physics, with energies ranging from approximately 0. 5 to 2. 4 Kelvin. These results suggest a transition between four-fold and two-fold degenerate Kondo effects, and importantly, the researchers demonstrate control over this behaviour using magnetic fields, paving the way for exploring a wide range of many-body quantum phenomena in this versatile material system.

This work investigates the emergence of Kondo physics in bilayer graphene quantum dots, where both spin and valley degrees of freedom are present. The team demonstrates that these quantum dots exhibit Kondo resonances, confirming the presence of unique electron interactions and offering potential for future spintronic devices.

Temperature and Field Dependence of Zero-Bias Peak

This supplementary material details the behaviour of a bilayer graphene quantum point contact (QPC) under various conditions, including different temperatures, magnetic fields, and gate voltages. The data reveals how a zero-bias peak evolves with temperature and gate voltage, with the peak disappearing at higher temperatures, suggesting thermally activated transport. Applying an in-plane magnetic field causes the peak to develop a minimum, consistent with previous observations. Crucially, an out-of-plane magnetic field causes valley splitting, breaking the degeneracy of the subbands and allowing quantitative determination of the valley g-factor. The clear observation of valley splitting and the determination of the valley g-factor are major findings, confirming the presence of valley degrees of freedom in the bilayer graphene QPC. This detailed characterization of the QPC’s electronic properties is essential for understanding its behaviour and for developing future devices based on bilayer graphene.

Kondo Physics Realized in Bilayer Graphene

Scientists have achieved a significant breakthrough in understanding electron interactions in bilayer graphene (BLG). Their work demonstrates the realization of Kondo physics within a BLG quantum point contact, carefully fabricated and controlled with precisely positioned gates. Experiments revealed a distinct subband, consistent with a phenomenon known as the ‘0. 7 anomaly’, and observed signatures of Kondo physics ranging from approximately 0. 5 to 2.

4 Kelvin. These measurements confirm a transition between different Kondo effect regimes, specifically from a four-fold degenerate state to a two-fold degenerate state, indicating a change in the symmetry of electron interactions. Remarkably, Kondo signatures persisted even with an applied magnetic field, suggesting a transition to a valley-polarized state. This demonstrates the versatility of BLG QPCs as a platform for exploring complex many-body physics and controlling electron behaviour at the quantum level.

Kondo Physics and Valley Polarization in Graphene

Researchers have demonstrated a unique form of electron interaction in bilayer graphene quantum point contacts, revealing a transition between different types of Kondo effects. The team observed a distinct subband exhibiting signatures of Kondo physics, suggesting a shift from a four-fold degenerate interaction to a two-fold degenerate interaction, and further to a valley-polarized state when a magnetic field is applied. This work provides valuable insight into the interplay between itinerant electrons and localized spins in novel materials. The study successfully breaks the valley degeneracy of the lowest subband using an out-of-plane magnetic field, confirming the versatility of bilayer graphene quantum point contacts for exploring many-body effects.