arXiv subjects

Koichi Kindo

Publications and source records attributed to Koichi Kindo.

At least 37 records · Page 2Linked to original sources

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

Electronic Structure of Kramers Nodal-Line Semimetal YAuGe and Anomalous Hall Effect Induced by Magnetic Rare-Earth Substitution

Nodal-line semimetals are a class of topological materials hosting one dimensional lines of band degeneracy. Kramers nodal-line (KNL) metals/semimetals have recently been theoretically recognized as a class of topological states inherent to all non-centrosymmetric achiral crystal lattices. We investigate the electronic structure of candidate KNL semimetal YAuGe by angle-resolved photoemission spectroscopy (ARPES) and quantum oscillations as well as by density functional theory (DFT) calculations. DFT has revealed that YAuGe hosts KNLs on the G-A-L-M plane of the Brillouin zone, that are protected by the time reversal and mirror-inversion symmetries. Through ARPES and quantum oscillations we identify signatures of hole bands enclosing the G point, and the observed splitting of quantum oscillation frequency with angle is attributed to spin-orbit-coupling-induced band splitting away from the KNLs. Furthermore, we show that the degeneracy of the nodal lines along the G-A line is lifted by the time-reversal-symmetry breaking when the Y is substituted by magnetic R ions (R = rare earth). This becomes a source of Berry curvature and contributes to the anomalous Hall effect in magnetic RAuGe. These findings establish RAuGe as a new class of KNL semimetals offering significant potential for engineering of anomalous magnetotransport properties via magnetic rare-earth substitution.

cond-mat.mtrl-sci↗

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$é$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

Anomalous Electrical Transport in the Kagome Magnet YbFe$_6$Ge$_6$

Two-dimensional (2D) kagome metals offer a unique platform for exploring electron correlation phenomena derived from quantum many-body effects. Here, we report a combined study of electrical magnetotransport and neutron scattering on YbFe$_6$Ge$_6$, where the Fe moments in the 2D kagome layers exhibit an $A$-type collinear antiferromagnetic order below $T_{\rm{N}} \approx 500$ K. Interactions between the Fe ions in the layers and the localized Yb magnetic ions in between reorient the $c$-axis aligned Fe moments to the kagome plane below $T_{\rm{SR}} \approx 63$ K. Our magnetotransport measurements show an intriguing anomalous Hall effect (AHE) that emerges in the spin-reorientated collinear state, accompanied by the closing of the spin anisotropy gap as revealed from inelastic neutron scattering. The gapless spin excitations and the Yb-Fe interaction are able to support a dynamic scalar spin chirality, which explains the observed AHE. Therefore, our study demonstrates spin fluctuations may provide an additional scattering channel for the conduction electrons and give rise to AHE even in a collinear antiferromagnet.

cond-mat.str-el↗

arXiv2025

Piezoelectric Transition in a Nonpyroelectric Gyroidal Metal-Organic Framework

Among the thirty-two crystallographic point groups, 432 is the only one that lacks an inversion center but does not exhibit piezoelectricity. A gyroidal structure belongs to point group 432 and shows characteristic physical properties attributed to its distinctive strong isotropic network. Here, we investigate a gyroidal cobalt oxalate metal-organic framework (MOF) with disordered orientations of SO4 tetrahedra. Synchrotron X-ray diffraction experiments using a single crystal reveal a cubic-to-cubic structural phase transition at TS = 120 K. This transition involves a change in the point group from nonpiezoelectric 432 to piezoelectric 23. The symmetry change arises from the ordering of distorted SO4 molecules, leading to a three-dimensional helical arrangement of electric dipole moments. Furthermore, pyroelectric current measurements using polycrystalline pellet samples reveal that electric polarization emerges below TS depending on the magnitude of the pelletizing pressure, demonstrating piezoelectricity. The gyroidal MOF offers an opportunity to explore unique dielectric properties induced by the helical ordering of molecules and structural flexibility.

cond-mat.mtrl-sci↗

arXiv2025

Crystal-field magnetostriction of the spin ice under ultrahigh magnetic fields

We present a comprehensive study of the magnetoelastic properties of the Ising pyrochlore oxide Ho$_{2}$Ti$_{2}$O$_{7}$, known as spin ice, by means of high-field magnetostriction measurements and numerical calculations. When a magnetic field is applied along the crystallographic <111> axis, the longitudinal magnetostriction exhibits a broad maximum in the low-field regime around 30 T, followed by a dramatic lattice contraction due to crystal-field (CF) level crossing at $B_{\rm cf} \sim 65$ T. The transverse magnetostriction exhibits a contrasting behavior, highlighting the anisotropic nature of the CF striction. We identify distinct timescales of spin dynamics and CF-phonon dynamics by applying a magnetic field with different field-sweep rates. Our mean-field calculations, based on a point-charge model, successfully reproduce the overall magnetostriction behavior, revealing the competition between the exchange striction and CF striction. A signature of the CF level crossing is also observed through adiabatic magnetocaloric-effect measurements, consistent with our magnetostriction data.

cond-mat.str-el↗

arXiv2024

Properties of an organic model $S=1$ Haldane chain system

We present the properties of a new organic $S=1$ antiferromagnetic chain system $m$-NO$_2$PhBNO (abbreviated BoNO). In this biradical system two unpaired electrons from aminoxyl groups are strongly ferromagnetically coupled ($|J_\text{FM}| /k_B \gtrsim 500$ K) which leads to the formation of an effective $S=1$ state for each molecule. The chains of BoNO biradicals propagate along the crystallographic $a$ axis. Temperature dependence of the $g$ factor and electron paramagnetic resonance (EPR) linewidth are consistent with a low-dimensional system with antiferromagnetic interactions. The EPR data further suggest that BoNO is the first known Haldane system with an almost isotropic $g$ factor ($2.0023 \pm 2 \unicode{x2030}$). The magnetization measurements in magnetic fields up to $40$ T and low-field susceptibility, together with $^1$H nuclear magnetic resonance (NMR) spectra, reveal a dominant intrachain antiferromagnetic exchange coupling of $J_\text{1D}/k_B = (11.3\pm0.1)$ K, and attainable critical magnetic fields of $μ_0 H_\text{c1} \approx 2$ T and $μ_0 H_\text{c2} \approx 33$ T. These measurements therefore suggest that BoNO is a unique Haldane system with extremely small magnetic anisotropy. Present results are crucial for a future in-depth NMR study of the low-temperature Tomonaga-Luttinger liquid (TLL) and magnetic field-induced phases, which can be performed in the entire phase space.

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

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↗

arXiv2024

Giant Linear Dichroism Controlled by Magnetic Field in FePS$_3$

Magnetic-field control of fundamental optical properties is a crucial challenge in the engineering of multifunctional microdevices. Van der Waals (vdW) magnets retaining a magnetic order even in atomically thin layers, offer a promising platform for hosting exotic magneto-optical functionalities owing to their strong spin-charge coupling. Here, we demonstrate that a giant optical anisotropy can be controlled by magnetic fields in the vdW magnet FePS$_3$. The giant linear dichroism ($\sim$11%), observed below $T_{\text{N}}\!\sim\!120$ K, is nearly fully suppressed in a wide energy range from 1.6 to 2.0 eV, following the collapse of the zigzag magnetic order above 40 T. This remarkable phenomenon can be explained as a result of symmetry changes due to the spin order, enabling minority electrons of Fe$^{2+}$ to hop in a honeycomb lattice. The modification of spin-order symmetry by external fields provides a novel route for controllable anisotropic optical micro-devices.

cond-mat.mtrl-sci↗

arXiv2024

Quantum spin state stabilized by coupling with classical spins

We introduce a model compound featuring a spin-1/2 frustrated square lattice partially coupled by spin-5/2. A significant magnetization plateau exceeding 60 T could be observed, indicating a quantum state formed by $S$ = 1/2 spins in the square lattice. The remaining $S$ = 5/2 spins exhibited paramagnetic behavior in the low-field regions. The numerical analysis confirmed that the observed quantum state is a many-body entangled state based on the dominant AF interactions and is strongly stabilized by coupling with spin-5/2. The stabilization of this quantum state can be attributed to a compensation effect similar to magnetic field-induced superconductivity, which serves as a strategy to control the stability of quantum spin states in magnetic fields.

cond-mat.str-el↗

arXiv2024

Geometric frustration and Dzyaloshinskii-Moriya interactions in a quantum star lattice hybrid copper sulfate

We study the magnetism of a layered, spin-$\frac12$ organic-inorganic copper sulfate, which is a close realization of the star lattice antiferromagnet, one of the playgrounds of geometric frustration and resonating valence bond physics in two spatial dimensions. Our thermodynamic measurements show no ordering down to 0.1 K and a characteristic field-induced entropic shift, revealing the presence of an infinite number of competing states down to very low energy scales. The response to external magnetic fields shows, in addition, a peculiar anisotropy, reflected in the formation of a 1/3 magnetization plateau (stable up to full saturation around 105 T) and a paramagnetic, Curie-like susceptibility for one direction of the field (${\bf H}\parallel{\bf c}$), and a completely different response in other field directions. Our first-principles density functional theory calculations and exact diagonalizations show that these experimental puzzles are distinctive signatures of a strong interplay between geometric frustration and sizable Dzyaloshinskii-Moriya interactions, and the emergence of a continuous U(1) symmetry at low energy scales.

cond-mat.str-el↗

arXiv2024

$J_{\rm{eff}}$ = 1/2 Hyperoctagon Lattice in Cobalt Oxalate Metal-Organic-Framework

We report the magnetic properties of a cobalt oxalate metal-organic-framework featuring the hyperoctagon lattice. Our thermodynamic measurements reveal the $J_{\rm{eff}}$ = 1/2 state of the high-spin Co$^{2+}$ (3$\textit{d}^{7}$) ion and the two successive magnetic transitions at zero field with two-stage entropy release. $^{13}$C-NMR measurements reveal the absence of an internal magnetic field in the intermediate temperature phase. Multiple field-induced phases are observed before full saturation at around 40 T. We argue the unique cobalt oxalate network gives rise to the Kitaev interaction and/or a bond frustration effect, providing an unconventional platform for frustrated magnetism on the hyperoctagon lattice.

cond-mat.str-el↗

arXiv2024

Quantum liquid states of spin solitons in a ferroelectric spin-Peierls state

In this study, we performed high-magnetic-field magnetization, dielectric, and ultrasound measurements on an organic salt showing a ferroelectric spin-Peierls (FSP) state, which is in close proximity to a quantum critical point. In contrast to the sparsely distributed gas-like spin solitons typically observed in conventional spin-Peierls (SP) states, the FSP state exhibits dense liquid-like spin solitons resulting from strong quantum fluctuations, even at low fields. Nevertheless, akin to conventional SP systems, a magnetic-field-induced transition is observed in the FSP state. In conventional high-field SP states, an emergent wave vector results in the formation of a spin-soliton lattice. However, in the present high-field FSP state, the strong quantum fluctuations preclude the formation of such a soliton lattice, causing the dense solitons to remain in a quantum-mechanically melted state. This observation implies the realization of a quantum liquid--liquid transition of topological particles carrying spin and charge in a ferroelectric insulator.

cond-mat.str-el↗

arXiv2023

Pseudogap formation in organic superconductors

The condensation of paired fermions into superfluid states changes progressively depending on the coupling strength. At the midpoint of the crossover between Bardeen--Cooper--Schrieffer (BCS) weak-coupling and Bose--Einstein condensate (BEC) strong-coupling limits, paired fermions condensate most robustly, thereby leading to the emergence of a pseudogap due to enhanced pairing fluctuations. In the case of electrons in solids, excessively strong interactions often induce competing electronic orders instead of strong-coupling superconductivity, and experimental comprehension of the pseudogap remains incomplete. In this study, we provide experimental evidence demonstrating the opening of a pseudogap, marking the incipient stage of the BCS-BEC crossover in the organic system $κ$-(BEDT-TTF)$_2$$X$. By controlling electron correlations, we investigate the thermodynamic properties of the BCS-BEC crossover and pseudogap phase. Since the superconductivity of $κ$-(BEDT-TTF)$_2$$X$ arises from a simple Fermi liquid that does not exhibit any other electronic orders, our study shed light on the inherent nature of the BCS-BEC crossover.

cond-mat.supr-con↗

arXiv2023

NiCrAl piston-cylinder cell for magnetic susceptibility measurements under high pressures in pulsed high magnetic fields

We developed a metallic pressure cell made of nickel-chromium-aluminum (NiCrAl) for use with a non-destructive pulse magnet and a magnetic susceptibility measurement apparatus with a proximity detector oscillator (PDO) in pulsed magnetic fields of up to 51 T under pressures of up to 2.1 GPa. Both the sample and sensor coil of the PDO were placed in the cell so that the magnetic signal from NiCrAl would not overlay the intrinsic magnetic susceptibility of the sample. A systematic investigation of the Joule heating originating from metallic parts of the pressure cell revealed that the temperature at the sample position remains at almost 1.4 K until approximately 80 $\%$ of the maximum applied magnetic field ($H_{\rm max}$) in the field-ascending process (e.g., 40 T for $H_{\rm max}$ of 51 T). The effectiveness of our apparatus was demonstrated, by investigating the pressure dependence of the magnetization process of the triangular-lattice antiferromagnet Ba$_3$CoSb$_2$O$_9$.

cond-mat.mtrl-sci↗

arXiv2023

Field-induced quantum phase in a frustrated zigzag-square lattice

This study presents the experimental realization of a spin-1/2 zigzag-square lattice in a verdazyl-based complex, namely ($m$-Py-V-2,6-F$_2$)$[$Cu(hfac)$_2]$. Molecular orbital calculations suggest the presence of five types of frustrated exchange couplings. Our observations reveal an incremental increase in the magnetization curve beyond a critical field, signifying a phase transition from the antiferromagnetic ordered state to a quantum state characterized by a 1/2 plateau. This intriguing behavior arises from the effective stabilization of a zigzag chain by the external fields. These results provide evidence for field-induced dimensional reduction in a zigzag-square lattice attributed to the effects of frustration.

cond-mat.str-el↗

arXiv2023

Fermi surface and light quasi particles in hourglass nodal chain metal \b{eta}-ReO2

Quantum oscillations in magnetic torque and electrical resistivity were measured to investigate the electronic structure of \b{eta}-ReO2, a candidate hourglass nodal chain metal (Dirac loop chain metal). All the de Haas-van Alphen oscillation branches measured at 30 mK in magnetic fields of up to 17.5 T were consistent with first-principles calculations predicting four Fermi surfaces (FSs). The small-electron FS of the four FSs exhibited a very small cyclotron mass, 0.059 times that of the free electrons, which is likely to be related to the linear dispersion of the energy band. The consistency between the quantum oscillation results and band calculations indicates the presence of the hourglass nodal chain predicted for \b{eta}-ReO2 in the vicinity of the Fermi energy.

cond-mat.str-el↗