Kagome metal-organic frameworks as a platform for strongly correlated electrons

Abstract

By using first-principles calculations we put forward the Cu-dicyanoanthracene lattice as a platform to investigate strong electronic correlations in the family of Kagome metal-organic frameworks. We show that the low-energy model is composed by molecular orbitals which arrange themselves in a typical Kagome lattice at n=2/3 filling, where the Fermi level lies at the Dirac point. The Coulomb interaction matrix expressed in this molecular orbitals basis, as obtained by large-scale constrained random-phase approximation calculations, is characterized by local U and non-local ¢ U parameters exceeding more than ten times the Kagome bandwidth. For such Kagome systems, our findings suggest the possible emergence of peculiar electron-electron collective phenomena, such as an exotic valence bond solid order characterized by modulated bond strengths.

Manifestation of Strongly Correlated Electrons in a 2D Kagome Metal–Organic Framework

“…Recent theoretical work has shown electron‐electron Coulomb interactions in the DCA 3 Cu 2 MOF with characteristic energies of ≈3 eV, several times larger than the kagome bandwidth—even at VB electron fillings where the Fermi level is far below the flat band. [ 4 ] We, therefore, explored the effect of electron correlations by including in our DFT calculations a Hubbard U term accounting for corrections due to on‐site Coulomb repulsion between electrons with Cu 3 d character (see Experimental Section). Figure 4b shows the spatial distribution of magnetic moments within the DCA 3 Cu 2 MOF on Ag(111) calculated for U = 3 eV (see Experimental Section), closely resembling that of the DCA LUMO on Ag(111) [ 24 ] and on Gr/Ir(111), [ 22 ] with some Cu contribution (due to partial hybridization between DCA LUMOs and Cu d orbitals).…”

“…In our calculations, the energy scale of these interactions ( U > ≈3 eV in DFT+ U , U MFH > ≈0.35 eV in MFH model) is comparable to or larger than the kagome bandwidth, consistent with previous calculations. [ 4 ] The spatial distribution of the experimentally observed ZBPs (Figure 2c,d) is similar to that of the DFT+ U ‐derived magnetic moments (Figure 4b), following the kagome‐arranged DCA LUMOs and Cu atoms, consistent with the dominant LUMO and slight Cu 3 d character of the VB (Figure 4a): on‐site Coulomb repulsion between electrons hopping among these states give rise to local magnetic moments that become Kondo‐screened by Ag(111) conduction electrons ( Figure ). Our calculations indicate a magnetic moment of ≈1/4 μ B per DCA (Figure 4c); Kondo screening has been demonstrated for fractional magnetic moments of similar magnitude, experimentally [ 34 ] and theoretically.…”

“…[ 1–3 ] These states, when filled gradually, give rise to increasing Coulomb electron‐electron interactions, with characteristic energies that can be significantly larger than the valence bandwidth. [ 4 ] These strong Coulomb interactions, tunable via the valence band (VB) electronic population, can lead to a vast range of quantum correlated electron phenomena, including magnetic ordering of unpaired electron spins (i.e., magnetic moments, resulting in, for example, ferro‐ [ 5 ] or antiferromagnetic phases [ 6 ] ), quantum spin liquids, [ 7 ] metal‐to‐insulator Mott transitions, [ 8 ] and non‐trivial topology. [ 9–11 ] This tunability between versatile and fundamentally diverse quantum phenomena offers promise for electronics and spintronics applications based on controllable electron correlations in 2D materials.…”

“…Recently, correlated electron phenomena and flat electronic bands have been observed in inorganic 2D kagome materials. [6,14,15] Metal-organic frameworks [4,9] (MOFs) and covalent organic frameworks [16] (COFs) with similar structure and electronic properties have been predicted, with the additional promise of tunable electron-electron interactions and topology via versatile, atomically precise synthesis approaches based on metal-organic coordination [17] and organic covalent bonding. [18] While 2D kagome MOFs and COFs have been synthesized, neither flat bands nor correlated electron phenomena have yet been observed in these systems.…”

Turbulent hydrodynamics in strongly correlated Kagome metals

“…These were recently synthesized successfully. They exhibit strong electronic correlations 31 and even superconductivity 32 . The fact that the number of available strongly correlated Kagome metals is growing significantly is encouraging in view of experimentally realizing our results, and outweighs the difficulties so far encountered in achieving conducting Herbertsmithite materials 33 – 35 .…”