Observation of Topological Flat Bands in the Kagome Semiconductor Nb3Cl8.

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.

Highly Spin-Frustrated Magnetism in the Topochemically Prepared Triangular Lattice Cluster Magnets Na3 A2 (MoO4 )2 Mo3 O8 (A=In, Sc).

The physical properties of novel cluster-based triangular lattice antiferromagnets Na3 A2 (MoO4 )2 Mo3 O8 (A=In, Sc), synthesized through a topochemical Na-intercalation to nonmagnetic Na2 A2 (MoO4 )2 Mo3 O8 , are reported. The S=1/2 [Mo3 ]11+ clusters form a regular triangular lattice, which gives the magnetic system a strong geometrical spin frustration effect. Despite the strong antiferromagnetic couplings among [Mo3 ]11+ clusters, they show no long-range magnetic orderings down to 0.5 K with the finite residual magnetic entropy. The ground states of Na3 A2 (MoO4 )2 Mo3 O8 have been characterized as a quantum spin liquid, owing to the strong spin frustration of cluster spins on the triangular lattice.

Magnetic-Nonmagnetic Phase Transition with Interlayer Charge Disproportionation of Nb3 Trimers in the Cluster Compound Nb3Cl8.

We grew 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 anti-ferromagnet. At low temperatures, Nb3Cl8 exhibits a magnetic-nonmagnetic phase transition driven by a charge disproportionation, in which the paramagnetic [Nb3]8+ clusters transform into alternating layers of nonmagnetic [Nb3]7+ and [Nb3]9+ clusters. The observed exotic phenomenon with the strong correlation between the magnetism and structure are based on the nature of the cluster magnetism.

High-temperature magnetic anomaly via suppression of antisite disorder through synthesis route modification in a Kitaev candidate Cu2IrO3.

By incorporating inert KCl into the Na2IrO3 + 2CuCl → Cu2IrO3 + 2NaCl topochemical reaction, we significantly reduced the synthesis temperature of Cu2IrO3 from the 350°C reported in previous studies to 170°C. This adjustment decreased the Cu/Ir antisite disorder concentration in Cu2IrO3 from ~19% to ~5%. Furthermore, magnetic susceptibility measurements of the present Cu2IrO3 sample revealed a weak ferromagnetic-like anomaly with hysteresis at a magnetic transition temperature of ~70 K. Our research indicates that the spin-disordered ground state reported in chemically disordered Cu2IrO3 is an extrinsic phenomenon, rather than an intrinsic one, underscoring the pivotal role of synthetic chemistry in understanding the application of Kitaev model to realistic materials.

Honeycomb lattice iridate on the verge of Mott-collapse.

A new honeycomb lattice iridate (La,Na)IrO3(≈LaNaIr2O6) is successfully synthesized from the spin-orbit coupled Mott insulator Na2IrO3by replacing the interlayer Na+ions with La3+ions. (La,Na)IrO3shows a finite Sommerfeld term in heat capacity and a -lnTdependence of resistivity, indicating a realization of a metallic state driven by a Mott collapse. Furthermore, crystal structure analysis reveals the formation of Ir zig-zag chains with metal-metal bonding, increasing kinetic energy resulting in the Mott collapse. This observation would be due to a Mott collapse induced in aJeff= 1/2 spin-orbit coupling Mott insulator with an Ir honeycomb lattice by topochemical control of the ionic configuration.

Spin-glass transition in the spin-orbit-entangled Jeff = 0 Mott insulating double-perovskite ruthenate.

We have successfully synthesized new Ru4+ double perovskite oxides SrLaInRuO6 and SrLaGaRuO6, which are expected to be a spin-orbit coupled Jeff = 0 Mott insulating ground state. Their magnetic susceptibility is much significant than that expected for a single Ru4+ ion for which exchange coupling with other ions is negligible. Their isothermal magnetization process suggests that there are about 20 percent isolated spins. These origins would be the Ru3+/Ru5+ magnetic defects, while the regular Ru4+ sites remain nonmagnetic. Moreover, SrLaGaRuO6 shows a spin-glass-like magnetic transition at low temperatures, probably caused by isolated spins. The observed spin-glass can be interpreted by the analogy of a dilute magnetic alloy, which can be seen as a precursor to the mobile Jeff = 1 exciton as a dispersive mode as predicted.

Metallic state in the vicinity of molecular orbital crystallization in thed1thiospinel ZnTi2S4prepared via a reductive ion-exchange reaction.

A novel Ti3+-based thiospinel ZnTi2S4is successfully synthesized via a low-temperature ion-exchange reaction. ZnTi2S4shows a signature of metallic ground state evidenced by a contribution of conduction electrons in the heat capacity and Pauli-like paramagnetic susceptibility. These observations contrast to the electronic state of similar Ti3+-based spinel MgTi2O4exhibiting the metal-insulator transition associated with a molecular orbital crystallization (MOC). Furthermore, the magnetic susceptibility of ZnTi2S4shows a pseudogap-like behavior indicated by a vast peak in the magnetic susceptibility around 110 K, likely originating from the MOC fluctuation. The origin of the difference in the electronic states of MgTi2O4and ZnTi2S4would be due to the different magnitude of overlap between Ti 3dandporbitals (O: 2pand S: 3p). The presence of a MOC state in the close vicinity of insulator-metal transition may suggest the importance of itinerancy in a MOC.

Tetramer spin singlet instability in the fluorine-substituted pyrochlore superconducting system Cd2Re2O7-x F x.

We synthesized polycrystalline samples of the fluorine-substituted pyrochlore rhenates Cd2Re2O7-x F x , and investigated their magnetic, transport and structural properties. The transition temperature T s1, where each Re4 tetrahedron in the Re pyrochlore network alternately expands and contracts, decreases with increasing x from 200 K at x = 0 to 100 K at x = 0.5. The strong x dependence of the magnetic and transport properties at the low-temperature phase indicates that the driving force of structural phase transition is fluctuations of the tetramer spin singlet formation in order to release the spin frustration in the pyrochlore lattice. Furthermore, we found unconventional superconducting properties in Cd2Re2O7-x F x . It was found that the superconducting phase transition temperature T c markedly decreases with increasing x, suggesting that the addition of imperfection suppresses a condensation of Cooper-pair. In addition, the estimated upper critical field at zero temperature exceeds the Pauli paramagnetic limit and increases with increasing x in spite of the reduction of T c. Hence, Cd2Re2O7-x F x is suggested to be an exotic superconductor realized in the itinerant electron systems on a spin frustrated lattice.