Speaker: Professor Agustin Schiffrin
Affiliation: Monash University


Two-dimensional (2D) and layered electronic materials with specific crystal structures can host electronic wavefunctions that interfere destructively, resulting in highly localised electronic states. When filled gradually, these localised states can give rise to increasing Coulomb electron-electron interactions. As a result, control over these Coulomb interactions via electron filling of the material’s valence band can lead to a vast range of quantum phases where electrons are strongly correlated. These correlated-electron phases can include metal-toinsulator transitions as well as a variety of different magnetically ordered configurations. One example of such a crystal structure is the kagome lattice, where atoms or molecules are arranged in corner-sharing equilateral triangles. Strong electronelectron interactions and correlated-electron quantum phases have been observed in inorganic kagome crystals; however, they remain elusive in organic and metalorganic materials. Such systems yield promise as functional electronic materials given their versatile synthesis protocols, including molecular self-assembly and metal-ligand coordination. Here, I will talk about strong electron-electron Coulomb interactions and correlated-electron phases in 2D kagome materials. In particular, I will focus on 2D metal-organic frameworks (MOFs). As an example, I will discuss our recent work [1] on a specific 2D MOF consisting of flat aromatic molecules (di-cyanoanthracene; DCA) arranged in a kagome structure via coordination with copper (Cu) atoms. When adsorbed on a weakly interacting metal surface, such 2D kagome MOF hosts local magnetic moments which, as shown by density functional theory and mean-field Hubbard modelling, are the consequence of strong electron-electron Coulomb interactions resulting from the kagome geometry. When adsorbed on an insulator, the same 2D kagome MOF shows metal-to-insulator transitions with insulating electronic bandgaps larger than ~100 meV. These findings pave the way for nanoelectronics and spintronics technologies based on controllable correlated-electron phases in 2D organic and metal-organic materials. [1] D. Kumar, J. Hellerstedt, B. Field, B. Lowe, Y. Yin, N.V. Medhekar and A. Schiffrin, “Manifestation of strongly correlated electrons in a 2D kagome metal–organic framework” Adv. Funct. Mater., 31, 2106474 (2021).

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