Spectroscopic evidence of flat bands in breathing kagome semiconductor Nb3I8

Abstract

Kagome materials have become solid grounds to study the interplay among geometry, topology, correlation, and magnetism. Recently, niobium halide semiconductors Nb3X8 (X = Cl, Br, I) have been predicted to be two-dimensional magnets and these materials are also interesting for their breathing kagome geometry. However, experimental electronic structure studies of these promising materials are still lacking. Here, we report the spectroscopic evidence of flat and weakly dispersing bands in breathing-kagome semiconductor Nb3I8 around 500 meV binding energy, which is well supported by our first-principles calculations. These bands originate from the breathing kagome lattice of niobium atoms and have niobium d-orbital character. They are found to be sensitive to the polarization of the incident photon beam. Our study provides insight into the electronic structure and flat band topology in an exfoliable kagome semiconductor, thereby providing an important platform to understand the interaction of geometry and electron correlations in two-dimensional materials.

Terahertz Spectroscopy and DFT Analysis of Phonon Dynamics of the Layered Van der Waals Semiconductor Nb3X8 (X = Cl, I)

“…The band gaps of Nb 3 Cl 8 and Nb 3 I 8 were experimentally reported to be 1.12 and ∼1 eV, respectively, , thereby indicating their potential in device applications. Additionally, Nb 3 Cl 8 and Nb 3 I 8 have been experimentally confirmed to possess topological flat bands. , …”

“…Additionally, Nb 3 Cl 8 and Nb 3 I 8 have been experimentally confirmed to possess topological flat bands. 18 , 20 …”

Topological Flat Bands in 2D Breathing‐Kagome Lattice Nb3TeCl7

“…Recently, FBs and Mott-like gap have been experimentally observed in Nb 3 X 8 . [39][40][41] The transition between band insulator and Mott insulator, as well as the molecular quantum spin liquid state, are theoretically predicted in Nb 3 X 8 . [42,43] These studies indicate that the flatness and topological properties of the FBs are predominated by electron hopping parameters and onsite energy of Nb sites in Nb 3 X 8 and Nb 3 QX 7 .…”

“…Nb 3 X 8 (X = Cl, Br, I) and Nb 3 QX 7 (Q = S, Se, Te; X = Cl, Br, I) possess 2D breathing-kagome lattice and weak van der Waals (vdW) interlayer interaction. [33][34][35][36][37][38][39][40][41][42][43][44][45][46][47][48][49][50] Their electronic bands in the close vicinity of the Fermi level are contributed by the metallic breathing-kagome layer, which is entirely composed of Nb atoms. These features make these materials appealing for exploring 2D FBs physics.…”

Enabling triferroics coupling in breathing kagome lattice Nb3X8 (X = Cl, Br, I) monolayers

“…It is worth noting that the layered Nb 3 I 8 has also been fabricated and studied. [23][24][25] Meanwhile, Dai et al theoretically demonstrated that the Nb 3 I 8 monolayer is a robust FM semiconductor with a moderate band gap, which exhibits the long-sought spontaneous valley polarization without requiring any external tuning. 26 Moreover, Ren et al proposed that the Ti 3 X 8 compounds with breathing kagome lattices are shown to form 2D intrinsic multiferroic semiconductors with the coexistence of ferroelectricity and ferromagnetism.…”