arXiv subjects

Yoshinori Okada

Publications and source records attributed to Yoshinori Okada.

At least 19 recordsLinked to original sources

arXiv2026

Anomalous vortex shape in a frustrated superconductor hosting chiral multicomponent order parameters

Multicomponent chiral superconductors can host spatially separated superconducting components within a single Abrikosov vortex, leading to unconventional, anisotropic vortex shapes forbidden in conventional single-component superconductors. Here, using spectroscopic scanning tunneling microscopy at 0.3 K, we investigate the mixed state of the spinel superconductor LiTi2O4 (Tc = 13 K). In the low-field regime, where vortex-vortex interactions are minimized and the intrinsic single-vortex shape is preserved, reasonably isolated triangular Abrikosov vortices are directly visualized with high statistical significance. Remarkably, the vortex orientation is locked to crystallographic domains rather than the magnetic-field direction, revealing a hidden chirality selectivity embedded in the zero-field electronic state.

cond-mat.supr-con↗

arXiv2026

Raman spectroscopy of van der Waals topological magnet GdGaI

We report polarization-resolved Raman spectroscopy of a van der Waals compound GdGaI that is a candidate for excitonic insulators. By combining the symmetry analysis with density functional theory calculations, we identify six Raman-active phonons. The spectra exhibit only the expected anharmonic hardening down to 4 K: no additional peaks, no soft modes, and no signatures of zone folding are observed. This result indicates that any lattice distortion is below our experimental sensitivity, supporting an electronically driven origin for the band reconstruction reported by angle-resolved photoemission spectroscopy rather than an electron-phonon-driven mechanism. Moreover, we observe a pronounced circular dichroism of the $A_{1g}$ modes under an out-of-plane magnetic field. Based on symmetry considerations, we attribute this dichroic response to chiral $A_{1g}$ phonons with opposite angular momenta generated by spin-phonon coupling in the time-reversal-broken state. The temperature evolution of the degree of circular polarization further suggests that circularly polarized Raman spectroscopy detects the emergence of short-range antiferromagnetic correlations. Our results highlight GdGaI as a promising platform in which excitonic order, magnetism, and circularly polarized phonons can be intertwined, and demonstrate that circular-polarization Raman provides a sensitive probe of spin-phonon coupling in excitonic systems.

cond-mat.mtrl-sci↗

arXiv2026

Highly fluctuating double-$q$ magnetic order in the van der Waals metal CeTe$_3$

CeTe$_3$ is a van der Waals antiferromagnet composed of magnetic [CeTe]$^+$ layers coupled to highly conducting Te$^{0.5-}$ square nets. Its simple quasi-two-dimensional electronic structure and cleavable nature make it an appealing platform for exploring correlated magnetism in reduced dimensions. To clarify the nature of its low-temperature state, we performed single-crystal neutron diffraction down to 0.3 K, complemented by scanning tunneling microscopy. A magnetic transition near 1.5 K gives rise to incommensurate Bragg peaks at $q_{\pm}\sim(\pm0.17,0,0.31)$, consistent with a double-$q$ magnetic order whose moments are predominantly aligned along the $c$ axis. The strongly reduced ordered moment is consistent with enhanced quantum fluctuations driven by $c$-$f$ hybridization, while the deviation of the propagation vectors from simple nesting suggests a coupling to residual charge-density-wave instabilities of the quasi-one-dimensional Te-derived bands. These results indicate that CeTe$_3$ hosts a correlated magnetic ground state where spin and itinerant charge degrees of freedom are intimately linked in the van der Waals limit.

cond-mat.str-el↗

arXiv2026

Robust topological surface states in skyrmion-host magnets Eu(Ga,Al)4: evidence for dual topology

The interplay between real-space topology such as magnetic skyrmions and momentum-space topology characterized by topological surface states (TSSs) is predicted to realize novel phenomena and functionalities, yet materials hosting both topologies are scarce. Skyrmion-hosting helimagnet family EuGa$_2$Al$_2$ and EuAl$_4$ has been a prime candidate for such a dual-topology system, but conclusive evidence for its momentum-space topology has remained elusive. We provide this evidence by directly observing TSSs that stem from bulk Dirac nodal lines using high-resolution angle-resolved photoemission spectroscopy. These TSSs are exceptionally robust against various perturbations such as a 2$\times$1 surface reconstruction, a chemical change in the termination of the crystal surface, and the onset of helical antiferromagnetic order. Crucially, below the Neel temperature, we observe replica bands driven by the magnetic ordering. Moreover, we demonstrate clear surface-termination dependence of this magneto-topological coupling. Our findings establish Eu(Ga$_{1-x}$Al$_x$)$_4$ as a dual-topology material and offer a rare platform to explore and control the interaction between the two fundamental topological realms.

cond-mat.str-el↗

arXiv2026

Spin Hall effect in van der Waals ferromagnet Fe$_{5}$GeTe$_{2}$

We investigate the spin Hall effect (SHE) in a van der Waals (vdW) ferromagnet Fe$_{5}$GeTe$_{2}$ (FGT) with a Curie temperature $T_{\rm C}$ of 310 K utilizing the spin-torque ferromagnetic resonance method. In synchronization with the emergence of the ferromagnetic phase resulting in the anomalous Hall effect (AHE), a noticeable enhancement in the SHE was observed below $T_{\rm C}$. On the other hand, the SHE shows a different temperature dependence from the AHE: the effective spin Hall conductivity is clearly enhanced with decreasing temperature unlike the anomalous Hall conductivity, reflecting the variation of band-structure accompanied by the complicated magnetic ordering of the FGT. The results provide a deep understanding of the SHE in magnetic materials to open a new route for novel functionalities in vdW materials-based spintronic devices.

cond-mat.mes-hall↗

arXiv2025

Defects Engineering of ZrTe5 for Stabilizing Ideal Topological States

ZrTe5 is a highly tunable, high-mobility topological material that hosts a rich variety of quantum phenomena, making it a promising platform for next-generation quantum technologies. Despite intensive research efforts, experimental studies have reported inconsistent and sometimes conflicting results for its electronic and topological states, largely due to variations in sample quality. Here, through systematic frst-principles investigations of all intrinsic point defects, we identify a practical route to achieving stable and ideal topological characteristics in ZrTe5. We show that the competition between two dominant charged defects, donor-like Zr interstitials and acceptor-like Te vacancies, governs the Fermi-level position. Furthermore, variations in defect density determine the topological phases of the samples. We theoretically propose and experimentally confrm that increasing the Te/Zr ratio during crystal growth effectively suppresses intrinsic defects and stabilizes ZrTe5 in a nearly ideal weak topological insulator state. These fndings provide clear guidance for defect control and sample optimization, paving the way toward robust and reproducible realization of topological quantum states in ZrTe5 for future quantum applications.

cond-mat.mtrl-sci↗

arXiv2025

Long-range magnetic perturbations at Bi2Te3/Cr2Te3 interfaces induced by chemical diffusion and proximity effects

The heterointerface between topological insulators and magnetic materials provides a crucial platform for investigating exotic electronic and magnetic phases, with implications for both fundamental studies and potential applications. A key challenge is determining the spatial extent of magnetic perturbation across the interface. In this study, we grew Bi2Te3 films with mixed thicknesses of 5 and 6 quintuple layers (QLs) on a thick ferromagnetic Cr2Te3 film. Chemical analysis indicates gradual Cr diffusion across the interface into Bi2Te3, reaching up to the 4th QL, while the 5th and 6th QL remain free of Cr. Despite the absence of direct Cr incorporation, quasiparticle interference imaging using spectroscopic scanning tunneling microscopy reveals strong backscattering on the 5th QL surface. This suggests an intrinsic magnetic proximity effect, which breaks time-reversal symmetry without magnetic doping, extending into the 5th QLs. Although variations may exist across different systems, the large magnetic perturbation length scale observed here is a valuable guiding principle for engineering exotic electronic and magnetic states at heterointerfaces between topological insulators and magnetic materials.

cond-mat.mtrl-sci↗

arXiv2024

Imaging Josephson Vortices on Curved Junctions

Understanding the nature of vortices in type-II superconductors is crucial for comprehending exotic superconductors and advancing the application of superconducting materials in future electronic devices. This study uses spectroscopic scanning tunneling microscopy to visualize Josephson vortices along crystalline domain boundaries in the superconducting spinel oxide LiTi2O4 (LTO). Our experimental results reveal that the local curvature of the Josephson junction dictates the positioning of Josephson vortices. Self-consistent solutions of the Bogoliubov-de Gennes and gap equations theoretically corroborate this observation. In addition to enhancing our understanding of the physics of Josephson vortex formation, this study offers potential guidelines for developing vortex-based superconducting devices.

cond-mat.supr-con↗

arXiv2024

Magnetotransport properties in van-der-Waals \textit{\textbf{R}}Te$_{3}$ (\textit{\textbf{R}} = La, Ce, Tb)

Rare-earth tritellurides are van-der-Waals antiferromagnets which have been attracting attention as materials not only with high mobility, but also with various states such as superconductivity under high pressure, incommensurate charge-density-wave (CDW) phase, and multiple antiferromagnetic phases. In this work, we performed longitudinal resistivity and Hall resistivity measurements simultaneously in exfoliated $R$Te$_{3}$ ($R$ =La, Ce, Tb) thin film devices, in order to investigate the influence of magnetic ordering on transport properties in van-der-Waals magnetic materials. We have obtained carrier mobility and concentration using a two-band model, and have observed an increase in carrier mobility in the antiferromagnets CeTe$_{3}$ and TbTe$_{3}$ due to the magnetic transition. Especially in CeTe$_{3}$, the carrier concentration has changed drastically below the magnetic transition temperature, suggesting the interaction between the CDW and antiferromagnetic phases. In addition, the analysis of the Shubnikov-de Haas oscillations in CeTe$_{3}$ supports the possibility of Fermi surface modulation by magnetic ordering. This research will pave the way not only for spintronic devices that take advantage of high mobility, but also for the study of the correlation between CDW and magnetism states in low-dimensional materials.

cond-mat.str-el↗

arXiv2023

Imaging emergent exotic quasiparticle state in a frustrated transition metal oxide

The existence of rich Fermiology in anomalous metal phase in exotic superconductors has attracted considerable interests, as exemplified in copper, iron-based, and intermetallic frustrated kagome-based compounds. A common feature in these cases is pseudo-gap opening or long-range lattice/electronic ordering above superconducting critical temperature Tc. As yet developed area is the potential existence of exotic Fermiology in superconducting transition metal oxides on a geometrically frustrated lattice. Here, we focus on the spinel oxide superconductor LiTi2O4, which can be viewed as the hole-doped side of the orbital ordered 3d1 Mott system on the Ti-derived pyrochlore frustrated network. By the in-situ combination of angle-resolved photoemission spectroscopy (ARPES) and epitaxial thin film growth, we discovered the abrupt flattening of near Fermi energy dispersion below the characteristic temperature T* ~ 150 K. While the emergent negative thermal expansion below T* strongly supports a distinct phase at low-temperature, absence of energy gap opening, splitting/folding of bands, nor long-range lattice distortion are seen across T*. We propose that the competition between growing instability towards orbital ordering and its inherent geometric frustration in the Ti-pyrochlore network results in a new quantum state of matter with robust high entropic nature below T*. Our findings collectively point to a unique Fermiology in frustrated three-dimensional transition metal oxides, and its connection to superconductivity below Tc is open as an interesting future challenge. Also, a potential guideline is unexpectedly provided for designing zero thermal expansion metal to develop future solid-state devices.

cond-mat.supr-con↗

arXiv2022

Large antiferromagnetic fluctuation enhancement of the thermopower at a critical doping in magnetic semimetal Cr1+dTe2

Cr1+dTe2 is a self-intercalated transition metal dichalcogenide that hosts tunable electronic filling and magnetism in its semimetallic band structure. Recent angle-resolved photoemission spectroscopy (ARPES) studies have unveiled a systematic shift in this semimetallic band structure relative to the chemical potential with increased Cr doping. This report presents the temperature and magnetic field dependence of the longitudinal thermopower Sxx for different Cr1+dTe2 compositions. We show that as doping increases, the sign of Sxx changes from positive to negative at the critical doping level of d ~ 0.5. This observed doping-dependent trend in the thermopower is consistent with the evolution of the semimetallic band structure from ARPES. Importantly, an anomalous enhancement of the thermoelectric response is also observed around d~0.5. Combining information from magnetometry and ARPES measurements, existence of the critical nature of the doping level dc (~0.5) is unveiled in magnetic semimetal Cr1+dTe2, where antiferromagnetic fluctuation and near-Fermi-energy pseudogap formation play a potential vital role in enhancing thermoelectric energy conversion.

cond-mat.mtrl-sci↗

arXiv2022

Metallic delafossite thin films for unique device applications

Metallic delafossites (ABO2) are layered oxides with quasi-two dimensional conduction layers. Metallic delafossites are among the most conducting materials with the in-plane conductivity comparable with that of elemental metals. In this perspective, we will discuss basic properties and future research prospects of metallic delafossites, mainly focusing on thin films and heterostructures. We exemplify the fascinating properties of these compounds such as high conductivity and surface polarity and discuss how it can be utilized in thin films and heterostructures.

cond-mat.str-el↗

arXiv2021

Visualizing superconductivity in an inversion-symmetry-broken doped Weyl semimetal

The Weyl semimetal MoTe$_2$ offers a rare opportunity to study the interplay between Weyl physics and superconductivity. Recent studies have found that Se substitution can boost the superconductivity up to 1.5K, but suppress the Td structure phase that is essential for the emergence of Weyl state. A microscopic understanding of possible coexistence of enhanced superconductivity and the Td phase has not been established so far. Here, we use scanning tunneling microscopy (STM) to study a optimally doped new superconductor MoTe$_{1.85}$Se$_{0.15}$ with bulk Tc ~ 1.5K. By means of quasiparticle interference imaging, we identify the existence of low temperature Td phase with broken inversion symmetry where superconductivity globally coexists. Consistently, we find that the superconducting coherence length, extracted from both the upper critical field and the decay of density of states near a vortex, is much larger than the characteristic length scale of existing dopant derived chemical disorder. Our findings of robust superconductivity arising from a Weyl semimetal normal phase in MoTe$_{1.85}$Se$_{0.15}$, makes it a promising candidate for realizing topological superconductivity.

cond-mat.supr-con↗

arXiv2021

Observation of near EF Fermi-arc van Hove singularity with prominent coupling to phonon in a van der Waals coupled Weyl semimetal

A van der Waals coupled Weyl semimetal material NbIrTe4 is investigated by combining scanning tunneling microscopy/spectroscopy and first principles calculations. We observe a sharp peak in the tunneling conductance near the zero bias energy, and its origin is ascribed to a van Hove singularity associated with a Lifshitz transition of the topologically none trivial Fermi arc states. Furthermore, tunneling spectroscopy measurements show a surprisingly large signature of electron boson coupling, which presumably represents anomalously enhanced electron phonon coupling through the enhanced charge susceptibility. Our finding in van der Waals coupled material is particularly invaluable due to applicable exfoliation technology for searching exotic topological states by further manipulating near Fermi energy van Hove singularity in nanometer scale flakes and their devices.

cond-mat.mes-hall↗

arXiv2021

Magnetic frustration in a van der Waals metal CeSiI

The realization of magnetic frustration in a metallic van der Waals (vdW) coupled material has been sought as a promising platform to explore novel phenomena both in bulk matter and in exfoliated devices. However, a suitable material platform has been lacking so far. Here, we demonstrate that CeSiI hosts itinerant electrons coexisting with exotic magnetism. In CeSiI, the magnetic cerium atoms form a triangular bilayer structure sandwiched by van der Waals stacked iodine layers. From resistivity and magnetometry measurements, we confirm the coexistence of itinerant electrons with magnetism with dominant antiferromagnetic exchange between the strongly Ising-like Ce moments below 7 K. Neutron diffraction directly confirms magnetic order with an incommensurate propagation vector k ~ (0.28, 0, 0.19) at 1.6 K, which points to the importance of further neighbor magnetic interactions in this system. The presence of a two-step magnetic-field-induced phase transition along c axis further suggests magnetic frustration in the ground state. Our findings provide a novel material platform hosting a coexistence of itinerant electron and frustrated magnetism in a vdW system, where exotic phenomena arising from rich interplay between spin, charge and lattice in low dimension can be explored.

cond-mat.str-el↗

arXiv2020

Three-dimensional electronic structure in ferromagnetic $\textrm{Fe}_3\textrm{Sn}_2$ with breathing kagome bilayers

A large anomalous Hall effect (AHE) has been observed in ferromagnetic $\textrm{Fe}_3\textrm{Sn}_2$ with breathing kagome bilayers. To understand the underlying mechanism for this, we investigate the electronic structure of $\textrm{Fe}_3\textrm{Sn}_2$ by angle-resolved photoemission spectroscopy (ARPES). In particular, we use both vacuum ultraviolet light (VUV) and soft x ray (SX), which allow surface-sensitive and relatively bulk-sensitive measurements, respectively, and distinguish bulk states from surface states, which should be unlikely related to the AHE. While VUV-ARPES observes two-dimensional bands mostly due to surface states, SX-ARPES reveals three-dimensional band dispersions with a periodicity of the rhombohedral unit cell in the bulk. Our data show a good consistency with a theoretical calculation based on density functional theory, suggesting a possibility that $\textrm{Fe}_3\textrm{Sn}_2$ is a magnetic Weyl semimetal.

cond-mat.mtrl-sci↗

arXiv2020

Quantum oscillations with magnetic hysteresis observed in CeTe$_{3}$ thin films

We have performed magnetotransport measurements in CeTe$_{3}$ thin films down to $0.2~{\rm K}$. It is known that CeTe$_{3}$ has two magnetic transitions at $T_{\rm N1} \approx 3~{\rm K}$ and $T_{\rm N2} \approx 1~{\rm K}$. A clear Shubnikov-de-Haas (SdH) oscillation was observed at $4~{\rm K}$, demonstrating the strong two-dimensional nature in this material. Below $T_{\rm N2}$, the SdH oscillation has two frequencies, indicating that the Fermi surface could be slightly modulated due to the second magnetic transition. We also observed a magnetic hysteresis in the SdH oscillation below $T_{\rm N1}$. Especially, there is a unique spike in the magnetoresistance at $B \approx 0.6~{\rm T}$ only when the magnetic field is swept from a high enough field (more than $2~{\rm T}$) to zero field.

cond-mat.mes-hall↗

arXiv2018

Disorder induced power-law gaps in an insulator-metal Mott transition

A correlated material in the vicinity of an insulator-metal transition (IMT) exhibits rich phenomenology and variety of interesting phases. A common avenue to induce IMTs in Mott insulators is doping, which inevitably leads to disorder. While disorder is well known to create electronic inhomogeneity, recent theoretical studies have indicated that it may play an unexpected and much more profound role in controlling the properties of Mott systems. Theory predicts that disorder might play a role in driving a Mott insulator across an IMT, with the emergent metallic state hosting a power law suppression of the density of states (with exponent close to 1; V-shaped gap) centered at the Fermi energy. Such V-shaped gaps have been observed in Mott systems but their origins are as yet unknown. To investigate this, we use scanning tunneling microscopy and spectroscopy to study isovalent Ru substitutions in Sr$_3$(Ir$_{1-x}$Ru$_x$)$_2$O$_7$ which drives the system into an antiferromagnetic, metallic state. Our experiments reveal that many core features of the IMT such as power law density of states, pinning of the Fermi energy with increasing disorder, and persistence of antiferromagnetism can be understood as universal features of a disordered Mott system near an IMT and suggest that V-shaped gaps may be an inevitable consequence of disorder in doped Mott insulators.

cond-mat.str-el↗