arXiv subjects

Ryuji Okazaki

Publications and source records attributed to Ryuji Okazaki.

At least 19 recordsLinked to original sources

arXiv2026

Characteristics of natural remanence records in fine-grained particles returned from asteroid Ryugu

Particles collected from the asteroid Ryugu by the Hayabusa2 spacecraft offer a unique opportunity to investigate the magnetic record of the primitive solar system, as any terrestrial magnetic contamination is minimal and can be accounted for. In previous studies, stepwise alternating field demagnetization (AFD) measurements of natural remanent magnetization (NRM) records have been conducted on seven Ryugu particles. However, due to the limited number of samples, there is no consensus regarding the interpretation of the results of these measurements. To address this problem, we performed stepwise AFD measurements of the NRM on 28 Ryugu particles. Twenty-three of the particles exhibited one or two stable NRM components, whereas the remaining five did not. Isothermal remanent magnetization-based paleointensity values derived from stable NRM components varied by more than one order of magnitude. These NRM characteristics were consistent with those observed in previous studies. Therefore, as a reflection of the original nature of the NRM record, some Ryugu particles exhibited stable NRM components, whereas others did not. The Ryugu particles investigated in this study and those from a previous study exhibited spatially inhomogeneous NRM directions within individual particles, constraining the NRM acquisition time to before the final solidification of the current Ryugu particles. A mechanism of remanence acquisition that can explain the observed NRM characteristics is a chemical remanent magnetization associated with the growth of framboidal magnetite during aqueous alteration in Ryugu's parent body.

astro-ph.EP↗

arXiv2025

Effects of successive annealing on high-field electrical transport and the upper critical field in S-substituted FeTe

Since iron-based superconductors have been discovered, many scientists have focused on their characteristic properties, such as an unconventional mechanism and a high upper critical field. Sulphur-substituted FeTe compounds are one of the members of the iron-based superconductors; however, chemical processes, such as O$_2$ annealing, are needed to induce superconductivity because of the existence of excess iron in as-grown crystals. Thus, the removal of excess iron and the obtaining of clean sulphur-substituted FeTe can play a key role in the understanding of the superconducting properties and the application to the superconducting devices. In this study, we present the successive annealing effects on sulphur-substituted FeTe compounds to investigate the electrical transport properties under high magnetic fields. Our measurements show that successive annealing processes improve the electrical transport properties in the superconducting states under magnetic fields. The removal of excess iron acting as magnetic impurities is indicated by the improvement of the upper critical field and its analysis.

cond-mat.supr-con↗

arXiv2025

Impurity-induced spin density wave in the thermoelectric layered cobaltite [Ca$_2$CoO$_3$]$_{0.62}$[CoO$_2$]

We investigate the Sn-substitution effect on the thermoelectric transport properties of the layered cobaltite [Ca$_2$CoO$_3$]$_{0.62}$[CoO$_2$] single crystals, which exhibit a non-monotonic temperature variation of the electrical resistivity and the Seebeck coefficient owing to the complex electronic and magnetic states. We find that the onset temperature of the short-range spin-density-wave (SDW) formation increases with the substituted Sn content, indicating the impurity-induced stabilization of the SDW order, reminiscent of the disorder/impurity-induced spin order in the cuprate superconductors. The Seebeck coefficient is well related to such impurity effects, as it is slightly enhanced below the onset temperatures, implying a decrease in the carrier concentration due to a pseudo-gap formation associated with the SDW ordering. We discuss the site-dependent substitution effects including earlier studies, and suggest that substitution to the conducting CoO$_2$ layers is essential to increase the onset temperature, consistent with the impurity-induced SDW picture realized in the conducting layers with the cylindrical Fermi surface.

cond-mat.str-el↗

arXiv2024

Transverse thermoelectric conversion in the mixed-dimensional semimetal WSi$_2$

Materials with axis-dependent conduction polarity are known as $p\times n$-type or goniopolar conductors that can be used for transverse thermoelectric devices, allowing the longitudinal thermal current to be converted into the transverse electrical current. Here, we performed experimental and computational studies on the transport properties of WSi$_2$ single crystals, in which such axis-dependent conduction polarity of the thermopower and the Hall coefficient have recently been reported, and demonstrated the transverse thermoelectric effect by applying the temperature gradient in a 45$^{\circ}$ rotated direction from the crystallographic axis. We have observed strongly sample-dependent transport properties, which have also been observed in previous studies, and together with first-principles calculations we show that such sample-dependent transport properties originate from the band-dependent scattering rates of carriers. The calculated band-resolved Peltier conductivity shows that the mixed-dimensional electronic structure consisting of a quasi-one-dimensional electron Fermi surface and a quasi-two-dimensional hole surface is a key property for the axis-dependent conduction polarity. The directly obtained transverse thermoelectric figure of merit is comparable to that of the anomalous Nernst materials, implying that the present material is a potential candidate for transverse thermoelectric conversion.

cond-mat.mtrl-sci↗

arXiv2024

The Ni isotopic composition of Ryugu reveals a common accretion region for carbonaceous chondrites

The isotopic compositions of samples returned from Cb-type asteroid Ryugu and Ivuna-type (CI) chondrites are distinct from other carbonaceous chondrites, which has led to the suggestion that Ryugu and CI chondrites formed in a different region of the accretion disk, possibly around the orbits of Uranus and Neptune. We show that, like for Fe, Ryugu and CI chondrites also have indistinguishable Ni isotope anomalies, which differ from those of other carbonaceous chondrites. We propose that this unique Fe and Ni isotopic composition reflects different accretion efficiencies of small FeNi metal grains among the carbonaceous chondrite parent bodies. The CI chondrites incorporated these grains more efficiently, possibly because they formed at the end of the disk's lifetime, when planetesimal formation was also triggered by photoevaporation of the disk. Isotopic variations among carbonaceous chondrites may thus reflect fractionation of distinct dust components from a common reservoir, implying CI chondrites and Ryugu may have formed in the same region of the accretion disk as other carbonaceous chondrites.

astro-ph.EP↗

arXiv2024

Thermoelectric transport properties of the quasi-one-dimensional dimer-Mott insulator $\beta'$-(BEDT-TTF)$_2$ICl$_2$

Low-dimensional materials, in which the electronic and transport properties are drastically modified in comparison to those of three-dimensional bulk materials, yield a key class of thermoelectric materials with high conversion efficiency. Among such materials, the organic compounds may serve peculiar properties owing to their unique molecular-based low-dimensional structures with highly anisotropic molecular orbitals. Here we present the thermoelectric transport properties of the quasi-one-dimensional dimer-Mott insulator $\beta'$-(BEDT-TTF)$_2$ICl$_2$, where BEDT-TTF stands for bis(ethylenedithio)-tetrathiafulvalene. We find that the thermopower exhibits typical activation-type temperature variation expected for insulators but its absolute value is anomalously large compared to the expected value from the activation-type temperature dependence of the electrical resistivity. Successively, the Jonker-plot analysis, in which the thermopower is usually scaled by the logarithm of the resistivity, shows an unusual relation among such transport quantities. We discuss a role of the low dimensionality for the enhanced thermopower along with recent observations of such a large thermopower in several low-dimensional materials.

cond-mat.str-el↗

arXiv2024

Negatively enhanced thermopower near a Van Hove singularity in electron-doped Sr$_2$RuO$_4$

The layered perovskite Sr$_2$RuO$_4$ serves as a model material of the two-dimensional (2D) Fermi liquid but also exhibits various emergent phenomena including the non-Fermi-liquid (NFL) behavior under external perturbations such as uniaxial pressure and chemical substitutions. Here we present the thermoelectric transport of electron-doped system Sr$_{2-y}$La$_{y}$RuO$_4$, in which a filling-induced Lifshitz transition occurs at the Van Hove singularity (VHS) point of $y\approx 0.2$. We find that the sign of the low-temperature thermopower becomes negative only near the VHS point, where the NFL behavior has been observed in the earlier work. This observation is incompatible with either a numerical calculation within a constant relaxation-time approximation or a toy-model calculation for the 2D Lifshitz transition adopting an elastic carrier scattering. As a promising origin of the observed negatively enhanced thermopower, we propose a skewed NFL state, in which an inelastic scattering with a considerable odd-frequency term plays a crucial role to negatively enhance the thermopower.

cond-mat.str-el↗

arXiv2024

Microscale Hydrogen, Carbon, and Nitrogen Isotopic Diversity of Organic Matter in Asteroid Ryugu

We report the H, C, and N isotopic compositions of microscale (0.2 to 2$\mu$m) organic matter in samples of asteroid Ryugu and the Orgueil CI carbonaceous chondrite. Three regolith particles of asteroid Ryugu, returned by the Hayabusa2 spacecraft, and several fragments of Orgueil were analyzed by NanoSIMS isotopic imaging. The isotopic distributions of the Ryugu samples from two different collection spots are closely similar to each other and to the Orgueil samples, strengthening the proposed Ryugu-CI chondrite connection. Most individual sub-$\mu$m organic grains have isotopic compositions within error of bulk values, but 2-8% of them are outliers exhibiting large isotopic enrichments or depletions in D, $^{15}$N, and/or $^{13}$C. The H, C and N isotopic compositions of the outliers are not correlated with each other: while some C-rich grains are both D- and $^{15}$N-enriched, many are enriched or depleted in one or the other system. This most likely points to a diversity in isotopic fractionation pathways and thus diversity in the local formation environments for the individual outlier grains. The observation of a relatively small population of isotopic outlier grains can be explained either by escape from nebular and/or parent body homogenization of carbonaceous precursor material or addition of later isotopic outlier grains. The strong chemical similarity of isotopically typical and isotopically outlying grains, as reflected by synchrotron x-ray absorption spectra, suggests a genetic connection and thus favors the former, homogenization scenario. However, the fact that even the least altered meteorites show the same pattern of a small population of outliers on top of a larger population of homogenized grains indicates that some or most of the homogenization occurred prior to accretion of the macromolecular organic grains into asteroidal parent bodies.

astro-ph.EP↗

arXiv2023

Fermi surface topology and electronic transport properties of a chiral crystal NbGe$_2$ with strong electron-phonon interaction

We report the electronic structures and transport properties of a chiral crystal NbGe$_2$, which is a candidate for a coupled electron-phonon liquid. The electrical resistivity and thermoelectric power of NbGe$_2$ exhibit clear differences compared to those of NbSi2 even though both niobium ditetrelides are isostructural and isoelectronic. We discuss the intriguing transport properties of NbGe$_2$ based on a van Hove-type singularity in the density of states. The analysis of de Haas-van Alphen oscillations measured by the field modulation and magnetic torque methods reveals the detailed shape of the Fermi surface of NbGe$_2$ by comparison with the results of energy band structure calculations using a local density approximation. The electron and hole Fermi surfaces of NbGe$_2$ split into two because of the anti-symmetric spin-orbit interaction. The temperature dependence of quantum oscillations indicates that the effective mass is isotropically enhanced in NbGe$_2$ due to strong electron-phonon interaction.

cond-mat.str-el↗

arXiv2023

Carrier filtering effect for enhanced thermopower in a body-centered tetragonal ruthenate

Charged carriers in solids diffuse from hot to cold sides under temperature gradient to induce the thermoelectric voltage. Carrier filtering effect, which only passes either electrons or holes for the conduction process, is an efficient method to enhance such voltage, although it is challenging to experimentally realize it especially in conventional metals with weak energy dependence of the density of states near the Fermi level. Here we measure the in-plane and out-of-plane thermopower of the layered perovskite Sr$_2$RuO$_4$ single crystals above room temperature, and find that the out-of-plane thermopower is largely enhanced with increasing temperature, while the in-plane one seems to remain a temperature-independent constant value which is expected from the Heikes formula. The observed large out-of-plane thermopower may originate from the recently proposed intriguing hole filtering effect in the body-centered tetragonal system, in which the carrier hopping through the centered atom is essential. Thus, the present carrier filtering effect may be a universal property to be applicable in various materials belonging to such crystal system.

cond-mat.str-el↗

arXiv2023

Mechanically exfoliated low-layered [Ca$_2$CoO$_3$]$_{0.62}$[CoO$_2$]: A single-crystalline p-type transparent conducting oxide

Transparent conducting oxides (TCOs) are essential components of optoelectronic devices and various materials have been explored for highly efficient TCOs having a combination of high transmittance and low sheet resistance. Here, we focus on a misfit thermoelectric oxide [Ca$_2$CoO$_3$]$_{0.62}$[CoO$_2$] and fabricate the transparent low-layered crystals by a mechanical tape-peeling method using the single-crystalline samples. From the transmittance measurement, we find that the thickness of low-layered samples is several orders of hundred nanometers, which is comparable with the estimation from the scanning electron microscopy images. Compared to the previous results on the polycrystalline and $c$-axis oriented transparent films, the electrical resistivity is reduced owing to the single-crystalline nature. The figure of merit for the transparent conducting materials in the present low-layered samples is then evaluated to be higher than the values in the previous reports. The present results on the low-layered single-crystalline [Ca$_2$CoO$_3$]$_{0.62}$[CoO$_2$] may offer a unique class of multi-functional transparent thermoelectric oxides.

cond-mat.mtrl-sci↗

arXiv2023

Monoaxial Dzyaloshinskii-Moriya interaction-induced topological Hall effect in a new chiral-lattice magnet GdPt$_2$B

We investigate the topological Hall effect (THE) in the monoaxial chiral crystal GdPt$_2$B, a recently discovered compound that exhibits putative helimagnetism below 87 K. The distinct THE was observed in GdPt2B in the magnetically ordered state. The scaling relations for anomalous and topological Hall conductivities differed from those of conventional models based on the scattering process. We further demonstrate the clear scaling behavior of the THE in a wide temperature range, which we attribute to the monoaxial Dzyaloshinskii-Moriya (DM) interaction under external magnetic fields perpendicular to the screw axis. The THE induced by the monoaxial DM interaction as well as the THE in a monoaxial chiral crystal of f-electron system are demonstrated in this study.

cond-mat.str-el↗

arXiv2022

Anomalous scaling law for thermoelectric transport of 2D-confined electrons in an organic molecular system

Confined electrons in low dimensions host desirable material functions for downscaled electronics as well as advanced energy technologies. Thermoelectricity is a most fascinating example, since the dimensionality modifies the electron density of states dramatically, leading to enhanced thermopower as experimentally examined in artificial two-dimensional (2D) structures. However, it is still an open question whether such an enhanced thermopower in low dimensions is realized in layered materials with strong 2D characters such as cuprates. Here, we report unusual enhancement of the thermopower in the layered organic compound $\alpha$-(BEDT-TTF)$_2$I$_3$, where BEDT-TTF stands for bis(ethylenedithio)-tetrathiafulvalene. We find that the slope in the Jonker plot (thermopower $S$ vs. logarithm of electrical conductivity $\log\sigma$) for $\alpha$-(BEDT-TTF)$_2$I$_3$ is significantly larger than that of conventional semiconductors. Moreover, the large slope is also seen in the related layered salt, demonstrating the impact of the 2D-confined carriers in the layered organics on thermoelectricity.

cond-mat.mtrl-sci↗

arXiv2022

New Gd-based magnetic compound GdPt$_2$B with a chiral crystal structure

Herein, we report the discovery of a novel Gd-based magnetic compound GdPt$_2$B with a chiral crystal structure. X-ray diffraction and chemical composition analyses reveal a CePt$_2$B-type crystal structure (space group: $P6_422$) for GdPt$_2$B. Moreover, we successfully grew single crystals of GdPt$_2$B using the Czochralski method. Magnetization measurements and the Curie$-$Weiss analysis demonstrate that the ferromagnetic interaction is dominant in GdPt$_2$B. A clear transition is observed in the temperature dependence of electrical resistivity, magnetic susceptibility, and specific heat at $T_{\rm O}$ = 87 K. The magnetic phase diagram of GdPt$_2$B, which consists of a field-polarized ferromagnetic region and a magnetically ordered region, resembles those of known chiral helimagnets. Furthermore, magnetic susceptibility measurements reveal a possible spin reorientation within the magnetically ordered phase in magnetic fields perpendicular to the screw axis. The results demonstrate that GdPt$_2$B is a suitable platform for investigating the competing effects of ferromagnetic and antisymmetric exchange interactions in rare-earth-based chiral compounds.

cond-mat.mtrl-sci↗

arXiv2022

Enhanced Seebeck coefficient through the magnetic fluctuations in Sr$_2$Ru$_{1-x}M_x$O$_4$ ($M = $ Co, Mn)

The layered perovskite Sr$_2$RuO$_4$ is a most intensively studied superconductor, but its pairing mechanism, which is often coupled intimately with magnetic fluctuations in correlated materials, is still an open question. Here we present a systematic evolution of the Seebeck coefficient in Co- and Mn-substituted Sr$_2$RuO$_4$ single crystals, in which ferromagnetic and antiferromagnetic glassy states respectively emerge in proximity to the superconducting phase of the parent compound. We find that the Seebeck coefficient $S$ divided by temperature $T$, $S/T$, shows a maximum near characteristic temperatures seen in the irreversible magnetization $M_{\rm ir}$ in both of the Co- and Mn-substituted crystals, demonstrating both of the ferromagnetic and antiferromagnetic fluctuations to enhance the Seebeck coefficient. Interestingly, $S/T$ increases with lowering temperature in the parent compound, reminiscent of non-Fermi-liquid behavior, indicating an essential role of coexisting ferromagnetic and antiferromagnetic fluctuations for the itinerant electrons in Sr$_2$RuO$_4$.

cond-mat.str-el↗

arXiv2022

Prominent electron-hole asymmetry in thermoelectric transport of LaCoO$_3$

We have measured the electrical resistivity and the thermopower of the electron-doped perovskite cobaltites LaCo$_{1-y}$Te$_y$O$_3$. In contrast to the hole-doped systems such as metallic ferromagnets La$_{1-x}M_x$CoO$_3$ ($M$ = Ca, Sr, Ba), the electron-doped samples show an insulating behavior even in a heavily doped range due to a spin-state blockade mechanism that an electron hopping from high-spin Co$^{2+}$ to low-spin Co$^{3+}$ site is energetically suppressed. We find that, despite the electron doping, the thermopower shows relatively large positive values above $y=0.05$, strikingly distinct from the hole-doped case where it comes close to zero with doping. This prominent electron-hole asymmetry seen in the thermopower originates from a bipolar conduction which consists of a slight amount of mobile holes and the main immobile electrons, demonstrating an impact of spin-state blockade on thermoelectric transport.

cond-mat.str-el↗

arXiv2022

Third-order Electrical Conductivity of the Charge-ordered Organic Salt $α$-(BEDT-TTF)$_2$I$_3$

We performed third-order electrical conductivity measurements on the organic conductor $α$-(BEDT-TTF)$_2$I$_3$ using an ac bridge technique sensitive to nonlinear signals. Third-order conductance $G_3$ is clearly observed even at low electric fields, and interestingly, $G_3$ is critically enhanced above the charge-order transition temperature $T_{\rm CO}=136$~K. The observed frequency dependence of $G_3$ is incompatible with a percolation model, in which a Joule heating in a random resistor network is relevant to the nonlinear conduction. We instead argue the nonlinearity of the relaxation time according to a phenomenological model on the mobility in materials with large dielectric constants, and find that the third-order conductance $G_3$ corresponds to the third-order electric susceptibility $χ_3$. Since the nonlinear susceptibility is known as a probe for higher-order multipole ordering, the present observation of the divergent behavior of $G_3$ above $T_{\rm CO}$ reveals an underlying quadrupole instability at the charge-order transition of the organic system.

cond-mat.str-el↗

arXiv2019

Enhanced Seebeck coefficient by a filling-induced Lifshitz transition in KxRhO2

We have systematically measured the transport properties in the layered rhodium oxide K$_{x}$RhO$_{2}$ single crystals ($0.5\lesssim x \lesssim 0.67$), which is isostructural to the thermoelectric oxide Na$_{x}$CoO$_{2}$. We find that below $x = 0.64$ the Seebeck coefficient is anomalously enhanced at low temperatures with increasing $x$, while it is proportional to the temperature like a conventional metal above $x=0.65$, suggesting an existence of a critical content $x^{*} \simeq 0.65$. For the origin of this anomalous behavior, we discuss a filling-induced Lifshitz transition, which is characterized by a sudden topological change in the cylindrical hole Fermi surfaces at the critical content $x^*$.

cond-mat.str-el↗