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UNICOSMOS |
Here is a summary of my MSCA project UNICOSMOS - Unravelling the intertwined correlated states of matter in moire' superlattices.
The recent discovery of various quantum correlated phases, including superconductivity and correlated insulating states, in twisted bilayers of 2D materials has generated significant interest and spurred extensive research into these phenomena, sparking an entire new field of research known as twistronics. The ability to engineer quantum states of matter with a few adjustable experimental parameters, such as the twist angle between two 2D layers, represents a major breakthrough in the field of so-called moiré materials. In particular, moiré materials composed of transition metal dichalcogenides (TMDs) have gained considerable traction as a robust platform for simulating quantum phases of matter on emergent 2D lattices.
Although it is widely recognized that enhanced electron-electron interactions drive the quantum phases in moiré materials, the precise quantum nature of many of these correlated phases remains poorly understood. First-principles theoretical and computational methods offer a powerful means to interpret experimental findings and predict new quantum phases. However, conventional methods such as density functional theory (DFT) are computationally prohibitive for moiré systems, which typically involve unit cells with thousands of atoms, and are generally insufficient for addressing the complexities of strongly correlated materials. A new approach is therefore essential.
In UNICOSMOS I propose to develop an efficient multi-scale framework that integrates various theoretical and computational techniques to investigate quantum phases in TMD moiré superlattices. Specifically, I will combine classical force field calculations, ab initio tight-binding methods, and many-body approaches to overcome the limitations of conventional first-principles methods, while preserving their predictive accuracy.
Schematics of charge-ordered state in a TMD moire trilayer, highlighting the emergence of triangular and honeycomb lattices
"The interplay of field-tunable strongly correlated states in a multi-orbital moiré system":
In this article, we explored the complex behavior of strongly correlated electronic states within a multi-orbital moiré superlattice consisting of a rotated MoSe2 monolayer onto a WSe2 2H bilayer. Using advanced experimental techniques and theoretical modeling, we investigated how correlated states emerge upon hole doping and how they can be controlled and tuned by applying external electric fields perpendicular to the plane of the trilayer. We found that by manipulating the perpendicular electric field, the system transition between &Gamma-derived and K/K'-derived correlated phases, revealing intricate interplay between orbital, spin and layer degrees of freedom and correlated states. This work has significant implications for the development of next-generation electronic devices, where precise control of strongly correlated systems is essential for applications such as quantum computing and advanced electronics.
Funding: This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement no. 101067977
Campbell, A. J., Vitale, V., Brotons-Gisbert, M., Baek, H., Borel, A., Ivanova, T. V., Taniguchi, T., Watanabe, K., Lischner, J., & Gerardot, B. D. The interplay of field-tunable strongly correlated states in a multi-orbital moiré system. Nature Physics 20(4), 589–596 (2024).
Link to Nature Physics Link to arXiv
"Short vs long range exchange interactions in twisted bilayer graphene":
This study examined the impact of long-range interactions using the self-consistent Hartree-Fock (HF) approximation, compared to short-range atomic Hubbard interactions, on the band structure of twisted bilayer graphene (TBG) at charge neutrality across various twist angles. Beginning with atomistic calculations, the quasi-particle band structure of TBG is determined for three types of magnetic orderings: modulated anti-ferromagnetic (MAFM), nematic anti-ferromagnetic (NAFM), and hexagonal anti-ferromagnetic (HAFM). We then extended this analysis by incorporating these magnetic orderings, along with the HF potential, into a continuum model. Away from the magic angle, magnetic ordering has a significant influence on the band structure, surpassing the effect of the HF potential. However, near the magic angle, the HF potential dominates the band structure, with HAFM and MAFM exerting a secondary influence, while NAFM continues to noticeably alter the electronic structure. These findings indicate that the spin-valley degenerate broken symmetry state commonly observed in HF calculations for charge-neutral TBG near the magic angle is likely to favor magnetic ordering, as the Hubbard interactions tend to break this symmetry in favor of spin polarization.
Funding: This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement no. 101067977
Jimeno-Pozo, A., Goodwin, Z. A. H., Pantaleón, P. A., Vitale, V., Klebl, L., Kennes, D. M., Mostofi, A. A., Lischner, J., & Guinea, F. Short versus long range exchange interactions in twisted bilayer graphene. Advanced Physics Research 2(12), 2300048 (2023).
Link Advanced Physics Research Link to arXiv
"Origin and properties of the flat band in NbOCl2 monolayer":
In this work we explore the intriguing electronic properties of a recently synthesized monolayer of niobium oxychloride (NbOCl2). This material features a unique flat band near the Fermi level, a characteristic that can lead to fascinating phenomena such as strong electron correlation effects and potential applications in condensed matter physics. The study explains that the flat band arises due to a combination of the Peierls distortion and the specific electronic structure of niobium atoms in the monolayer. This distortion reduces the symmetry of the lattice, stabilizing the monolayer's structure and contributing to its semiconducting behavior. The research also highlights that the material maintains structural stability even under strain, which suggests potential for integration into flexible electronic devices. Additionally, the article delves into the magnetic and optical properties of the NbOCl2 monolayer. It shows that hole doping can induce a transition from a semiconductor to a ferromagnetic material, with possible applications in spintronics. The presence of a bright exciton with a relatively high binding energy also points to its potential use in optoelectronic devices.
Funding: This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement no. 101067977
Mohammad Ali Mohebpour, Sahar Izadi Vishkayi, Valerio Vitale, Nicola Seriani, and Meysam Bagheri Tagani Phys. Rev. B 110, 035429 (2024)
