arXiv subjects

Takuya Aoyama

Publications and source records attributed to Takuya Aoyama.

15 recordsLinked to original sources

arXiv2026

Interplay of Magnetic Order, Structural Stability, and Orbital Ordering in BaFe2X3 (X = S and Se)

BaFe2X3 (X= S and Se) are quasi-one-dimensional Mott insulators with a ladder structure that exhibit Stripe- and Block-type antiferromagnetic order, respectively. The ladder arrangement of Fe atoms and strong electron correlations give rise to rich magnetic and orbital phenomena, including pressure-induced superconductivity and orbital-selective electronic states. Both compounds also show resistivity anomalies above the Neel temperature, suggesting that orbital degrees of freedom play an important role in their electronic properties. To clarify the interplay among magnetic order, structural stability, orbital ordering, and transport properties, we performed resistivity measurements and first-principles calculations for BaFe2S3 and BaFe2Se3, systematically comparing candidate magnetic and crystal structures. We find that the magnetic configuration strongly influences the stable crystal structure, orbital ordering, and transport anisotropy, providing a microscopic understanding of the contrasting electronic properties of the two compounds.

cond-mat.str-el↗

arXiv2026

Pressure-induced PT Symmetry Breaking in LaMnSi

We investigate the magnetotransport properties of the antiferromagnetic metal LaMnSi, in which the collinear magnetic order breaks both spatial inversion (P) and time-reversal (T) symmetry yet preserves their combined PT symmetry. High pressure is found to suppress this PT-symmetric antiferromagnetic phase, inducing a transition into a PT-broken state characterized by a large anomalous Hall effect. Based on symmetry analysis, we propose a candidate magnetic structure for the high-pressure phase. Subsequent band calculations for this structure reveal the emergence of band splitting and orbital-dependent spin polarization. Our results establish LaMnSi as an ideal platform for controlling PT symmetry breaking via external parameters.

cond-mat.str-el↗

arXiv2026

Magnetotransport Properties of Iron-Based Ladder Compounds BaFe2S3 and BaFe2Se3 under High Pressure

The magnetotransport properties of the iron-based ladder compounds BaFe2S3 and BaFe2Se3 under various pressures were investigated. BaFe2S3 had a T^2 dependence in electrical resistivity and a normal magnetoresistance near an insulator-metal transition. Therefore, it had Fermi-liquid-like properties in the normal phase. However, BaFe2Se3 had a T^3/2 dependence in electrical resistivity and a negative magnetoresistance. This suggested that BaFe2Se3 had non-Fermi-liquid properties near the insulator-metal transition. For both materials, the Hall resistivities indicated a hole-dominant magnetic field dependence. During a pressure-induced insulator-metal transition, BaFe2Se3 had a rapid decrease in electrical resistivity above 15 GPa attributed to an isomorphic structural transition previously reported in X-ray diffraction experiments. This transition had a strong impact on the electronic properties.

cond-mat.str-el↗

arXiv2026

Piezomagnetic effect of a rare-earth-based altermagnet TbPt6Al3

We have investigated the piezomagnetic (PZM) effect of the rare-earth-based g-wave altermagnet TbPt6Al3 by magnetization measurements of single-crystalline samples under uniaxial stress sigma. The magnetization in magnetic field along the trigonal a axis increases linearly with sigma for T < TN, indicating the emergence of PZM effect, while the theoretically predicted nonlinear PZM effect was not observed. PZM coefficient of Q11 at 2 K is obtained as 9.1 times 10^-3 mu_B/(f.u. MPa), which is larger by more than two orders of magnitude than those for other altermagnets and noncollinear antiferromagnets. Temperature dependence of Q11 below TN yielded the critical component beta as 0.28, whose value is close to that of the magnetic moment estimated by the neutron powder diffraction. We propose that the large Q11 and the large poling field of 10000 Oe to achieve the single-domain state in TbPt6Al3 are due to the strong relativistic spin-orbit coupling of the 4f electrons in the Tb3+ ions.

cond-mat.str-el↗

arXiv2026

Magneto-optical imaging of macroscopic altermagnetic domains in MnTe

Altermagnets are a new class of magnets accompanying global time-reversal symmetry breaking (TRSB) without net magnetization. The TRSB results in formation of novel altermagnetic domains. Features of altermagnetic domains, in particular their responses to external stimuli, are essentially important but yet unexplored. Here, we report visualization of bulk altermagnetic domains in MnTe based on scanning magneto-optical Kerr-effect microscopy using telecom infrared wavelength. We found two distinct TRSB domains with large Kerr rotations that do not scale with its tiny bulk magnetization. We also revealed controllability and stability of domains against magnetic or thermal perturbations. Our first observation of altermagnetic domains using a laboratory-scale simple optical technique showing their movable nature provide firm bases for future fundamental and application studies of altermagnets.

cond-mat.mtrl-sci↗

arXiv2025

Piezomagnetic effect in 5$d$ transition metal oxides Y$_2$Ir$_2$O$_7$ and Cd$_2$Os$_2$O$_7$ with all-in/all-out magnetic order

We investigated the piezomagnetic effect in pyrochlore-type oxides Y$_2$Ir$_2$O$_7$ and Cd$_2$Os$_2$O$_7$, which show a non-coplanar magnetic structure called the all-in/all-out antiferromagnetic order at low temperatures. The all-in/all-out magnetic order can be viewed as a ferroic order of the $xyz$-type magnetic octupoles. We observed a linear development of magnetization with applying stress for both materials. We then estimated the powder-averaged piezomagnetic tensor at 50 K to be $Q = 5.74 \times 10^{-6}$ $μ_{\text{B}}$/Ir$\cdot$MPa for Y$_2$Ir$_2$O$_7$, and $Q = 3.49 \times 10^{-7}$ $μ_{\text{B}}$/Os$\cdot$MPa for Cd$_2$Os$_2$O$_7$. We discuss the microscopic mechanism of the piezomagnetic effect in terms of the stress-induced modification of the g-tensor anisotropy and Dzyaloshinskii-Moriya (D-M) interactions. This work paves the way for the further development of piezomagnetic materials using a strategy based on magnetic multipoles.

cond-mat.str-el↗

arXiv2023

Thermal Magnetoelectrics in all Inorganic Quasi-Two-Dimensional Halide Perovskites

From lithium-ion batteries to high-temperature superconductors, oxide materials have been widely used in electronic devices. However, demands of future technologies require materials beyond oxides, as anion chemistries distinct from oxygen can expand the palette of mechanisms and phenomena, to achieve superior functionalities. Examples include nitride-based wide bandgap semiconductors and halide perovskite solar cells, with MAPbBr3 being a representation revolutionizing photovoltaics research. Here, we demonstrate magnetoelectric behaviour in quasi-two-dimensional halides (K,Rb)3Mn2Cl7 through simultaneous thermal control of electric and magnetic polarizations by exploiting a polar-to-antipolar displacive transition. Additionally, our calculations indicate a possible polarization switching path including a strong magnetoelectric coupling, indicating halides can be excellent platforms to design future multiferroic and ferroelectric devices. We expect our findings to broaden the exploration of multiferroics to non-oxide materials and open access to novel mechanisms, beyond conventional electric/magnetic control, for coupling ferroic orders.

cond-mat.mtrl-sci↗

arXiv2023

Piezomagnetic Properties in Altermagnetic MnTe

We examined the piezomagnetic effect in an antiferromagnet composed of MnTe, which is a candidate material for altermagnetism with a high critical temperature. We observed that the magnetization develops with the application of stress and revealed that the piezomagnetic coefficient Q is 1.38$\times10^{-8}$ $μ$B/MPa at 300 K. The poling-field dependence of magnetization indicates that the antiferromagnetic domain can be controlled using the piezomagnetic effect. We demonstrate that the piezomagnetic effect is suitable for detecting and controlling the broken time reversal symmetry in altermagnets.

cond-mat.mtrl-sci↗

arXiv2021

Strongly electron-correlated semimetal RuI$_3$ with a layered honeycomb structure

A polymorph of RuI$_3$ synthesized under high pressure was found to have a two-layered honeycomb structure. The resistivity of RuI$_3$ exhibits a semimetallic behavior, in contrast to insulating properties in $α$-RuCl$_3$. In addition, Pauli paramagnetic behavior was observed in the temperature dependence of a magnetic susceptibility and a nuclear spin-lattice relaxation rate 1/$T_1$. The band structure calculations indicate that contribution of the I 5$p$ components to the low-energy $t_\mathrm{2g}$ bands effectively decreases Coulomb repulsion, leading to semimetallic properties. The physical properties also suggest strong electron correlations in RuI$_3$.

cond-mat.str-el↗

arXiv2021

Magnetism of Kitaev spin-liquid candidate material RuBr$_3$

The ruthenium halide $α$-RuCl$_{3}$ is a promising candidate for a Kitaev spin liquid. However, the microscopic model describing $α$-RuCl$_{3}$ is still debated partly because of a lack of analogue materials for $α$-RuCl$_{3}$, which prevents tracking of electronic properties as functions of controlled interaction parameters. Here, we report a successful synthesis of RuBr$_{3}$. The material RuBr$_{3}$~possesses BiI$_3$-type structure (space group: $R\overline{3}$) where Ru$^{3+}$ form an ideal honeycomb lattice. Although RuBr$_{3}$ has a negative Weiss temperature, it undergoes a zigzag antiferromagnetic transition at $T_\mathrm{N}=34$ K, as does $α$-RuCl$_{3}$. Our analyses indicate that the Kitaev and non-Kitaev interactions can be modified in ruthenium trihalides by changing the ligand sites, which provides a new platform for exploring Kitaev spin liquids.

cond-mat.str-el↗

arXiv2020

Dichotomy Between Orbital and Magnetic Nematic Instabilities in BaFe2S3

Nematic orders emerge nearly universally in iron-based superconductors, but elucidating their origins is challenging because of intimate couplings between orbital and magnetic fluctuations. The iron-based ladder material BaFe2S3, which superconducts under pressure, exhibits antiferromagnetic order below TN ~ 117K and a weak resistivity anomaly at T* ~ 180K, whose nature remains elusive. Here we report angle-resolved magnetoresistance (MR) and elastoresistance (ER) measurements in BaFe2S3, which reveal distinct changes at T*. We find that MR anisotropy and ER nematic response are both suppressed near T*, implying that an orbital order promoting isotropic electronic states is stabilized at T*. Such an isotropic state below T* competes with the antiferromagnetic order, which is evidenced by the nonmonotonic temperature dependence of nematic fluctuations. In contrast to the cooperative nematic orders in spin and orbital channels in iron pnictides, the present competing orders can provide a new platform to identify the separate roles of orbital and magnetic fluctuations.

cond-mat.supr-con↗

arXiv2019

Hydrated lithium intercalation into the Kitaev spin liquid candidate material $α-$RuCl$_3$

We study on transport and magnetic properties of hydrated and lithium-intercalated $α$-RuCl$_3$, Li$_x$RuCl$_3 \cdot y$H$_2$O, for investigating the effect on mobile-carrier doping into candidate materials for a realization of a Kitaev model. From thermogravitometoric and one-dimensional electron map analyses, we find two crystal structures of this system, that is, mono-layer hydrated Li$_x$RuCl$_3 \cdot y$H$_2$O~$(x\approx0.56, y\approx1.3)$ and bi-layer hydrated Li$_x$RuCl$_3 \cdot y$H$_2$O~$(x\approx0.56, y\approx3.9)$. The temperature dependence of the electrical resistivity shows a temperature hysteresis at 200-270 K, which is considered to relate with a formation of a charge order. The antiferromagnetic order at 7-13 K in pristine $α$-RuCl$_3$~ is successfully suppressed down to 2 K in bi-layer hydrated Li$_x$RuCl$_3 \cdot y$H$_2$O, which is sensitive to not only an electronic state of Ru but also an interlayer distance between Ru-Cl planes.

cond-mat.str-el↗

arXiv2019

Polar State induced by Block-type Lattice Distortions in BaFe2Se3 with Quasi-One-Dimensional Ladder Structure

Temperature dependent crystal structures of the quasi-one-dimensional ladder material BaFe2Se3 are examined. Combining the optical second harmonic generation (SHG) experiments and neutron diffraction measurements, we elucidate the crystal structure with Pmn2_1 space group in the low-temperature phase below Ts2 = 400 K, further above Neel temperature. This low-temperature phase loses the spatial inversion symmetry, where a resultant macroscopic polarization emerges along the rung direction. The transition is characterized by block-type lattice distortions with the magneto-striction mechanism. Change in the electrical resistivity and the magnetic susceptibility across the polar-nonpolar transition also suggests a modification of the electronic states reflecting the structural instability. Consistency and discrepancy with the existing theory are discussed.

cond-mat.str-el↗

arXiv2017

Orbital order and fluctuations in the two-leg ladder materials BaFe$_2X_3$ ($X$ = S and Se) and CsFe$_2$Se$_3$

The electronic structure of BaFe$_2X_3$ ($X$ = S and Se) and CsFe$_2$Se$_3$ in which two-leg ladders are formed by the Fe sites are studied by means of x-ray absorption and resonant inelastic x-ray scattering spectroscopy. The x-ray absorption spectra at the Fe L edges for BaFe$_2X_3$ exhibit two components, indicating that itinerant and localized Fe 3$d$ sites coexist. Substantial x-ray linear dichroism (XLD) is observed in polarization dependent spectra, indicating the existence of orbital order or fluctuation in the Fe-ladder even above the Néel temperature $T_N$. Direct exchange interaction along the legs of the Fe-ladder stabilizes the orbital and antiferromagnetic orders in BaFe$_2$S$_3$, while the ferromagnetic molecular orbitals are realized between the rungs in CsFe$_2$Se$_3$.

cond-mat.supr-con↗

arXiv2016

The magnetic precursor of the pressure-induced superconductivity in Fe-ladder compound

The pressure effects on the antiferromagentic orders in iron-based ladder compounds CsFe$_2$Se$_3$ and BaFe$_2$S$_3$ have been studied using neutron diffraction. With identical crystal structure and similar magnetic structures, the two compounds exhibit highly contrasting magnetic behaviors under moderate external pressures. In CsFe$_2$Se$_3$ the ladders are brought much closer to each other by pressure, but the stripe-type magnetic order shows no observable change. In contrast, the stripe order in BaFe$_2$S$_3$, undergoes a quantum phase transition where an abrupt increase of N$\acute{e}$el temperature by more than 50$\%$ occurs at about 1 GPa, accompanied by a jump in the ordered moment. With its spin structure unchanged, BaFe$_2$S$_3$ enters an enhanced magnetic phase that bears the characteristics of an orbital selective Mott phase, which is the true neighbor of superconductivity emerging at higher pressures.

cond-mat.supr-con↗