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arXiv · 2412.13456

Multiferroic Dark Excitonic Mott Insulator in the Breathing-Kagome Lattice Material Nb$_3$Cl$_8$

Mahtab A. Khan·Naseem Ud Din·Dmitry Skachkov·Dirk R. Englund·Michael N. Leuenberger
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Abstract

Flat electronic bands strongly enhance Coulomb interactions and can stabilize unconventional insulating states. Motivated by the recent discovery of flat bands in breathing Kagome lattices, we use first-principles GW--Bethe--Salpeter theory to investigate the excitonic spectrum of single-layer Nb$_3$Cl$_8$. We find a dark spin-triplet Frenkel exciton whose spectral peak lies at negative energy ($-0.14$~eV) relative to the quasiparticle gap, directly signaling a preformed bound state and an excitonic Mott insulating phase potentially stable at room temperature. Bright excitons appear at $0.94$~eV and $1.21$~eV, with ultra-large binding energies of $2.05$~eV and $1.77$~eV. By mapping the low-energy dynamics onto a spin-1 Hubbard model on a triangular lattice, we show that frustrated antiferromagnetic and ferroelectric tendencies naturally emerge. These results identify Nb$_3$Cl$_8$ as a candidate multiferroic dark excitonic insulator, opening a pathway to correlated quantum phases in two dimensions.

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Mahtab A. Khan, Naseem Ud Din, Dmitry Skachkov, Dirk R. Englund, Michael N. Leuenberger. 2025-11-01. Multiferroic Dark Excitonic Mott Insulator in the Breathing-Kagome Lattice Material Nb$_3$Cl$_8$. https://arxiv.org/abs/2412.13456

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