[Paper Review] Relativistic Artificial Molecules Realized by Two Coupled Graphene Quantum Dots

Zhong-Qiu Fu, Yueting Pan+7 more|arXiv (Cornell University)|Aug 19, 2019

Graphene research and applications39 references4 citations

TL;DR

This study demonstrates relativistic artificial molecules formed by two coupled graphene quantum dots using scanning tunneling microscopy and spectroscopy. The researchers observe bonding and antibonding states of massless Dirac fermions, with magnetic fields lifting degeneracy and splitting molecular states into two peaks, revealing relativistic quantum behavior in a tunable nanoscale system.

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.

Motivation & Objective

• To explore the formation of artificial molecules with relativistic fermions in graphene quantum dots.
• To investigate how relativistic quantum effects manifest in coupled quantum dot systems.
• To experimentally observe and visualize bonding and antibonding molecular states in a system governed by Dirac fermions.
• To examine the influence of external magnetic fields on the degeneracy of angular-momentum states in such relativistic artificial molecules.

Proposed method

• Scanning tunneling microscopy (STM) and spectroscopy (STS) are used to image and probe electronic states in two circular graphene quantum dots.
• The graphene quantum dots are fabricated on a hexagonal boron nitride substrate to preserve Dirac fermion behavior.
• Tunneling spectroscopy measures the local density of states to identify bonding and antibonding molecular states.
• External magnetic fields are applied to probe Landau level splitting and lift degeneracy in the molecular states.
• The system's electronic structure is analyzed to confirm the relativistic nature of the molecular orbitals.
• Theoretical modeling supports the observed splitting patterns and confirms the role of massless Dirac fermions.

Experimental results

Research questions

• RQ1 Can relativistic artificial molecules be formed in coupled graphene quantum dots?
• RQ2 How do bonding and antibonding states of Dirac fermions manifest in such a system?
• RQ3 To what extent does an external magnetic field lift the degeneracy of angular-momentum states in these relativistic molecular systems?
• RQ4 What is the spatial distribution of massless Dirac fermions in the molecular states?
• RQ5 How do the spectroscopic signatures of these states differ from those in nonrelativistic artificial molecules?

Key findings

• Bonding and antibonding molecular states are directly visualized in two coupled graphene quantum dots using STM/STS.
• The molecular states are formed by massless Dirac fermions, confirming relativistic quantum behavior in the system.
• Magnetic fields split both bonding and antibonding states into two distinct peaks, lifting degeneracy of angular-momentum states.
• The splitting pattern is consistent with relativistic Landau level quantization in graphene.
• The spatial distribution of Dirac fermions is significantly altered by the formation of molecular states.
• The observed spectroscopic features match theoretical predictions for relativistic artificial molecules, validating the system as a tunable platform for relativistic quantum phenomena.