Monoclinic distortion in hyperhoneycomb Kitaev material β\beta-ZnIrO3_3 revealed by improved sample quality

The sample quality of the hyperhoneycomb lattice Kitaev magnet $\beta$-ZnIrO$_3$ was successfully improved by removing the maximum amount of moisture from the reaction ampoule. The X-ray diffraction structural analysis of the high-quality sample confirmed the presence of $P$2$_1$/$c$ superlattice peaks of the Fddd structure in the original structural model. These peaks could not be distinguished due to the presence of impurities in the low-quality sample in a previous study. The structural analysis based on this monoclinic crystal structure model showed no chemical disorder, suggesting that the observed spin liquid type behavior is an intrinsic property unrelated to bond randomness. The details of the $\beta$-ZnIrO$_3$ structure revealed in this study will stimulate the further investigation of Kitaev physics.Comment: 4pages, 3figure

Honeycomb lattice iridate on the verge of Mott-collapse

A new honeycomb lattice iridate (La,Na)IrO$_3$ ($\approx$ LaNaIr$_2$O$_6$) is successfully synthesized from the spin-orbit coupled Mott insulator Na$_2$IrO$_3$ by replacing the interlayer Na$^+$ ions with La$^{3+}$ ions. (La,Na)IrO$_3$ shows a finite Sommerfeld term in heat capacity and a $-\ln T$ dependence 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 a $J_{\mathrm{eff}} = 1/2$ spin-orbit coupling Mott insulator with an Ir honeycomb lattice by topochemical control of the ionic configuration.Comment: 7pages, 5 figures, accepted in Journal of Physics: Condensed Matte

Magnetic ordering in the JeffJ_{\rm eff} = 0 Nickelate NiRh2_2O4_4 prepared via a solid-state metathesis

In spinel-type nickelate NiRh$_2$O$_4$, magnetic ordering is observed upon the sample synthesized via kinetically controlled low-temperature solid-state metathesis, as opposed to previously-reported samples obtained through conventional solid-state reaction. Our findings are based on a combination of bulk susceptibility and specific heat measurements that disclose a N$\'e$el transition temperature of $T_N$ = 45 K in this material, which might feature spin-orbit entanglement in the tetragonally-coordinated $d^8$ Mott insulators. The emergence of magnetic ordering upon alteration of the synthesis route indicates that the suppression of magnetic ordering in the previous sample was rooted in the cation-mixing assisted by the entropy gain that results from high-temperature reactions. Furthermore, the $J_{\rm eff}$ = 0 physics, instead of solely the spin-only $S = 1$, describes the observed enhancement of effective magnetic moment well. Overseeing all observations and speculations, we propose that the possible mechanism responsible for the emergent magnetic orderings in NiRh$_2$O$_4$ is the condensation of $J_{\rm eff}$ = 0 exciton, driven by the interplay of the tetragonal crystal field and superexchange interactions.Comment: 7 pages, 5 figures, accepted in Physical Review Material

Perfect kagome-lattice antiferromagnets with Jeff_{eff} = 1/2: The Co2+^{2+}-analogs of copper minerals volborthite and vesignieite

We report the synthesis, crystal structure, and magnetic properties of Co$^{2+}$ kagome magnets Co$_3$V$_2$O$_7$(OH)$_2$$\cdot$2H$_2$O and BaCo$_3$(VO$_4$)$_2$(OH)$_2$, which can be recognized as Co-analogues of the intensively researched quantum kagome magnet volborthite Cu$_3$V$_2$O$_7$(OH)$_2$$\cdot$2H$_2$O and vesignieite BaCu$_3$(VO$_4$)$_2$(OH)$_2$. For each compound, the ground state is seemingly A-type antiferromagnetic order. At low temperatures, applying a magnetic field causes a metamagnetic-like transition described by the transition in which antiferromagnetically-aligned canted moments change to ferromagnetically-aligned ones. These ground and field-induced states include a canted ferromagnetic component perpendicular to the kagome planes favored by Dzyaloshinskii-Moriya interactions. These magnetic properties are well characterized by the J$_{eff}$ = 1/2 physics. Our findings will be the first step toward clarifying the J$_{eff}$ = 1/2 kagome physics, which has been little studied experimentally or theoretically.Comment: 9 pages, 7+1 figures, accepted for publication in Physical Review

Observation of topological flat bands in the kagome semiconductor Nb3_3Cl8_8

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 Fe$_3$Sn$_2$, FeSn, CoSn, and YMn$_6$Sn$_6$. Nonetheless, kagome materials that are both exfoliable and semiconducting are lacking, which seriously hinders their device applications. Herein, we show that Nb$_3$Cl$_8$, 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 TFB and a moderate band gap in Nb$_3$Cl$_8$. By mechanical exfoliation, we successfully obtain monolayers of Nb$_3$Cl$_8$ and confirm that they are stable under ambient conditions. In addition, our calculations show that monolayers of Nb$_3$Cl$_8$ have a magnetic ground state, thus providing opportunities to study the interplay between geometry, topology, and magnetism.Comment: 6 pages, 4 figure

Case of plasmablastic lymphoma of the sigmoid colon and literature review

Plasmablastic lymphoma (PBL) is a rare form of non-Hodgkin\u27s lymphoma that is associated with human immunodeficiency virus (HIV) infection. Although PBL is most commonly observed in the oral cavity of HIV-positive patients, it can also be observed at extra-oral sites in HIV-negative patients. This report represents an unusual case of HIV-negative PBL that occurred in the sigmoid colon. This patient had a history of systemic lupus erythematosus and an underlying immunosuppressive state from long term steroid therapy. The lymphoma cells were positive for CD138, kappa light chain restriction and Epstein-Barr virus and negative for CD20/L26, CD3, CD79a, UCHL1 (CD45RO) and cytokeratin (AE1/AE3). The patient died approximately 2 mo after the operation. In the present paper, we review cases of PBL of the colon in HIVnegative patients