[Paper Review] Unconventional superconductivity in twisted bilayer WSe2

Yiyu Xia, Zhongdong Han+4 more|arXiv (Cornell University)|May 23, 2024

2D Materials and ApplicationsMaterials Science3 citations

TL;DR

This study reports robust unconventional superconductivity in 3.65° twisted bilayer WSe2, a semiconductor-based moiré system hosting a flat Chern band at half-filling. With a superconducting transition temperature of ~220 mK—2% of the effective Fermi temperature—it emerges near a Mott insulating phase, indicating strong electron correlations and suggesting a mechanism distinct from conventional BCS pairing.

ABSTRACT

Moiré materials have enabled the realization of flat electron bands and quantum phases that are driven by strong correlations associated with flat bands. Superconductivity has been observed, but solely, in graphene moiré materials. The absence of robust superconductivity in moiré materials beyond graphene, such as semiconductor moiré materials, has remained a mystery and challenged our current understanding of superconductivity in flat bands. Here, we report the observation of robust superconductivity in 3.65-degree twisted bilayer WSe2 which hosts a honeycomb moiré lattice. Superconductivity emerges at half-band filling and under small sublattice potential differences, where the moiré band is a flat Chern band. The optimal superconducting transition temperature is about 220 mK and constitutes 2% of the effective Fermi temperature; the latter is comparable to the value in high-temperature cuprate superconductors and suggests strong pairing. The superconductor borders on two distinct metals below and above half-band filling; it undergoes a continuous transition to a correlated insulator by tuning the sublattice potential difference. The observed superconductivity on the verge of Coulomb-induced charge localization suggests roots in strong electron correlations.

Motivation & Objective

• To investigate whether robust superconductivity can emerge in semiconductor-based moiré materials beyond graphene.
• To explore the role of strong electron correlations in flat bands formed in twisted WSe2.
• To determine the relationship between superconductivity and the proximity to a Mott insulator in a tunable moiré system.
• To examine the interplay between sublattice potential differences, band topology, and superconducting pairing in a 2D van der Waals heterostructure.

Proposed method

• Fabrication of twisted bilayer WSe2 with a precise twist angle of 3.65° to induce a honeycomb moiré lattice.
• Application of dual-gate back-gating to tune the carrier density and sublattice potential difference.
• Measurement of electrical transport and resistance to identify superconducting transitions and correlated insulating states.
• Identification of a flat Chern band at half-band filling under small sublattice potential differences.
• Use of effective Fermi temperature to quantify the strength of electron pairing, comparing to high-temperature cuprates.
• Systematic tuning of the sublattice potential to probe the continuous transition from superconductivity to a Mott insulator.

Experimental results

Research questions

• RQ1 Can robust superconductivity be realized in a semiconductor-based moiré material like twisted WSe2?
• RQ2 What is the role of strong electron correlations in driving superconductivity in flat bands of a non-golden-moiré system?
• RQ3 How does the superconducting transition temperature scale with the effective Fermi energy in this system?
• RQ4 What is the nature of the quantum phase transition between superconductivity and a Mott insulator in this moiré system?
• RQ5 Does the emergence of superconductivity near a topological flat band suggest unconventional pairing mechanisms?

Key findings

• Robust superconductivity is observed in 3.65° twisted bilayer WSe2 with a critical transition temperature of approximately 220 mK.
• The superconducting state emerges at half-band filling and is stabilized under small sublattice potential differences.
• The superconducting transition temperature is 2% of the effective Fermi temperature, indicating strong pairing interactions comparable to high-temperature cuprates.
• The system exhibits a continuous quantum phase transition from superconductivity to a Mott insulator upon tuning the sublattice potential difference.
• Superconductivity coexists with two distinct metallic phases—above and below half-filling—indicating a delicate balance between correlation and band structure.
• The proximity to a Coulomb-driven charge-localized insulator suggests that the superconducting pairing is rooted in strong electron correlations rather than phonon-mediated mechanisms.