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[en] We study the one-dimensional Kitaev-Heisenberg model as a possible realization of magnetic degrees of freedom of the K-intercalated honeycomb-lattice ruthenium trichloride $\alpha$-${\mathrm{RuCl}}_3$, denoted as ${\mathrm{K}}_{0.5}$${\mathrm{RuCl}}_m$. First, we discuss the possible charge ordering pattern in ${\mathrm{K}}_{0.5}$${\mathrm{RuCl}}_m$, where half of the $j=\frac{1}{2}$ spins are replaced by nonmagnetic ions in the honeycomb layer. Next, we investigate the low-energy excitations of the 1D Kitaev-Heisenberg model by calculating the dynamical spin structure factor using the Lanczos exact-diagonalization method. In the vicinity of Kitaev limit, there exist two well-separated dispersions. The bandwidth of each dispersion depends on the Heisenberg and Kitaev terms. This result may be relevant to the low-lying magnetic excitations of ${\mathrm{K}}_{0.5}$${\mathrm{RuCl}}_m$.

[en] Magnetic insulators in the regime of strong spin–orbit coupling exhibit intriguing behaviors in external magnetic fields, reflecting the frustrated nature of their effective interactions. We review the recent advances in understanding the field responses of materials that are described by models with strongly bond-dependent spin exchange interactions, such as Kitaev’s celebrated honeycomb model and its extensions. We discuss the field-induced phases and the complex magnetization processes found in these theories and compare with experimental results in the layered Mott insulators -RuCl₃ and Na₂IrO₃, which are believed to realize this fascinating physics. (topical review)

[en] Hyoscine butyl bromide (HBB) is an anti-cholinergic and anti-muscarinic drug used to treat pain and discomfort caused by abdominal cramps etc. It is semi-synthetic derivative alkaloid hyoscyamine having a broad spectrum of activity. Hence the Ru(III) catalyzed the oxidation of hyoscine butyl bromide drug by diperiodatocuprate(III) (DPC) in aqueous alkaline medium was investigated and monitored spectrophotometrically (max = 415 nm) at a constant ionic strength of 0.1 mol dm–3 and 301 K. The reaction shows 1:2 stoichiometry between HBB and DPC. The reaction is of the first order in [DPC] and [Ru(III)] and less than unit order in [HBB] and [alkali], periodate has a retarding effect on the reaction rate. The ionic strength, dielectric constant of the medium and added products has no significant effect on the rate of the reaction but Ru(III) increases the reaction rate. The main oxidation products of HBB were identified by spectral studies. The active forms of catalyst and oxidant were detected as Ru(III), [Cu(H2IO6)(H2O)2] respectively. The rate law and reaction mechanism was proposed. The equilibrium constants and rate constants were calculated. The activation parameters were deliberated for catalyzed and uncatalyzed reactions. (author)

[en] The core task for Mott insulators includes how rigid distributions of electrons evolve and how these induce exotic physical phenomena. However, it is highly challenging to chemically dope Mott insulators to tune properties. Herein, we report how to tailor electronic structures of the honeycomb Mott insulator RuCl${}_3$ employing a facile and reversible single-crystal to single-crystal intercalation process. The resulting product (NH${}_4$)${}_{0.5}$RuCl${}_3$⋅1.5 H${}_2$O forms a new hybrid superlattice of alternating RuCl${}_3$ monolayers with NH${}_4$${}^{+}$ and H${}_2$O molecules. Its manipulated electronic structure markedly shrinks the Mott-Hubbard gap from 1.2 to 0.7 eV. Its electrical conductivity increases by more than 10${}^3$ folds. This arises from concurrently enhanced carrier concentration and mobility in contrary to the general physics rule of their inverse proportionality. We show topotactic and topochemical intercalation chemistry to control Mott insulators, escalating the prospect of discovering exotic physical phenomena. (© 2023 Wiley‐VCH GmbH)

[en] Electrical signatures for quantum magnetic phases of α-RuCl${}_3$ sensed by an adjacent graphene field-effect transistor (FET) are reported. The gate-voltage dependence of the graphene FET reveals p-type doping beneath α-RuCl${}_3$ caused by the charge transfer due to the work function difference. Furthermore, the gate-voltage hysteresis exhibits a marked change in the transport behavior with the temperature variation. The high-T simple paramagnetic phase and the low-T zigzag antiferromagnetic phase of α-RuCl${}_3$ exhibit negligible hysteresis. In sharp contrast, a huge hysteresis of the graphene FET at intermediate temperatures is observed. The possibility that the observed conductance hysteresis is associated with the intriguing continuum excitations of the thermally induced Kitaev paramagnet state is discussed. The results show that the proximity effect-based electric approach can be utilized for investigating charge-neutral quasiparticles in quantum magnetic insulators. (© 2024 Wiley‐VCH GmbH)

[en] We report measurements of optical absorption in the zigzag antiferromagnet α-RuCl₃ as a function of temperature T , magnetic field B , and photon energy ℏ ω in the range ~ 0.3 –8.3 meV, using time-domain terahertz spectroscopy. Polarized measurements show that threefold rotational symmetry is broken in the honeycomb plane from 2 to 300 K. We find a sharp absorption peak at 2.56 meV upon cooling below the Néel temperature of 7 K at B = 0 that we identify as the magnetic-dipole excitation of a zero-wave-vector magnon, or antiferromagnetic resonance (AFMR). With the application of B , the AFMR broadens and shifts to a lower frequency as long-range magnetic order is lost in a manner consistent with transitioning to a spin-disordered phase. From a direct, internally calibrated measurement of the AFMR spectral weight, we place an upper bound on the contribution to the dc susceptibility from a magnetic excitation continuum.

[en] We study the influence of Cr doping on magnetic properties of α-RuCl ₃ single crystals in detail. With increasing Cr content, the c-axial lattice parameter increases gradually, implying that the Cr doping may weaken the interlayer interactions. The magnetism of Ru _{1 – x} Cr_xCl ₃ single crystals evolves from a long-range AFM order to a possible spin-glass state with Cr doping. The appearance of a possible spin-glass state can be explained by the introduction of FM interaction by Cr ³⁺ ions, which competes with the AFM interaction between Ru ³⁺ ions. Moreover, the larger magnetic moment of Cr ³⁺ ion with S = 3/2 than Ru ³⁺ ion with J _eff = 1/2 also results in a monotonic increase of the effective moment of Ru _{1 – x} Cr_xCl ₃ single crystal. (paper)

[en] The exactly solvable Kitaev model on the honeycomb lattice has recently received enormous attention linked to the hope of achieving novel spin-liquid states with fractionalized Majorana-like excitations. In this review, we analyze the mechanism proposed by Jackeli and Khaliullin to identify Kitaev materials based on spin-orbital dependent bond interactions and provide a comprehensive overview of its implications in real materials. We set the focus on experimental results and current theoretical understanding of planar honeycomb systems (Na₂IrO₃, α-Li₂IrO₃, and α-RuCl₃), three-dimensional Kitaev materials (β- and γ-Li₂IrO₃), and other potential candidates, completing the review with the list of open questions awaiting new insights. (topical review)

[en] Layered electrodes of electrodeposited RuO_2 (eRuO_2) and polyaniline (ePAn) are prepared on a platinum substrate to yield ePAn-on-eRuO_2 and eRuO_2-on-ePAn electrodes and their supercapacitive properties are investigated. The eRuO_2 electrode forms a compact stratified structure with nearly flat surfaces by electrodeposition from a RuCl_3·3H_2O-based solution, whereas the ePAn electrode forms densely connected particles or particle aggregates by the electrodeposition from aniline solution. In the scan rate range of 20–1000 mV s"−"1, specific capacitance (810–575 F g"−"1) of ePAn-on-eRuO_2 electrode is superior by the higher probability to occur redox reaction and proton diffusion due to particulate surface and inner pores of the upper layer (ePAn), respectively. In contrast, the eRuO_2-on-ePAn electrode loses the specific capacity (680–550 F g"−"1) but achieves higher capacity retention ratio (81%) by flat compact surface of the upper layer (eRuO_2).