Researchers from Delft University of Technology, the University of Manchester, and the National Institute for Materials Science in Japan have made significant strides in understanding and manipulating the Kondo effect in bilayer graphene. This effect, which is a type of quantum phenomenon, could have important implications for the development of next-generation electronic devices, including those used in the energy sector for sensing and control.
The Kondo effect occurs when electrons interact with localized spins in a material, leading to a range of electronic phenomena. In particular, the researchers were interested in the regime where the interacting electrons exhibit both spin and valley degeneracy, resulting in what is known as SU(4) Kondo physics. This regime is challenging to realize in typical mesoscopic systems because it requires a strong interaction between electrons, resulting in a Kondo temperature significantly larger than the spin and valley splittings.
To study this effect, the researchers conducted conductance measurements of a quantum point contact (QPC) in bilayer graphene. Beyond the expected quantized conductance plateaus, which reflect spin and valley degeneracy, they observed an additional subband known as the “0.7 anomaly.” This anomaly exhibited signatures of Kondo physics and a Kondo temperature ranging from approximately 0.5 up to 2.4 K at zero magnetic field, corresponding to Kondo energies between 40 and 200 microelectronvolts. Given that the spin-orbit splitting in bilayer graphene is between 40 and 80 microelectronvolts, the researchers argue that these results are consistent with a transition between four-fold degenerate SU(4) and two-fold degenerate spin-valley locked SU(2) Kondo effects.
Furthermore, the researchers broke the valley degeneracy of the lowest subband by applying an out-of-plane magnetic field. They found that Kondo signatures remained present, indicating a transition from SU(4) to a valley-polarized SU(2) Kondo effect. This demonstrates the versatility of bilayer graphene QPCs for exploring many-body effects.
The practical applications of this research for the energy sector are still in the early stages of exploration. However, the ability to manipulate and control the Kondo effect in bilayer graphene could lead to the development of new types of sensors and control devices that are highly sensitive and energy-efficient. Additionally, the fundamental understanding of many-body effects in bilayer graphene could pave the way for the development of new materials and devices for energy storage and conversion.
The research was published in the journal Nature Communications.
This article is based on research available at arXiv.