[Paper Review] Observation of superconductivity with Tc onset at 12K in electrically tunable twisted double bilayer graphene

Cheng Shen, Na Li+12 more|arXiv (Cornell University)|Mar 16, 2019

Graphene research and applications26 citations

TL;DR

This study demonstrates electrically tunable superconductivity in twisted double bilayer graphene with a superconducting transition temperature onset at 12 K. By applying a displacement field, the researchers tune the flat bands in the system, observing correlated insulating states at half-filling and evidence of spin polarization via a g-factor of ~2, establishing the platform as highly tunable for studying quantum many-body phenomena.

ABSTRACT

Electron-electron interactions play an important role in graphene and related systems and can induce exotic quantum states, especially in a stacked bilayer with a small twist angle. For bilayer graphene where the two layers are twisted by a magic angle, flat band and strong many-body effects lead to correlated insulating states and superconductivity. In contrast to monolayer graphene, the band structure of untwisted bilayer graphene can be further tuned by a displacement field, providing an extra degree of freedom to control the flat band that should appear when two bilayers are stacked on top of each other. Here, we report the discovery and characterization of such displacement-field tunable electronic phases in twisted double bilayer graphene. We observe insulating states at a half-filled conduction band in an intermediate range of displacement fields. Furthermore, the resistance gap in the correlated insulator increases with respect to the in-plane magnetic fields and we find that the g factor according to spin Zeeman effect is ~2, indicating spin polarization at half filling. These results establish the twisted double bilayer graphene as an easily tunable platform for exploring quantum many-body states.

Motivation & Objective

• To explore quantum many-body states in twisted double bilayer graphene, a system with tunable flat bands.
• To investigate the role of electron-electron interactions in inducing correlated insulating and superconducting phases.
• To determine how an applied displacement field influences the electronic structure and emergence of correlated states.
• To characterize the spin polarization of the insulating state using in-plane magnetic field dependence.
• To establish twisted double bilayer graphene as a highly tunable platform for studying correlated electron physics.

Proposed method

• Fabrication of twisted double bilayer graphene heterostructures with controlled twist angles between bilayers.
• Application of a displacement field via top and back gates to tune the band structure and induce flat bands.
• Measurement of electrical transport properties, including resistance and Hall effect, under varying displacement fields and magnetic fields.
• Use of in-plane magnetic fields to probe spin polarization via the Zeeman effect and extract the g-factor.
• Analysis of the resistance gap evolution under magnetic fields to infer the nature of the insulating state.
• Correlation of half-filled band filling with the emergence of insulating and superconducting phases.

Experimental results

Research questions

• RQ1 Can superconductivity be induced and tuned in twisted double bilayer graphene via an electric displacement field?
• RQ2 How does the application of a displacement field affect the formation of flat bands and correlated insulating states?
• RQ3 What is the spin polarization nature of the insulating state at half-filling, as revealed by in-plane magnetic field response?
• RQ4 What is the value of the g-factor in the insulating state, and what does it imply about spin Zeeman splitting?
• RQ5 How does the resistance gap in the correlated insulator evolve under in-plane magnetic fields?

Key findings

• Superconductivity with a Tc onset at 12 K is observed in twisted double bilayer graphene under electric field tuning.
• Correlated insulating states emerge at half-filling of the conduction band within an intermediate range of displacement fields.
• The resistance gap in the insulator increases with in-plane magnetic field, indicating enhanced energy splitting.
• The extracted g-factor from Zeeman splitting is approximately 2, indicating strong spin polarization at half filling.
• The system exhibits tunable flat bands via displacement field, enabling control over correlated electronic phases.
• Twisted double bilayer graphene supports a rich phase diagram of correlated states, including superconductivity and spin-polarized insulators.