Understanding unconventional superconductivity in twisted flatlands

Project Information

SuperTwist

Grant agreement ID: 852927

DOI

10.3030/852927

Project closed

EC signature date 4 December 2019

Start date 1 March 2020

End date 28 February 2026

Funded under

• EXCELLENT SCIENCE - European Research Council (ERC)

Total cost

€ 1 780 728,00

EU contribution

€ 1 780 728,00

Coordinated by

LUDWIG-MAXIMILIANS-UNIVERSITAET MUENCHEN
Germany

Periodic Reporting for period 3 - SuperTwist (Understanding unconventional superconductivity in twisted flatlands)

Reporting period: 2023-05-01 to 2024-10-31

Summary of the context and overall objectives of the project

It is widely believed that the development of room temperature superconductivity is one of biggest challenges of modern physics and will lead to a technological revolution. The question of how superconductivity (SC) arises in unconventional superconductors, is one of the major unsolved problems in condensed matter physics, and believed to be key on the path towards room temperature SC. In a breakthrough discovery this year, a group at MIT found a radically new way to create unconventional SC by inter-layer hybridization of two stacked graphene sheets, twisted by a “magic” angle of only 1.1º. This discovery has created unprecedented excitement and multi-disciplinary research activity. As graphene crystals are ultra-clean, highly tuneable and its parent state is well understood, the hope is that the study of these compounds will allow an advance towards the understanding of the microscopic mechanisms of unconventional SCs, and pave the way towards the development of SCs with higher critical temperatures.

In the beginning of this project we knew almost nothing about the exact nature of the SC state in these “magic” angle graphene bilayers, and how far its similarities to high temperature (high-Tc) SCs go. One big roadblock to solve the secrets of the SC in MAG, however, is the fact that most experimental techniques, which were instrumental in studying SC in bulk systems, are not applicable for these truly two-dimensional (2D) nano-scale materials. In this project, we wanted to uncover the nature of SC in “magic” angle bilayer graphene, by experimentally addressing its SC order parameter. While no method alone can definitively reveal it, we proposed an interplay of various experimental techniques, that will develop a radically new, multidisciplinary approach between material science and development of key measurement techniques for 2D materials.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

We have achieved a multitude of seminal scientific results in the general investigation of magic angle twisted bilayer graphene, where we have discovered many new electronic phases, and also have shown novel control knobs for quantum phases in these systems.

In particular we have observed clear topological Chern insulator phases in this system, which is summarized in a series of papers "Symmetry-broken Chern insulators and Rashba-like Landau-level crossings in magic-angle bilayer graphene" (Nature Physics volume 17, pages710–714 (2021)), "Twisted bilayer graphene. IV. Exact insulator ground states and phase diagram" (Physical Review B, 103, 205414 (2021)), "Competing Zero-Field Chern Insulators in Superconducting Twisted Bilayer Graphene" (Phys. Rev. Lett. 127, 197701 (2021)), "Chern mosaic and Berry-curvature magnetism in magic-angle graphene" (Nature Physics volume 18, pages885–892 (2022)).

We have further investigated magic angle graphene devices in ultra-high magnetic fields, where we have observed re-entrant correlated insulating phases at one magnetic flux through the moire unit cell " Observation of re-entrant correlated insulators and interaction driven Fermi surface reconstructions at one magnetic flux quantum per moiré unit cell in magic-angle twisted bilayer graphene" (Physical Review Letters, 128, 217701 (2022)), "Reentrant correlated insulators in twisted bilayer graphene at 25T (2π flux)" (Physical Review Letters, 129, 076401 (2022)).

We have show that superconductivity in twisted bilayer graphene originates from a non-Fermi liquid like metallic state, in analogy to a strange metal state "Quantum-critical behavior in magic-angle twisted bilayer graphene" (Nature Physics, 18, 633 (2022)).

We have constructed gate defined Josephson junctions from magic angle graphene devices, where we have demonstrated their symmetry broken ground states "Symmetry Broken Josephson Junctions and Superconducting Diodes in Magic Angle Twisted Bilayer Graphene" (Nature Communications, 14, 2396 (2023)), "φ0-Josephson junction in twisted bilayer graphene induced by a valley-polarized state" (Physical Review Research, 5, 023029 (2023)).

We have investigate the thermal conductivity of the electrons in the superconducting state in magic angle graphene, where we have demonstrated its power law dependence, which could be consistent with a nodal superconducting gap structure "Revealing the thermal properties of superconducting magic-angle twisted bilayer graphene" (Nano Letters, 22, 6465 (2022)).

We have further constructed a new type of magic angle trilayer graphene system, which show similar superconducting and correlated phases as magic angle bilayer graphene. In this system we were able to show a new method to measure the correlated gaps in the system "Dirac spectroscopy and strongly correlated phases in twisted trilayer graphene" (Nature Materials, 22, 336 (2023)).

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

We have followed mostly the research directions described in the project, however we were also very much following the ongoing trends and developments in this young research field. In particular we have improved the fabrication protocols of these devices and are credit so far with the production of the most homogeneous magic angle graphene devices in this research field. Towards this end we are currently planning to write a detailed paper, which will describe in great detail all the procedures that enables the fabrication of these devices.

The above mentioned work on the reentrant correlated states in high magnetic fields, the study of strange metal and the highly successful observation of the Chern insulator properties in magic angle graphene devices go beyond the initially conceived project plans. However in this young and fast moving research field, these general properties crystalized as absolutely general and fundamental to the system, and their investigation become key to the overall plans of the projects, and to understanding the superconducting properties of this system.

Moreover we have ventured beyond twisted bilayer graphene, as similar new systems have been discovered after the beginning of this project. This entails in particular our efforts on twisted trilayer graphene devices, which we very successfully have adopted as well. Some of these materials platform will be further studied and help to shine light on the most important attributes of twisted bilayer graphene complimentary to the main study of this system.

1.jpg