ABSTRACT:We have grown large single crystals of the cluster magnet Nb3Cl8 with a magnetic triangular lattice, and investigated its magnetic properties and crystal structure. In Nb3Cl8, the [Nb3] 8+ cluster has a single unpaired spin, making it an S = 1/2 triangular lattice antiferromagnet. At low temperatures, Nb3Cl8 exhibits a magnetic-nonmagnetic phase transition driven by a charge disproportionation, in which the paramagnetic clusters. The observed exotic phenomenon with the strong correlation between the magnetism and structure are based on the nature of the cluster magnetism.

We synthesized the novel S = 1/2Mo 3 cluster magnet Li 2 ScMo 3 O 8 as well as the isomorphic compound Li 2 InMo 3 O 8 , and investigated their macroscopic and microscopic physical properties. Li 2 InMo 3 O 8 showed magnetic ordering at T N ∼ 12 K with a 120 • structure, while no magnetic ordering is observed down to 0.5 K in Li 2 ScMo 3 O 8 despite the strong antiferromagnetic interaction among clusters probed by the Weiss temperature. Results of the 7 Li nuclear magnetic resonance spectra and the nuclear spin-lattice relaxation rate (1/T 1 ) measurement show direct microscopic proof of the spin-disordered ground state. We discuss the difference in the ground state between Li 2 ScMo 3 O 8 and Li 2 InMo 3 O 8 using the description of localized magnetism based on spin frustration within the triangular lattice and that of inter-and intracluster charge fluctuations.

The destructive interference of wavefunctions in a kagome lattice can give rise to topological flat bands (TFBs) with a highly degenerate state of electrons. Recently, TFBs have been observed in several kagome metals, including Fe3Sn2, FeSn, CoSn, and YMn6Sn6. Nonetheless, kagome materials that are both exfoliable and semiconducting are lacking, which seriously hinders their device applications. Herein, we show that Nb3Cl8, which hosts a breathing kagome lattice, is gapped out because of the absence of inversion symmetry, while the TFBs survive because of the protection of the mirror reflection symmetry. By angle-resolved photoemission spectroscopy measurements and first-principles calculations, we directly observe the TFBs and a moderate band gap in Nb3Cl8. By mechanical exfoliation, we successfully obtain monolayer Nb3Cl8, which is stable under ambient conditions. In addition, our calculations show that monolayer Nb3Cl8 has a magnetic ground state, thus providing opportunities to study the interplay among geometry, topology, and magnetism.

We investigated the magnetic properties of the ilmenite-type iridate CdIrO3 with a honeycomb lattice formed by Ir 4+ ions prepared via a solid-state metathesis. The magnetization measurements with using the powder sample reveal a large effective magnetic moment and a fairly strong antiferromagnetic interaction, indicating a deviation from the Kitaev model. Considering the relationship between magnetism and crystal structure in CdIrO3 with comparing with the other ilmenite-type iridates ZnIrO3 and MgIrO3, we conclude that insulating CdIrO3 cannot be describe as a Jeff = 1/2 Mott state owing to a metathetically-stabilized large trigonal distortion of IrO6 octahedra.

We have successfully synthesized the novel antiferromagnets with Ir 4+ honeycomb lattice ZnIrO3 and MgIrO3 and investigated their magnetic and thermodynamic properties. The two iridates are isomorphic but exhibit qualitatively different magnetic properties. ZnIrO3 shows antiferromagnetic ordering below 46.6 K, whereas MgIrO3 displays weak ferromagnetic behavior below 31.8 K owing to formation of a canted antiferromagnetic ordering. The measurement of magnetic susceptibility with using an oriented powder sample revealed the presence of an XY-like magnetic anisotropy and a tilting magnetic structure which is possibly stabilized by the Kitaev interaction. Moreover, magnetization curves of MgIrO3 and ZnIrO3 up to 60T show different behaviors, demonstrating that each magnetic ground state is different with each other. We discuss the difference in the ground state between MgIrO3 and ZnIrO3 from the viewpoint a magnetic model consisting of the Kitaev and Dzyaloshinskii-Moriya interactions with the spin frustration effect on the honeycomb lattice.