A team led by Prof. Guo Guangcan from the University of Science and Technology of China (USTC) has demonstrated the ability to electrically manipulate the spin filling sequence in a bilayer graphene (BLG) quantum dot (QD). This achievement, published in Physical Review Letters , showcases the potential to control the spin degree of freedom in BLG, a material with promising applications in quantum computing and advanced electronics.
BLG has drawn extensive attention in recent years due to its unique properties. When an out-of-plane electric field is applied, it can generate a tunable band gap. Moreover, the trigonal warping effect, caused by the skew interlayer coupling, gives rise to additional minivalley degeneracy, greatly influencing the behavior of charge carriers. Quantum dot devices, which can precisely control the number of charge carriers, have become a crucial tool for studying these phenomena at the single-particle level.
The research team delved into the intricate dynamics of electron shell structures within bilayer graphene quantum dot, focusing on how these structures can be manipulated through the trigonal warping effect, a unique feature of bilayer graphene. They employed a highly tunable quantum dot device, which provided the means to control the electron filling sequence. They began by applying a small perpendicular electric field, observing that the s-shell filled with four electrons, two with spin-up and two with spin-down, each from opposite valleys.
However, the crux of the study was the observation made when the electric field was increased. The researchers noted a significant change: the s-shell's capacity to hold electrons expanded to 12, with the first six electrons all having the same spin polarization. This shift from a fourfold to a twelvefold degeneracy was attributed to the trigonal warping effect, which became pronounced under the influence of a stronger electric field.
To further elucidate the interplay between spin, valley, and minivalley degrees of freedom, the researchers conducted magnetotransport measurements under external magnetic fields. These measurements provided insights into the spin and valley filling sequences, revealing that the spin filling sequence could be electrically changed from "2 + 2 + 4 + 4" to "6 + 6". This transition indicated that the minivalley degree of freedom could be leveraged to electrically manipulate the spin degree of freedom, a finding with profound implications for quantum control and manipulation of electron states.
The study illustrates that the electrical manipulation of the spin filling sequence can be realized by utilizing the interactions between minivalleys and spin. This threefold minivalley degree of freedom holds promise for generating 3-spin states and simulating SU(3) symmetry. It also paves the way for the exploration of exotic electronic phases in trigonally warped BLG.
