arXiv subjects

Yuya Haraguchi

Publications and source records attributed to Yuya Haraguchi.

At least 19 recordsLinked to original sources

arXiv2026

Spin--orbit-enhanced correlation sensitivity and anomalous magnetic response in non-dimerized $4d^4$ ilmenite CdRuO$_3$

We report the synthesis and physical properties of CdRuO$_3$, a nominal Ru$^{4+}$ $4d^4$ ilmenite with edge-sharing RuO$_6$ honeycomb layers. Powder X-ray diffraction establishes a crystallographically non-dimerized $R\bar{3}$ structure with equivalent Ru-Ru bonds and a strongly distorted RuO$_6$ environment. The compacted-pellet resistivity is nonmetallic but non-Arrhenius, while the heat capacity contains a finite residual linear term. Matched nonmagnetic calculations show that PBE+$U$ without spin-orbit coupling remains metallic or semimetallic up to $U_{\mathrm{eff}} = 3$ eV, whereas PBE+SOC+$U$ exhibits a strong $U_{\mathrm{eff}}$ dependence and opens a direct gap of approximately 55 meV at $\Gamma$ for $U_{\mathrm{eff}} = 2.5$ eV. Spin-orbit coupling therefore markedly enhances the correlation sensitivity of the non-cubic Ru $t_{2g}$ manifold. After subtraction of a dilute Curie-Weiss defect contribution, the susceptibility remains weakly nonmonotonic and is inconsistent with both an ordinary Pauli response and independent spin-only $S = 1$ moments. CdRuO$_3$ thus realizes the non-dimerized structural branch predicted for ruthenium ilmenites, but not a simple robust multiorbital metal.

cond-mat.str-el↗

arXiv2026

Tetrahedrally ferromagnetic correlations and a glassy-freezing anomaly in the breathing pyrochlore magnet $\mathrm{AgInCr_4S_8}$ with partial $A$-site disorder

We investigate the chromium breathing pyrochlore sulfide $\mathrm{AgInCr_4S_8}$, a chromium-based thiospinel, by synchrotron x-ray and neutron powder diffraction, dc magnetization, and heat capacity. Diffraction confirms the $F\bar{4}3m$ breathing structure with alternating large and small $\mathrm{Cr_4}$ tetrahedra, a large breathing ratio ($d^\prime/d = 1.106$ at 300 K), and substantial Ag/In intermixing on the $A$ sublattice ($\sim 16\%$). No structural transition or magnetic Bragg peaks are detected down to 1.5 K. An enlarged low-angle difference plot between the 1.5 and 20 K neutron diffraction patterns shows a weak broad diffuse-like enhancement, consistent with short-range or frozen correlated moments within the sensitivity of the present data. Susceptibility yields a positive Weiss temperature $\theta_{\mathrm{W}} = +92$ K and a moment enhancement in 30--60 K, while the magnetic entropy released by $\sim 30$ K approaches a scale of order $R\ln 13$, together consistent with the development of short-range tetrahedral ferromagnetic correlations and an effective $S = 6$ cluster-moment picture. A broad susceptibility cusp with ZFC--FC bifurcation and a low-temperature specific heat anomaly near 9 K indicate a phenomenological glassy-freezing anomaly without long-range order. $\mathrm{AgInCr_4S_8}$ provides a benchmark for the interplay of strong breathing distortion and quenched $A$-site disorder in chromium breathing pyrochlores.

cond-mat.str-el↗

arXiv2026

Frustrated Magnetism of the $S = 1$ Trillium-Lattice Oxide Li$_2$NiGe$_3$O$_8$

We report magnetization and heat-capacity measurements on the ordered-spinel oxide Li$_2$NiGe$_3$O$_8$, where Ni$^{2+}$ ions with $S = 1$ form a single three-dimensional trillium lattice. Powder x-ray diffraction confirms a cubic ordered-spinel structure with space group $P4_{1}32$ or $P4_{3}32$. The inverse susceptibility $H/M$ follows Curie--Weiss behavior above 50 K with an effective magnetic moment $\mu_{\mathrm{eff}} = 3.124(4)\,\mu_{\mathrm{B}}$ per Ni and a Weiss temperature $\theta_{\mathrm{W}} = -0.21(1)$ K, but deviates smoothly below about 10 K. The magnetic heat capacity $C_{\mathrm{mag}}/T$ shows a broad maximum near 3 K with a wide tail to about 10 K, and the entropy recovered between 2 and 40 K is about 88% of $R \ln 3$. The broad heat-capacity maximum is compared with Monte Carlo results for the local ferromagnetic Ising model on the trillium lattice using a characteristic scale $J$ of order 7 K, while the inverse susceptibility shows only qualitative similarity to the theoretical curve. These results establish Li$_2$NiGe$_3$O$_8$ as a rare $S = 1$ single-trillium oxide with frustrated magnetic correlations. The present data provide an experimental platform for discussing the relation between Heisenberg-like and spin-ice-like regimes on the trillium lattice.

cond-mat.str-el↗

arXiv2026

Selective Octahedral Accommodation of Cr$^{3+}$ and Weak Magnetic Connectivity in the Sugilite Analogue KNa$_2$Cr$_2$Li$_3$Si$_{12}$O$_{30}$

We report the synthesis of the Cr analogue of sugilite, KNa$_2$Cr$_2$Li$_3$Si$_{12}$O$_{30}$, in the milarite-type framework. Rietveld refinement of a composition-conserving antisite model gives $x = 0.0024(18)$ in KNa$_2$[Cr$_{2-x}$Li$_x$][Li$_{3-x}$Cr$_x$]Si$_{12}$O$_{30}$, corresponding to a T2-site Cr occupancy of $0.0008(6)$. X-ray MEM analysis shows no detectable Cr-like density at T2. Magnetic susceptibility indicates weak antiferromagnetic interactions with $\theta_{\mathrm{W}} = -4.78(7)$ K and no ordering above 1.8 K.

cond-mat.mtrl-sci↗

arXiv2026

Evolution of crystal field and intraionic interactions in the ilmenite $A$IrO$_3$ ($A$ = Mg, Zn, Cd) and hyperhoneycomb $\beta$-ZnIrO$_3$

Spin-orbit Mott insulators with the $t_{2g}^5$ electron configuration are promising platforms for the Kitaev spin liquid, yet fine-tuning of their crystal structures is essential to suppress non-Kitaev interactions. Here, we investigate the local electronic structures of the ilmenite iridates $A\mathrm{IrO}_3$ ($A = \mathrm{Mg}, \mathrm{Zn}, \mathrm{Cd}$) and the hyperhoneycomb $\beta\text{-}\mathrm{ZnIrO}_3$ using Ir $L_3$-edge resonant inelastic x-ray scattering (RIXS). Multiplet analysis of the RIXS spectra reveals a systematic evolution of the crystal field and intraionic interaction parameters upon chemical substitution at the $A$-site. We observe an enhancement of the trigonal distortion with increasing $A$-site ionic radius. This provides a microscopic explanation for the deviation from the ideal $J=1/2$ state and the antiferromagnetic interactions identified in $\mathrm{CdIrO}_3$. Furthermore, the local multiplet parameters of ilmenite $\mathrm{ZnIrO}_3$ and hyperhoneycomb $\beta\text{-}\mathrm{ZnIrO}_3$ are found to be nearly identical, demonstrating that their different magnetic ground states are primarily governed by their distinct lattice structures rather than the single-ion properties. These findings establish a solid foundation for understanding how local crystal-field distortions control the magnetic Hamiltonian in Kitaev candidate materials.

cond-mat.str-el↗

arXiv2026

Ilmenite-Type Ca$_x$IrO$_3$ via Topochemical Ion Exchange: Stacking Faults and Low-Temperature Magnetic Anomaly

We report the synthesis of an ilmenite-type polymorph of Ca$_x$IrO$_3$ distinct from the known post-perovskite and perovskite phases, via low-temperature topochemical Ca$^{2+}$/2Na$^+$ exchange from Na$_2$IrO$_3$. Powder X-ray diffraction is indexable in $R\bar{3}$, and whole pattern modelling that includes layer glide faults indicates that the selective broadening can be captured by a first order Markov stacking description based on stochastic switching between two symmetry equivalent lateral stacking steps, with explicit model dependence and an uncertainty of at least several percent. A freezing-like bulk magnetic anomaly is suggested at $T^* \sim 25$ K (defined by the onset of a ZFC/FC bifurcation at $\mu_{0}H = 10$ mT), accompanied by a broad heat capacity feature and Curie-Weiss behavior with a large negative Weiss temperature of $\theta_W \sim -98$ K. The effective moment $\mu_{\rm eff} = 1.68 \mu_{\rm B}$ per Ir is consistent with $J_{\rm eff} = 1/2$ for an Ir$^{4+}$. SEM-EDX suggests an A-site content below unity (Ca/Ir $<$ 1); accordingly, we describe the ion-exchanged product using the nonstoichiometric formula Ca$_x$IrO$_3$. These results identify ilmenite-type CaIrO$_3$ as a honeycomb iridate in which stacking disorder can be quantified (with caveats regarding model and instrument correlations) and related to its low-temperature magnetic behavior.

cond-mat.mtrl-sci↗

arXiv2025

Crystalline water intercalation into the Kitaev honeycomb cobaltate Na$_2$Co$_2$TeO$_6$

We herein report the successful intercalation of water molecules into the layered honeycomb lattice of Na$_2$Co$_2$TeO$_6$, a Kitaev-candidate compound, to obtain the hydrated phase Na$_2$Co$_2$TeO$_6$$\cdot$$y$H$_2$O ($y \sim$ 2.4). Fourier transform infrared spectroscopy, thermogravimetric analysis, differential scanning calorimetry, and Rietveld refinements indicate that crystalline water resides between the cobalt-based honeycomb layers. This insertion of neutral molecules significantly alters the crystal structure, increasing the interlayer spacing and modifying the local bonding environment. Magnetization measurements reveal an antiferromagnetic transition at $T_N \sim 17.2$ K, accompanied by a discernible weak ferromagnetic component. The application of moderate magnetic fields induces a spin-flop reorientation at $\mu_0H \sim 5.7$ T. The $\lambda$-type anomaly and long-range order persist up to 9 T, showing the reconfiguration of the ground state as opposed to its suppression. Heat-capacity analysis reveals the full $2R\ln2$ magnetic entropy expected for two $J_{\rm eff} = 1/2$ moments per formula unit, confirming the pseudospin description. These findings demonstrate that water intercalation is a robust strategy for tuning the magnetic properties of honeycomb lattice materials. Overall, this study highlights neutral-molecule insertion as a promising route toward the discovery and engineering of quantum magnets based on layered transition metal oxides.

cond-mat.str-el↗

arXiv2025

Frustrated $J_1-J_2$ Diamond Lattice Antiferromagnet Co$_2$Ti$_3$O$_8$ with a Vacancy-ordered Spinel Structure Synthesized via a Topochemical Reaction

Metastable Co$_2$Ti$_3$O$_8$ was synthesized through a topochemical reaction using Li$_2$CoTi$_3$O$_8$ as the precursor, resulting in a vacancy-ordered spinel structure. Crystal structure analysis confirmed that Co ions selectively occupy the A-site, giving rise to a frustrated diamond lattice. Magnetic susceptibility and heat capacity measurements revealed antiferromagnetic order at 4.4 K, which is markedly suppressed compared to the negative Weiss temperature of ${\sim}-27$ K, indicating a high degree of frustration effects. Pulsed high-field magnetization measurements revealed a four-step successive magnetic phase transition, demonstrating that Co$_2$Ti$_3$O$_8$ is a promising candidate for a frustrated $J_1-J_2$ diamond lattice. Additionally, the $J_2/J_1$ ration estimated from the molecular field approximation suggests the possibility of a spiral ordered ground state. These observations highlight the potential of frustrated magnetism in ordered spinel structures to expand the material search space for quantum magnetism, including magnetic skyrmions.

cond-mat.str-el↗

arXiv2025

One-third magnetization plateau in a spin-1 kagome magnet BaNi$_3$(AsO$_4$)$_2$(OH)$_2$

We investigate the structural and magnetic properties of BaNi$_3$(AsO$_4$)$_2$(OH)$_2$, focusing on its spin-1 kagome lattice and the intricate coexistence of ferromagnetic and antiferromagnetic interactions. Powder x-ray diffraction analysis confirms a highly crystalline trigonal structure. Detailed Rietveld refinement identifies a single crystallographic Ni site, indicative of a perfect kagome lattice. Magnetic susceptibility measurements suggest predominantly ferromagnetic interactions with an effective magnetic moment consistent with Ni$^{2+}$ spins, yet the system undergoes antiferromagnetic ordering at a N$\'e$el temperature of 5.8 K. Isothermal magnetization measurements reveal a series of metamagnetic transitions culminating in a plateau-like phase near one-third of the total saturation magnetization. Analysis of the phase boundaries shows that the antiferromagnetic phase supports a substantial net moment in each kagome layer, comparable to that of the one-third plateau. This observation challenges the conventional model-where a 120$^\circ$ ground state transitions to an up-up-down configuration-commonly assumed for kagome antiferromagnets. Instead, our findings indicate that both the zero-field ground state and the field-induced phases exhibit in-plane ferrimagnetic spin arrangements on the kagome lattice, with the metamagnetic transition corresponding to a shift from layer-by-layer antiferromagnetically aligned net moments to ferromagnetically aligned ones. This configuration is stabilized by bond frustration, a network of competing interactions that can favor both ferromagnetic and antiferromagnetic couplings, highlighting the essential role of frustration in governing the low-temperature magnetic behavior of spin-1 kagome systems.

cond-mat.str-el↗

arXiv2025

Polymorphism and Magnetism in a Kitaev Honeycomb Cobaltate KCoAsO$_4$

We report the synthesis, crystal structure, and magnetic properties of a new Kitaev honeycomb cobaltate, KCoAsO$_4$, which crystallizes in two distinct forms: $P2/c$ and $R\bar{3}$ space groups. Magnetic measurements reveal ordering temperatures of $\sim$14 K for the $P2/c$ structure and $\sim$10.5 K for the $R\bar{3}$ structure. The $P2/c$-type KCoAsO$_4$ sample exhibits a complex temperature-field phase diagram, including a field-induced phase, while the $R\bar{3}$-type KCoAsO$_4$ shows a simpler phase diagram with a single magnetically ordered phase. The observed differences in magnetic properties are attributed to subtle structural variations, strongly suggesting that local structural changes play a crucial role in determining the magnetism of cobaltate-based Kitaev materials.

cond-mat.str-el↗

arXiv2024

CaCo$_2$TeO$_6$: A topochemically prepared 3$d^7$ honeycomb Kitaev magnet

We report the magnetic properties of CaCo$_2$TeO$_6$ as a Kitaev candidate. CaCo$_2$TeO$_6$ was synthesized through a topochemical process, wherein all Na$^+$ ions in Na$_2$Co$_2$TeO$_6$ were replaced with half the amount of Ca$^{2+}$ ions. This substitution brings the CoO$_6$ octahedra closer to an approximate cubic symmetry. CaCo$_2$TeO$_6$ exhibits antiferromagnetic ordering at $T_N \sim 13$ K, which is lower than $\sim$ 27 K observed for Na$_2$Co$_2$TeO$_6$. Notably, its magnetic order is suppressed in a relatively low magnetic field of approximately 4 T, indicating that non-Kitaev interactions can be restrained by reducing trigonal distortion. Our findings highlight the potential of CaCo$_2$TeO$_6$ as a viable platform for exploring Kitaev quantum spin liquids and pave the way for a deeper understanding of the fundamental mechanisms in Kitaev physics.

cond-mat.str-el↗

arXiv2024

Magnetic ordering of the Mo$_3$O$_8$-type cluster Mott insulator Na$_3$Sc$_2$(MoO$_4$)$_2$Mo$_3$O$_8$ with spin-1/2 triangular lattice prepared via optimal synthesis

We detail the optimized synthesis of the Mo$_3$O$_8$-type cluster Mott insulator (CMI) Na$_3$Sc$_2$(MoO$_4$)$_2$Mo$_3$O$_8$, which has been considered a candidate for realizing the spin liquid ground state. The optimized Na$_3$Sc$_2$(MoO$_4$)$_2$Mo$_3$O$_8$, characterized by x-ray diffraction, energy-dispersive x-ray spectroscopy, and magnetic and heat capacity measurements, exhibited an effective magnetic moment close to the ideal 1.73 $\mu_{\mathrm B}$ for $S = 1/2$ spin and magnetic ordering at $\sim$ 5 K. These observations categorize Na$_3$Sc$_2$(MoO$_4$)$_2$Mo$_3$O$_8$ as the second Mo$_3$O$_8$-type CMI to achieve a magnetic ground state, following Li$_2$InMo$_3$O$_8$. They highlight the stabilization of the magnetic ground state over the theoretically anticipated quantum spin liquid state through precise valence and chemical disorder tuning. Our findings challenge the existing theory that the magnetic ground state of Mo$_3$O$_8$-type CMIs is determined by the breathing parameter, instead showing that magnetic order is suppressed by spin defects. This study underscores the crucial role of chemical precision in investigating quantum magnetism. It suggests that precise tuning of valence states could induce magnetic ordering in previously nonmagnetic Mo$_3$O$_8$-type CMIs. Additionally, the negative findings regarding the existence of quantum spin liquids highlight the need for applied research and a reevaluation of our fundamental understanding of electronic states from both theoretical and experimental aspects.

cond-mat.str-el↗

arXiv2024

Molecular Orbital Electronic Instability in the van der Waals Kagome Semiconductor Nb$_3$Cl$_8$: Exploring Future Directions

Nb$_3$Cl$_8$, a cluster Mott insulator with a distinctive magnetic molecular orbital structure organized into a breathing kagome lattice, showcases critical phase transitions under specialized conditions. By transitioning from paramagnetic to nonmagnetic states below 90 K, we clarified this behavior through combined nuclear magnetic resonance and low-temperature X-ray diffraction studies, pointing to charge disproportionation as the driving force. Subsequent investigations via angle-resolved photoemission spectroscopy and first-principles calculations have disclosed topologically flat bands, confirming advanced electronic characteristics in Nb$_3$Cl$_8$. These discoveries not only deepen our comprehension of Mott insulators but also broaden our grasp of the dynamic interrelations among topology, electron interactions, and quantum phenomena in two-dimensional systems. The research on Nb$_3$Cl$_8$ thus lays foundational knowledge for advancing the exploration of quantum states in complex material systems, marking it as a critical model in the ongoing evolution of condensed matter physics.

cond-mat.mtrl-sci↗

arXiv2024

High-temperature magnetic anomaly via suppression of antisite disorder through synthesis route modification in a Kitaev candidate Cu$_2$IrO$_3$

By incorporating inert KCl into the Na$_2$IrO$_3$ + 2CuCl $\to$ Cu$_2$IrO$_3$ + 2NaCl topochemical reaction, we significantly reduced the synthesis temperature of Cu$_2$IrO$_3$ from the 350$^\circ$C reported in previous studies to 170$^\circ$C. This adjustment decreased the Cu/Ir antisite disorder concentration in Cu$_2$IrO$_3$ from $\sim$19$\%$ to $\sim$5$\%$. Furthermore, magnetic susceptibility measurements of the present Cu$_2$IrO$_3$ sample revealed a weak ferromagnetic-like anomaly with hysteresis at a magnetic transition temperature of $\sim$70 K. Our research indicates that the spin-disordered ground state reported in chemically disordered Cu$_2$IrO$_3$ 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.

cond-mat.mtrl-sci↗

arXiv2023

Impact of Oxidation State on the Valence-bond-glass Physics in the lithium-intercalated Mo$_3$O$_8$ Cluster Mott Insulators

We have successfully synthesized four Mo$_3$O$_8$-type cluster Mott insulators (CMI) by intercalating lithium into nonmagnetic precursors to regulate the Mo$_3$ cluster valence. The resulting materials are Li$_{1+x}$$R$Mo$_3$O$_8$ ($R$ = Sc, Y, Lu) and Li$_x$Zn$_2$Mo$_3$O$_8$. Our magnetic susceptibility measurements revealed that these materials display characteristics akin to a valence bond glass state and suggest the presence of short-range ordering when the Mo$_3$ cluster valence approximates its ideal value. These findings challenge the prevailing belief that the plaquette charge ordering state is an inherent feature of Mo$_3$O$_8$-type CMI. Instead, they underscore the importance of Mo$_3$ cluster valence in determining the physical properties of these systems. These insights furnish a fresh understanding of the Mo$_3$O$_8$-type CMI and open new research opportunities in highly frustrated magnetism.

cond-mat.str-el↗

arXiv2023

Monoclinic distortion in hyperhoneycomb Kitaev material $\beta$-ZnIrO$_3$ 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.

cond-mat.str-el↗

arXiv2023

Magnetic ordering in the $J_{\rm eff}$ = 0 Nickelate NiRh$_2$O$_4$ 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.

cond-mat.str-el↗

arXiv2023

Magnetic field induced valence change in Eu(Co$_{1-x}$Ni$_{x}$)$_{2}$P$_{2}$ up to 60 T

The solid solution 122 compounds, Eu(Co$_{1-x}$Ni$_{x}$)$_{2}$P$_{2}$, show valence transition between divalent state and intermediate valence states at Eu, which is firmly correlated to multiple degrees of freedom in the solid such as the isostructural transition between the collapsed tetragonal (cT) and uncollapsed tetragonal (ucT) structures, $3d$ magnetism, and the formation of P-P dimers. To gain insights into the correlated behavior, we investigate the effect of high magnetic fields on the samples of $x = 0.4$ and $0.5$ using magnetostriction and magnetization measurements up to 60 T. The samples are in the Eu valence fluctuating regime, where the possible structural transition from cT to ucT may be induced by the Eu valence change under the magnetic fields. For both samples, magnetostriction smoothly increases with increasing magnetic fields. The behavior is in good agreement with the calculated results using the interconfigurational fluctuation (ICF) model that describes the valence change. This indicates that $\Delta L$ represents the change of the Eu valence state in these compounds. Magnetization curves for both compounds show good agreement with the ICF model at high magnetic fields. In contrast, in the low magnetic field region, magnetization curves do not agree with the ICF model. These results indicate that the Eu valence changes manifest themselves in the magnetization curves at high magnetic fields and that the magnetism of the $3d$ electrons manifests itself in the magnetization at low magnetic fields. Hence, we conclude that the valence change occurs within the Eu valence fluctuation regime coupled with the cT structure. Thereby, we believe that the transition to ucT structure which is firmly coupled with the divalent Eu state does not occur within the magnetic field range of the present study. (Continued)

cond-mat.str-el↗