Relativistic Artificial Molecules Realized by Two Coupled Graphene Quantum Dots

Abstract

Coupled quantum dots (QDs), usually referred to as artificial molecules, are important not only in exploring fundamental physics of coupled quantum objects, but also in realizing advanced QD devices. However, previous studies have been limited to artificial molecules with nonrelativistic fermions. Here, we show that relativistic artificial molecules can be realized when two circular graphene QDs are coupled to each other. Using scanning tunneling microscopy (STM) and spectroscopy (STS), we observe the formation of bonding and antibonding states of the relativistic artificial molecule and directly visualize these states of the two coupled graphene QDs. The formation of the relativistic molecular states strongly alters distributions of massless Dirac fermions confined in the graphene QDs. Because of the relativistic nature of the molecular states, our experiment demonstrates that the degeneracy of different angular-momentum states in the relativistic artificial molecule can be further lifted by external magnetic fields. Then, both the bonding and antibonding states are split into two peaks.

Coexistence of electron whispering-gallery modes and atomic collapse states in graphene/WSe2 heterostructure quantum dots

“…Obviously, the WSe2 QD generates a circular p-n junction, i.e., a GQD, on graphene. The almost equally spaced peaks in the spectrum are the quasibound states confined in the GQD via the WGMs (9)(10)(11)(12)(13)(14)(15). Such a result is further confirmed by carrying out STS mapping at different resonance energies (Fig.…”

“…Because of the "small" velocity of the Dirac fermions, a Coulomb impurity in graphene with a charge Z ≥ 1 can result in the formation of atomic collapse states (ACSs) around it (16,17). In previous experiments, pronounced resonances of the two types of the quasibound states were clearly observed (9)(10)(11)(12)(13)(14)(15)(16)(17)(18). Due to their distinct underlying origins, the two quasibound states are expected to be observed in the two different systems.…”

“…For example, the massless Dirac fermions nature of the charge carriers in graphene enables us to demonstrate several oddball predictions by quantum electrodynamics (QED), among which the Klein tunneling (5) and atomic collapse (6)(7)(8) are the two most famous effects that have attracted much attention. Very recently, it was demonstrated that the two effects lead to the formation of two types of quasibound states in graphene (9)(10)(11)(12)(13)(14)(15)(16)(17)(18). The Klein tunneling, i.e., the anisotropic transmission of the massless Dirac fermions across the potential barrier, in graphene leads to the formation of quasibound states in circular p-n junctions, i.e., graphene quantum dots (GQDs), via whispering-gallery modes (WGMs) (9)(10)(11)(12)(13)(14)(15).…”

“…Very recently, it was demonstrated that the two effects lead to the formation of two types of quasibound states in graphene (9)(10)(11)(12)(13)(14)(15)(16)(17)(18). The Klein tunneling, i.e., the anisotropic transmission of the massless Dirac fermions across the potential barrier, in graphene leads to the formation of quasibound states in circular p-n junctions, i.e., graphene quantum dots (GQDs), via whispering-gallery modes (WGMs) (9)(10)(11)(12)(13)(14)(15). Because of the "small" velocity of the Dirac fermions, a Coulomb impurity in graphene with a charge Z ≥ 1 can result in the formation of atomic collapse states (ACSs) around it (16,17).…”

Recent progresses of quantum confinement in graphene quantum dots

“…Recently, researchers have successfully realized the relativistic arti-ficial molecule by using two coupled Klein GQDs with relaticistic Dirac fermions. The bonding and antibonding states are detected and imaged in the relativistic artificial molecule by STM measurements [113]. the lowest quasi-bound state is observed to split into two peaks with the seperation of about 30 meV [113].…”