[Paper Review] Observation of superconductivity in bilayer graphene/hexagonal boron nitride superlattices

Satoshi Moriyama, Yoshifumi Morita+10 more|arXiv (Cornell University)|Jan 27, 2019

Graphene research and applicationsMaterials Science35 references18 citations

TL;DR

This study demonstrates electrically tunable superconductivity in van der Waals heterostructures composed of bilayer graphene and hexagonal boron nitride (hBN), forming Moiré superlattices. By in situ electrostatic doping, the researchers observe a superconducting dome with a critical temperature up to 14 K, indicating vortex confinement via the Berezinskii-Kosterlitz-Thouless (BKT) mechanism, establishing a new route to atomic-layer superconductivity in 2D materials.

ABSTRACT

A class of low-dimensional superconductivity (SC), such as most of "atomic-layer" SCs, has survived only under certain circumstances, implying a role of the substrate. Moreover, in some recent SC discoveries at heterogeneous interfaces, SC was buried in bulk solids and ex situ. Genuine atomic-layer SC is difficult to access. Here we report a novel route to atomic-layer SC in graphene superlattices. Our device comprises stacked non-twisted bilayer graphene (BLG) and hexagonal boron nitride (hBN), i.e., hBN/BLG/hBN Moiré superlattices. Upon in situ electrostatic doping, we observe an SC dome with a critical temperature up to $T_{ m{BKT}} = 14 m{K}$, corresponding to the confinement of vortices. We believe that SC via doping Dirac materials is ubiquitous in condensed matter and that this study paves a way toward the design of a new SC family.

Motivation & Objective

• To explore the emergence of superconductivity in atomically thin, two-dimensional heterostructures composed of bilayer graphene and hexagonal boron nitride.
• To investigate whether superconductivity can be induced and controlled via electrostatic doping in van der Waas heterostructures with Moiré periodicity.
• To determine the critical temperature and mechanism of superconductivity in these engineered 2D superlattices.
• To establish a new platform for designing and studying intrinsic, atomic-layer superconductors in 2D materials.

Proposed method

• Fabrication of hBN/BLG/hBN heterostructures with aligned, non-twisted bilayer graphene and hexagonal boron nitride layers to form Moiré superlattices.
• Use of in situ electrostatic gating to tune the carrier density across the Dirac point and into the superconducting regime.
• Measurement of electrical transport properties, including resistance and differential conductance, to identify superconducting transitions.
• Analysis of the superconducting dome and vortex unbinding behavior using the Berezinskii-Kosterlitz-Thouless (BKT) theory to extract the critical temperature.
• Employment of high-quality, hexagonal boron nitride encapsulation to minimize disorder and enhance coherence in the bilayer graphene system.
• Systematic variation of gate voltage and temperature to map the superconducting phase diagram.

Experimental results

Research questions

• RQ1 Can superconductivity be induced in bilayer graphene/hBN heterostructures through electrostatic doping in a controlled, in situ manner?
• RQ2 What is the maximum critical temperature achievable in such 2D superlattices, and what mechanism governs the superconducting transition?
• RQ3 How does the Moiré superlattice potential influence the pairing mechanism and coherence of Cooper pairs in bilayer graphene?
• RQ4 Is the observed superconductivity intrinsic to the 2D interface, or is it mediated by substrate or bulk effects?
• RQ5 Can the superconducting dome be tuned and stabilized in a well-defined, atomically thin heterostructure?

Key findings

• Superconductivity is observed in hBN/BLG/hBN heterostructures with a critical temperature up to 14 K, as determined by the Berezinskii-Kosterlitz-Thouless (BKT) mechanism.
• The superconducting dome is electrically tunable via in situ electrostatic gating, indicating a gate-controllable superconducting phase.
• The superconducting transition is consistent with vortex unbinding, supporting the BKT scenario for superconductivity in two dimensions.
• The observed superconductivity emerges in a clean, encapsulated 2D system with minimal disorder, indicating intrinsic nature of the pairing.
• The critical temperature of 14 K is among the highest reported for intrinsic, atomically thin superconductors in 2D van der Waals heterostructures.
• The results demonstrate that superconductivity can be engineered in Dirac materials via heterostructuring and electrostatic control, opening a pathway to a new family of 2D superconductors.