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

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

Dynamical breakdown of the Ising spin-glass order under a magnetic field

The dynamical magnetic properties of an Ising spin glass Fe$_{0.55}$Mn$_{0.45}$TiO$_3$ are studied under various magnetic fields. Having determined the temperature and static field dependent relaxation time $\tau(T;H)$ from ac magnetization measurements under a dc bias field by a general method, we first demonstrate that these data provide evidence for a spin-glass (SG) phase transition only in zero field. We next argue that the data $\tau(T;H)$ of finite $H$ can be well interpreted by the droplet theory which predicts the absence of a SG phase transition in finite fields.Comment: 4 pages, 5 figure

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

Magnetic Phase Transition and Magnetization Plateau in Cs2_2CuBr4_4

The crystal structure of Cs$_2$CuBr$_4$ is the same as that of Cs$_2$CuCl$_4$, which has been characterized as a spin-1/2 quasi-two-dimensional frustrated system. The magnetic properties of Cs$_2$CuBr$_4$ were investigated by magnetization and specific heat measurements. The phase transition at zero magnetic field was detected at $T_{\rm N}=1.4$ K. It was observed that the magnetization curve has a plateau at about one-third of the saturation magnetization for magnetic field $H$ parallel to the $b$- and $c$-axes, while no plateau was observed for $H\parallel a$. The field-induced phase transition to the plateau state appears to be of the first order. The mechanism leading to the magnetization plateau is discussed.Comment: 6 pages, 4 figures, 4 eps files, ptptex, will appear in Supplement of Progress in Theoretical Physic

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

Magnetic phase diagram of the diluted metamagnet Fe\u3csub\u3e0.95\u3c/sub\u3eMg\u3csub\u3e0.05\u3c/sub\u3eBr\u3csub\u3e2\u3c/sub\u3e

The axial magnetic phase diagram of the antiferromagnet Fe0.95Mg0.05Br2 is studied by specific heat, superconducting quantum interference device, and Faraday rotation techniques. The diamagnetic impurities give rise to random-field criticality along the second-order phase line Hc(T) between TN=13.1 K and a multicritical point at Tm≈5 K, and to a spin-flop line between Tm and the critical end-point temperature Te≈3.5 K. The phase line H1(T)c(T) ending at Tm is probably due to symmetric nondiagonal exchange