arXiv subjects

K. Ohgushi

Publications and source records attributed to K. Ohgushi.

At least 19 recordsLinked to original sources

arXiv2026

Three-dimensional zigzag correlations in the van der Waals Kitaev magnet RuBr$_3$

Ruthenium trihalides Ru$X_3$ ($X$ = Cl, Br, I) provide a tunable platform for Kitaev magnetism in two-dimensional van der Waals materials. Despite their similar crystal structures and zigzag antiferromagnetic order, RuBr$_3$ exhibits a higher N\'eel temperature ($T_N$) than RuCl$_3$, suggesting their distinct proximity to the Kitaev quantum spin liquid phase. Using Ru $L_3$-edge resonant x-ray scattering, we show that, while the long-range zigzag order in RuBr$_3$ disappears at $T_N$, the zigzag correlations that persist well above $T_N$ show a pronounced spectral weight redistribution along the interlayer direction. These results suggest that the enhanced interlayer magnetic interactions driven by the extended Br 4$p$ orbitals stabilize three-dimensional zigzag correlations in RuBr$_3$.

cond-mat.str-el↗

arXiv2025

Orbital magnetic moments in FeCr2S4 studied by x-ray magnetic circular dichroism

We have investigated the element specific magnetic characteristics of single-crystal FeCr2S4 using x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD). We have found that the Fe L2,3-edge XAS spectra do not exhibit clear multiplet structures, indicating strong hybridization between the Fe 3d and S 3p orbitals, leading to delocalized rather than localized electronic states. The Fe 3d and Cr 3d spin moments are antiferromagnetically coupled, consistent with the Goodenough-Kanamori rule. The orbital magnetic moments of Fe and Cr are determined to be -0.23 and -0.017 μB/ion, respectively. The large orbital magnetic moment of Fe is due to the d6 configuration under the relatively weak tetrahedra crystal field at the Fe site, and the delocalized Fe electrons maintain the orbital degree of freedom in spite of their itinerant nature. To understand phenomena such as the gigantic Kerr rotation, it is essential to consider not only the orbital degrees of freedom but also the role of spin-orbit coupling, which induces a finite orbital magnetic moment through t2 and e level hybridization under the tetrahedral crystal field. This finite orbital moment serves as a direct indicator of spin-orbit interaction strength and links element-specific orbital magnetism to the large Kerr rotation. On the other hand, the octahedral crystal-field splitting of the Cr 3d level is large enough to result in the quenching of the orbital moment of the Cr ion in FeCr2S4.

cond-mat.str-el↗

arXiv2025

Circular Dichroism in Resonant Inelastic X-ray Scattering: Probing Altermagnetic Domains in MnTe

X-ray magnetic circular dichroism provides a means to identify ferromagnetic, chiral, and altermagnetic orders via their time-reversal-symmetry ($\mathcal{T}$) breaking. However, differentiating magnetic domains related by crystallographic symmetries remains a technical challenge. Here we reveal a circular dichroism (CD) in the resonant inelastic x-ray scattering (RIXS) spectra from the altermagnetic MnTe. The azimuthal dependence of the RIXS-CD intensity of the magnon excitations indicates a dominant occupation of a single altermagnetic domain. The RIXS-CD in our scattering geometry is ascribed to the mirror-symmetry breaking associated with the $\mathcal{T}$-broken altermagnetic order. Our results establish RIXS-CD as a domain-sensitive probe of elementary excitations in quantum materials.

cond-mat.str-el↗

arXiv2024

$J=1/2$ Pseudospins and $d$-$p$ Hybridization in the Kitaev Spin Liquid Candidates Ru$X_3$ ($X$ = Cl, Br, I)

The recent synthesis of ruthenium trihalides Ru$X_3$ ($X$ = Cl, Br, I) has enlarged the set of material candidates for Kitaev spin liquid. The realization of Kitaev model necessitates the formation of $J=1/2$ pseudospins in the octahedral crystal field. We use Ru $L_3$-edge resonant inelastic x-ray scattering to investigate the evolution of multiplet structures in Ru$X_3$. We identified quasi-elastic magnetic correlations and spin-orbit transitions to the $J=3/2$ states without discernible trigonal splitting, thereby validating the $J=1/2$ description of magnetism in the Ru$X_3$ family. Lineshape broadening in RuI$_3$ provides evidence for its bulk metallicity in the vicinity of a bandwidth-controlled metal-insulator transition. Our results highlight the pivotal role of halogen $p$ orbitals in controlling the electronic properties of Ru$X_3$.

cond-mat.str-el↗

arXiv2024

Observation of Giant Band Splitting in Altermagnetic MnTe

We performed angle-resolved photoemission spectroscopy (ARPES) on hexagonal MnTe, a candidate for an altermagnet with a high critical temperature (TN=307 K). By utilizing photon-energy-tunable ARPES in combination with first-principles calculations, we found that the band structure in the antiferromagnetic phase exhibits a strongly anisotropic band-splitting associated with the time-reversal-symmetry breaking, providing the first direct experimental evidence for the altermagnetic band-splitting. The magnitude of the splitting reaches 0.8 eV at non-high-symmetry momentum points, which is much larger than the spin-orbit gap of ~0.3 eV along the GK high-symmetry cut. The present result paves the pathway toward realizing exotic physical properties associated with the altermagnetic spin-splitting.

cond-mat.mtrl-sci↗

arXiv2023

Magnetism in Kitaev Quantum Spin Liquid Candidate RuBr$_3$

The present studies show that long-range magnetic order takes place in RuBr$_3$ at $\approx$ 34 K. The observations of clear oscillations in the muon time spectra demonstrate the presence of well-defined internal fields at the muon sites. The magnetic ordering appears to be very robust and static suggesting a more conventional nature of magnetic ordering in the RuBr$_3$ system at zero field. Present investigations prove that in RuBr$_3$ the Kitaev interactions are likely to be weakened at zero field in comparison to the $α$-RuCl$_3$ system. This proves that it is possible to tune the Kitaev interactions by replacing Cl with heavier halogen elements such as Br.

cond-mat.str-el↗

arXiv2022

Light-induced magnetization driven by interorbital charge motion in a spin-orbit assisted Mott insulator alpha-RuCl3

In a honeycomb-lattice spin-orbit assisted Mott insulator α-RuCl3, an ultrafast magnetization is induced by circularly polarized excitation below the Mott gap. Photo-carriers play an important role, which are generated by turning down the synergy of the on-site Coulomb interaction and the spin-orbit interaction realizing the insulator state. An ultrafast 6- fs measurement of photo-carrier dynamics and a quantum mechanical analysis clarify the mechanism, according to which the magnetization emerges from a coherent charge motion between different t2g orbitals (dyz-dxz-dxy) of Ru3+ ions. This ultrafast magnetization is weakened in the antiferromagnetic (AF) phase, which is opposite to the general tendency that the inverse Faraday effect is larger in AF compounds than in paramagnetic ones. This temperature dependence indicates that the interorbital charge motion is affected by pseudo-spin rotational symmetry breaking in the AF phase.

cond-mat.str-el↗

arXiv2022

Pressure-induced superconductivity in iron-based spin-ladder compound BaFe2+δ(S1-Sex)3

The iron-based superconductors had a significant impact on condensed matter physics. They have a common structural motif of a two-dimensional square iron lattice and exhibit fruitful physical properties as a strongly correlated electron system. During the extensive investigations, quasi-one-dimensional iron-based spin-ladder compounds attracted much attention as a platform for studying the interplay between magnetic and orbital ordering. In these compounds, BaFe2S3 and BaFe2Se3 were found to exhibit superconductivity under high pressure, having a different crystal and magnetic structure at low temperature. We report a brief review of the iron-based spin-ladder compound and recent studies for BaFe2+d(S1-xSex)3. BaFe2(S0.75 Se0.25)3 is in the vicinity of the boundary of two different magnetic phases and it is intriguing to perform high pressure experiments for studying superconductivity, since effects of large magnetic fluctuations on superconductivity are expected. The effect of iron stoichiometry on the interplay between magnetism and superconductivity is also studied by changing the iron concentration in BaFe2+dSe3.

cond-mat.supr-con↗

arXiv2018

Quantum paramagnet near spin-state transition

Spin-state transition, also known as spin crossover, plays a key role in diverse systems, including minerals and biological materials. In theory, the boundary range between the low- and high-spin states is expected to enrich the transition and give rise to unusual physical states. However, no compound that realizes a nearly degenerate critical range as the ground state without requiring special external conditions has yet been experimentally identified. This study reports that, by comprehensive measurements of macroscopic physical properties, X-ray diffractometry, and neutron spectroscopy, the Sc substitution in LaCoO$_3$ destabilizes its nonmagnetic low-spin state and generates an anomalous paramagnetic state accompanied by the enhancement of transport gap and magneto-lattice-expansion as well as the contraction of Co--O distance with the increase of electron site-transfer. These phenomena are not well described by the mixture of conventional low- and high-spin states, but by their quantum superposition occurring on the verge of a spin-state transition. The present study enables us to significantly accelerate the design of new advanced materials without requiring special equipment based on the concept of quantum spin-state criticality.

cond-mat.str-el↗

arXiv2015

Orbital anisotropy underlying the superconducting dome in BaFe$_2$(As$_{1-x}$P$_x$)$_2$ superconductors

We investigate the in-plane anisotropy of Fe 3d orbitals occurring in a wide temperature and composition range of BaFe2(As1-xPx)2 system. By employing the angle-resolved photoemission spectroscopy, the lifting of degeneracy in dxz and dyz orbitals at the Brillouin zone corners can be obtained as a measure of the orbital anisotropy. In the underdoped regime, it starts to evolve on cooling from high temperatures above both antiferromagnetic and orthorhombic transitions. With increasing x, it well survives into the superconducting regime, but gradually gets suppressed and finally disappears around the non-superconducting transition (x = 0.7). The observed spontaneous in-plane orbital anisotropy, possibly coupled with anisotropic lattice and magnetic fluctuations, implies the rotational-symmetry broken electronic state working as the stage for the superconductivity in BaFe2(As1-xPx)2.

cond-mat.supr-con↗

arXiv2015

Investigation of the Magnetic Model in Multiferroic NdFe$_{3}$(BO$_{3}$)$_{4}$ by Inelastic Neutron Scattering

We performed inelastic neutron scattering measurements on single crystals of NdFe$_{3}$($^{11}$BO$_{3}$)$_{4}$ to explore the magnetic excitations, to establish the underlying Hamiltonian, and to reveal the detailed nature of hybridization between the 4$f$ and 3$d$ magnetism. The observed spectra exhibiting a couple of key features, i.e., anti-crossing of Nd- and Fe-excitations and anisotropy gap at the antiferromagnetic zone center, are explained by the magnetic model including spin interaction in the framework of weakly-coupled Fe$^{3+}$ chains, interaction between the Fe$^{3+}$ and Nd$^{3+}$ moments, and single-ion anisotropy derived from Nd$^{3+}$ crystal field. The combination of the measurements and calculations reveals that the hybridization between 4$f$ and 3$d$ magnetism propagates the local magnetic anisotropy of the Nd$^{3+}$ ion to the Fe$^{3+}$ network, resulting in the bulk structure of multiferroics.

cond-mat.str-el↗

arXiv2015

Coexistence of localized and itinerant electrons in BaFe2X3 (X = S and Se) revealed by photoemission spectroscopy

We report a photoemission study at room temperature on BaFe2X3 (X = S and Se) and CsFe2Se3 in which two-leg ladders are formed by the Fe sites. The Fe 2p core-level peaks of BaFe2X3 are broad and exhibit two components, indicating that itinerant and localized Fe 3d sites coexist similar to KxFe2-ySe2. The Fe 2p core-level peak of CsFe2Se3 is rather sharp and is accompanied by a charge-transfer satellite. The insulating ground state of CsFe2Se3 can be viewed as a Fe2+ Mott insulator in spite of the formal valence of +2.5. The itinerant versus localized behaviors can be associated with the stability of chalcogen p holes in the two-leg ladder structure.

cond-mat.str-el↗

arXiv2014

Evidence of a universal relation between electron-mode coupling and Tc in Ba1-xKxFe2As2 superconductor from Laser ARPES

We performed a Laser angle-resolved photoemission spectroscopy (ARPES) study on a wide doping range of Ba1-xKxFe2As2 (BaK) iron-based superconductor. We observed a robust low-binding energy (BE) kink structure in the dispersion which is doping dependent where its energy peaks at the optimally-doped (OP) level (x~0.4) and decreases towards the underdoped (UD) and overdoped (OD) sides. It is also temperature-dependent and survives up to ~90K. We attribute this kink to electron-mode coupling in good agreement with the inelastic neutron scattering (INS) and scanning tunneling microscopy (STM) results on the same compound which observed a similar bosonic mode associated with spin excitations. The relation between the mode energy (Ω) and the SC transition temperature (Tc) deduced from our Laser ARPES data follow the universal relation deduced from INS and STM. In addition, we could resolve another kink at higher BE showing less doping and temperature dependence and may thus be of different origin.

cond-mat.supr-con↗

arXiv2014

CaIrO3: a Spin-Orbit Mott Insulator Beyond the jeff = 1/2 Ground State

In CaIrO3 electronic correlation, spin-orbit coupling, and tetragonal crystal field splitting are predicted to be of comparable strength. However, the nature of its ground state is still object of debate, with contradictory experimental and theoretical results. We probe the ground state of CaIrO3 and assess the effective tetragonal crystal field splitting and spin-orbit coupling at play in this system by means of resonant inelastic x-ray scattering. We conclude that insulating CaIrO3 is not a jeff = 1/2 iridate and discuss the consequences of our finding to the interpretation of previous experiments. In particular, we clarify how the Mott insulating state in iridates can be readily extended beyond the jeff = 1/2 ground state.

cond-mat.str-el↗

arXiv2013

Pseudogap formation above the superconducting dome in iron-pnictides

The nature of the pseudogap in high transition temperature (high-Tc) superconducting cuprates has been a major issue in condensed matter physics. It is still unclear whether the high-Tc superconductivity can be universally associated with the pseudogap formation. Here we provide direct evidence of the existence of the pseudogap phase via angle-resolved photoemission spectroscopy in another family of high-Tc superconductor, iron-pnictides. Our results reveal a composition dependent pseudogap formation in the multi-band electronic structure of BaFe2(As1-xPx)2. The pseudogap develops well above the magnetostructural transition for low x, persists above the nonmagnetic superconducting dome for optimal x and is destroyed for x ~ 0.6, thus showing a notable similarity with cuprates. In addition, the pseudogap formation is accompanied by inequivalent energy shifts in xz/yz orbitals of iron atoms, indicative of a peculiar iron orbital ordering which breaks the four-fold rotational symmetry.

cond-mat.supr-con↗

arXiv2012

Magnetoelasticity in ACr2O4 spinel oxides

Dynamical properties of the lattice structure was studied by optical spectroscopy in ACr2O4 chromium spinel oxide magnetic semiconductors over a broad temperature region of T=10-335K. The systematic change of the A-site ions (A=Mn, Fe, Co, Ni and Cu) showed that the occupancy of 3d orbitals on the A-site, has strong impact on the lattice dynamics. For compounds with orbital degeneracy (FeCr2O4, NiCr2O4 and CuCr2O4), clear splitting of infrared-active phonon modes and/or activation of silent vibrational modes have been observed upon the Jahn-Teller transition and at the onset of the subsequent long-range magnetic order. Although MnCr2O4 and CoCr2O4 show multiferroic and magnetoelectric character, no considerable magnetoelasticity was found in spinel compounds without orbital degeneracy as they closely preserve the high-temperature cubic spinel structure even in their magnetic ground state. Besides lattice vibrations, intra-atomic 3d-3d transitions of the A$^{2+}$ ions were also investigated to determine the crystal field and Racah parameters and the strength of the spin-orbit coupling.

cond-mat.str-el↗

arXiv2012

Anomalous two peak structure in the Angle-resolved Photoemission Spectra of Ba1-xKxFe2As2

The electronic structure near the Fermi level (EF) of Ba1-xKxFe2As2 (BaK122 ; x = 0.2 - 0.7) is studied using laser ultrahigh-resolution angle-resolved photoemission spectroscopy(ARPES). For the optimally doped case of x = 0.4, we clearly observe two peaks below Tc in the ARPES spectra at a binding energies (BE) of 5 meV and 13meV. The former is assigned to a superconducting (SC) coherence peak since it appears and evolves below the bulk SC transition at Tc (= 36 K), accompanying a gap opening centered at EF. In contrast, the latter peak, which appears below ~ 90 K without any gap formation, is interpreted to be not directly related to a SC coherence peak. This high-BE peak is observed from x = 0.2 to 0.6, reduces in energy with overdoping (x > 0.4) and is absent for x = 0.7. The temperature(T)- and doping-dependent ARPES results suggest that the high-BE peak originates from coupling to a bosonic mode of energy ~ 8 meV.

cond-mat.supr-con↗

arXiv2012

Tetrahedral Magnetic Order and the Metal-Insulator Transition in the Pyrochlore Lattice of Cd2Os2O7

Cd2Os2O7 shows a peculiar metal-insulator transition at 227 K with magnetic ordering in a frustrated pyrochlore lattice, but its magnetic structure in the ordered state and the transition origin are yet uncovered. We observed a commensurate magnetic peak by resonant x-ray scattering in a high-quality single crystal. X-ray diffraction and Raman scattering experiments confirmed that the transition is not accompanied with any spatial symmetry breaking. We propose a noncollinear all-in/all-out spin arrangement on the tetrahedral network made of Os atoms. Based on this we suggest that the transition is not caused by Slater mechanism as believed earlier but by an alternative mechanism related to the formation of the specific tetrahedral magnetic order on the pyrochlore lattice in the presence of strong spin-orbit interactions.

cond-mat.str-el↗