Unravelling the intertwined correlated states of matter in moiré superlattices

Project Information

UNICOSMOS

Grant agreement ID: 101067977

DOI

10.3030/101067977

Project closed

EC signature date 13 June 2022

Start date 1 September 2022

End date 31 August 2024

Funded under

• Marie Skłodowska-Curie Actions (MSCA)

Total cost

No data

EU contribution

€ 172 750,08

Coordinated by

UNIVERSITA DEGLI STUDI DI TRIESTE
Italy

Periodic Reporting for period 1 - UNICOSMOS (Unravelling the intertwined correlated states of matter in moiré superlattices)

Reporting period: 2022-09-01 to 2024-08-31

Summary of the context and overall objectives of the project

The UNICOSMOS project is focused on understanding the quantum properties of materials known as moiré superlattices or moiré materials from an atomistic point of view. Moiré superlattices are formed when two-dimensional (2D) materials such as graphene or transition-metal dichalcogenides (TMDs) are stacked on top of each other with a slight twist or mismatch in their lattice structures. This results in a nano-scale, periodic pattern called a moiré pattern.
In these moiré superlattices, the mismatch in alignment alters the electronic, optical, and mechanical properties of the original 2D materials. Since 2018, experimentalists have successfully created moiré superlattices, leading to discoveries such as twisted bilayer graphene's unconventional superconductivity and other correlated phases. This has generated tremendous excitement and given rise to the new field of twistronics.
These correlated phenomena are linked to the formation of "electronic flat bands," where electron movement is restricted, leading to an enhancement of electron-electron correlation and novel quantum states. However, theoretical understanding of the interplay between these quantum states remains limited, especially due to the computational challenges involved in modeling the intricate quantum interactions within such materials containing thousands of atoms.
The project's objective is to develop computational methods to better understand these emerging quantum phases and explore their potential applications in quantum technologies like quantum computing and optoelectronics. The research leverages a combination of computational approaches, including tight-binding models and classical force field methods, to simulate these complex materials in ways that current state-of-the-art methods struggle to achieve.
The UNICOSMOS project is organized around three primary research objectives (ROs) to explore and understand the complex quantum phases in moiré superlattices:
1. RO1: Develop tight-binding for moiré systems of transition metal dichalcogenides (TMDs) to study the formation of flat bands and their evolution with twist angle and chemical composition.
2. RO2: Use this framework to extract parameters for relevant model Hamiltonians of moiré flat bands.
3. RO3: Apply advanced methods to solve correlated models and study the phase diagram of experimentally relevant TMDs and other moiré superlattices.

The political and strategic context of the project is set within the European Union's strong focus on quantum research and 2D materials, as evidenced by initiatives like the Graphene Flagship. Europe's investment in these technologies reflects their potential to revolutionize fields like information processing and energy efficiency, making moiré materials a crucial area of study for maintaining Europe's leadership in quantum technologies.

Scientific Impact: Expanding the understanding of the underlying physics of correlated electron systems in moiré materials, leading to potential breakthroughs in superconductivity, topological states, nano-photonics and optoelectronics and quantum information.
Technological Impact: Moiré materials could enable the design of new, highly tunable materials for quantum devices, particularly for quantum computing and low-energy electronics, addressing current technological limitations in these fields.
Economic and Societal Impact: The ability to design and manipulate materials with quantum properties can lead to innovations in sectors like data transmission, storage, and energy-efficient technologies, potentially driving the next generation of quantum industries in Europe and beyond.

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

The project primarily focused on investigating the physics of correlated phases in moiré materials, especially those based on transition metal dichalcogenides and graphene. Studying these correlated phases is generally computationally demanding, even for small systems with only a few dozen atoms. However, moiré materials often involve unit cells with thousands of atoms, making standard first-principles calculations impractical due to their high computational cost. The key challenge was to develop computational frameworks capable of overcoming these challanges by leveraging advanced high-performance computing resources and efficient approximation methods, such as linear-scaling DFT, tight-binding models and classical Montecarlo algorithm (RO1 + RO2). In collaboration with collaborators, these tools were used to interpret the optical signatures of correlated insulating states moiré superlattice (RO3).

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 developed a multiscale computational framework that combines classical force fields with an ab initio tight-binding model, capable of accurately describing the flat bands that emerge in both the valence and conduction bands of moiré transition metal dichalcogenides (TMDs). This approach has been extended to multilayer moiré TMD systems (RO1), enabling to really go beyond the limitations of conventional first-principles methods. Additionally, we interfaced our tight-binding code with the Wannier90 code, enabling the construction of model Hamiltonians for flat bands (RO2), which facilitates the application of more advanced computational techniques within this reduced subspace. Our ability to describe both valence and conduction bands accurately also paves the way for computing optical properties, such as solving the Bethe-Salpeter equation within the tight-binding basis. We applied this framework, along with other computational methods, to analyze the optical signatures of correlated insulating states in a trilayer moiré superlattice—specifically, a natural 2H bilayer of WSe2 stacked on top of a slightly rotated (~3°) MoSe2 layer—and explored how these correlated states can be manipulated by an external electric field. This opens the way to the manipulation and the engineering of correlated phases in strongly correlated multi-orbital systems, with spin, layer and orbital degrees of freedom.

Schematics of electron localization in a moiré TMD with emergent triangular and honeycomb lattices