arXiv subjects

Masaki Uchida

Publications and source records attributed to Masaki Uchida.

At least 19 recordsLinked to original sources

arXiv2026

EuIn2Sb2: epitaxially stabilized axion insulator candidate with strong spin-orbit coupling

Eu triangular lattice layer compounds described by the general formula EuA2X2 have attracted growing attention due to a wide range of layered crystal structures and corresponding magnetic topological phases, arising from the coexistence of large Eu magnetic moments and energetically inverted A and X bands. Among the EuA2X2 family, EuIn2Sb2 has been predicted to host robust axion insulator and higher-order topological insulator states by stronger spin-orbit coupling but has not been experimentally realized, including the structural identification. Here we report the epitaxial stabilization of EuIn2Sb2 by adopting the molecular beam epitaxy technique. Structural characterization reveals that EuIn2Sb2 films are based on a unique In-on-In stacking, forming a new trilayer structure distinct from previously predicted structures in the EuA2X2 family. In addition to the in-plane antiferromagnetic ordering at TN=10.5K, the stronger spin-orbit coupling and enhanced In-In hybridization in EuIn2Sb2 make it an ideal candidate of axion insulator and higher-order topological insulator phases. These results establish EuIn2Sb2 as a new member of the EuA2X2 family and offer an advanced platform for exploring these magnetic topological phases.

cond-mat.mtrl-sci↗

arXiv2026

Observation of in-plane anomalous Nernst effect

The Nernst effect, which enables the conversion of a heat current into a transverse voltage under magnetic field or spin magnetization, holds significant promise for energy harvesting and thermal management in future electronics. However, the conventional Nernst effect is fundamentally constrained by the orthogonality requirement that the applied field or spontaneous magnetization must be perpendicular to the plane defined by the temperature gradient and the induced voltage. Here we report that symmetry-tailored ultrathin films of a prototypical ferromagnetic oxide exhibit anomalous Nernst effect arising from intrinsic coupling to spontaneous in-plane spin magnetization. Systematic magnetothermoelectric measurements under spherical rotations of the magnetic field reveal that a pronounced Nernst signal, comparable in magnitude to the out-of-plane response, emerges robustly associated with out-of-plane orbital magnetization. Our findings demonstrate that the anomalous Nernst effect is no longer limited by the orthogonality condition, opening new opportunities for more flexible designs of magnetothermoelectric materials and devices.

cond-mat.str-el↗

arXiv2026

Magnetic structure of EuZn$_2$Sb$_2$ single-crystal thin-film

Magnetic topological materials are a class of compounds which can host massless electrons controlled by the magnetic order. One such compound is EuZn$_2$Sb$_2$, which has recently garnered interest due to its strong interplay between the Eu magnetism and charge carriers. However the topology of the electronic band structure, which depends on the ground state magnetic configuration of the europium sublattice, has not been determined. Based on our \textit{ab-initio} calculations, we find that an in-plane and out-of-plane \textit{A}-type antiferromagnetic (AFM) order generates a topological crystalline insulator and Dirac semimetal respectively, whereas a ferromagnetic (FM) order stabilizes a Weyl semimetal. Our resonant x-ray elastic scattering measurements of single-crystal thin film EuZn$_2$Sb$_2$ reveal both a sharp magnetic peak at $\textit{\textbf{Q}}$=$(0,0,\frac{1}{2})$ and broad $\textit{\textbf{Q}}$=$(0,0,1)$ below $T_{\mathrm{N}}=12.9$\,K, which is associated with an \textit{A}-type AFM and FM order, respectively. Our measurements indicate that the FM and AFM layers are spatially separated along the crystal $c$ axis, with the former limited to the top three atomic layers. We propose that EuZn$_2$Sb$_2$ behaves as a Weyl semimetal in the surface FM layers, and as a topological crystalline insulator in the lower AFM layers.

cond-mat.str-el↗

arXiv2025

Structural origin of resonant diffraction in RuO$_2$

We report Ru L$_3$-edge resonant X-ray diffraction studies on single crystal and (001) epitaxial films of RuO$_2$. We investigate the distinct $\mathbf{Q} = (100)$ and $(001)$ reflections as a function of incident energy, azimuthal angle, and temperature. The results show that the observed resonant diffraction in RuO$_2$ is fully consistent with a resonant charge anisotropy signal of structural origin permitted by the parent (non-magnetic) rutile $P4_2/mnm$ space group. These results significantly constrain the magnetic contribution to the resonant diffraction signal and indicate the unlikely existence of $\mathbf{k} = 0$ antiferromagnetic order in RuO$_2$.

cond-mat.str-el↗

arXiv2025

Magneto-cubic and magneto-linear dependence observed in an in-plane anomalous Hall magnet

The Hall effect, particularly that arising from in-plane magnetic field, has recently emerged as a sensitive probe of quantum geometric properties in solids. Especially in trigonal systems, in-plane anomalous Hall effect (AHE) can be explicitly induced by nontrivial off-diagonal coupling between the magnetic field and the Hall vector on the principal plane. Here we elucidate multipolar dependence of the off-diagonal coupling in the in-plane AHE, by systematically measuring on the (001) principal plane of trigonal antiferromagnet EuCd2Sb2 thin films for each magnetic phase. Around zero field, magneto-cubic dependence of anomalous Hall resistivity is clearly observed not only in the paramagnetic phase but also even in the antiferromagnetic phase. An off-diagonal component of the octupolar tensor also exhibits unconventional decay above the magnetic ordering temperature, roughly depending on the inverse temperature to the third power. In the forced ferromagnetic phase, on the other hand, magneto-linear dependence dominantly appears and notably persists up to very high fields. Our findings clarify key aspects of the off-diagonal coupling in the in-plane AHE, paving the way for its future investigations and potential applications beyond conventional expectations about the Hall effect.

cond-mat.str-el↗

arXiv2025

Stark difference in the in-plane anomalous Hall response in Zintl compounds EuA2Sb2 (A = Zn, Cd) thin films

Recent observation of the in-plane anomalous Hall effect in magnetic Weyl semimetal EuCd2Sb2 has drawn attention to out-of-plane orbital magnetization induced by an in-plane field component. Here we study EuZn2Sb2, a sister compound of EuCd2Sb2, to demonstrate sensitive changes of the in-plane anomalous Hall effect on the band modulation. The Hall resistivity measured with rotating the magnetic field within the (001) principal plane of EuZn2Sb2 films exhibits a clear three-fold component corresponding to the in-plane anomalous Hall effect, which is distinct from the two-fold component of the planar Hall effect. The in-plane anomalous Hall effect of EuZn2Sb2 is highly contrasting to EuCd2Sb2, especially in terms of its opposite sign and field dependence, which can be explained by model calculations with different band inversion parameters. Our results pave the way for systematically controlling the in-plane anomalous Hall effect and orbital magnetization through elaborate band engineering.

cond-mat.mtrl-sci↗

arXiv2025

Anomalous Hall effect in Dirac semimetal probed by in-plane magnetic field

Intrinsic anomalous Hall effect (AHE) formulated by geometric properties of Bloch wavefunctions is a ubiquitous transport phenomenon not limited to magnetic systems but also allowed in non-magnetic ones under an external field breaking time-reversal symmetry. On the other hand, detection of field-induced AHE is practically challenging because the band modulation through the Zeeman and spin-orbit couplings is typically small compared to other contributions as induced by the Lorentz force. Here, we demonstrate on Dirac semimetal Cd$_3$As$_2$ films that the field-induced AHE in non-magnetic systems can be quantitatively probed by applying and rotating the magnetic field within the Hall deflection plane. Measurements on the Cd$_3$As$_2$ (112) plane reveal that AHE emerges as a clear three-fold symmetric component for the in-plane field rotation. This intrinsic response becomes more pronounced in ultralow-electron-density films where significant variations in the geometric properties are expected under the magnetic field. Our findings open new opportunities in the research of Hall responses manifested as orbital magnetization in non-magnetic systems.

cond-mat.mtrl-sci↗

arXiv2025

Spontaneous in-plane anomalous Hall response observed in a ferromagnetic oxide

Recent observation of anomalous Hall effect (AHE) induced by magnetic field or spin magnetization lying in the Hall deflection plane has sparked interest in diverse mechanisms for inducing the Hall vector component perpendicular to the applied magnetic field. Such off-diagonal coupling, which is strictly constrained by symmetry of the system, provides new degrees of freedom for engineering Hall responses. However, spontaneous response as extensively studied for out-of-plane AHE remains unexplored. Here we elucidate in-plane AHE in a typical ferromagnetic oxide SrRuO$_3$. The (111)-orientated ultrathin films with in-plane easy axes of spin magnetization exhibit spontaneous AHE at zero field, which is intrinsically coupled to the in-plane spin magnetization and controllable via its direction. Systematic measurements by varying azimuthal and polar field angles further reveal complex Hall responses shaped by higher-order terms allowed by trigonal distortion of the films. Our findings highlight versatile and controllable in-plane Hall responses with out-of-plane orbital ferromagnetism.

cond-mat.mtrl-sci↗

arXiv2024

Distinct topological Hall responses in CeCu$_2$-type EuZn$_2$ and EuCd$_2$ films

Rare earth intermetallic compounds crystallized in AlB$_2$-type and its low-symmetry derivative CeCu$_2$-type structures potentially host diverse frustrated magnetic structures and rich magnetotransport phenomena. We report the film growth of CeCu$_2$-type EuZn$_2$ by molecular beam epitaxy and the observation of topological Hall responses highly contrastive to isostructural EuCd$_2$. While their magnetization curves are rather similar, the topological Hall effect observed in EuZn$_2$ is simpler, with the only one component enhanced at the magnetic transition field. EuZn$_2$ may be a unique system for studying the magnetic domain boundary effect on topological Hall responses among the CeCu$_2$-type rare-earth intermetallic compounds.

cond-mat.mtrl-sci↗

arXiv2024

Unconventional two-dimensional quantum oscillations in three-dimensional thick SrRuO$_3$ films

SrRuO$_3$ is a prototypical transition metal oxide which hosts rich physical properties including itinerant ferromagnetism, high conductivity, and intrinsic Hall effect originating in the Weyl points. Recently, high-quality SrRuO$_3$ films with residual resistivity ratios of more than 50 have been reported to exhibit quantum oscillations at low temperatures in spite of its strong electron correlation. While the origin of the oscillations has been discussed in relation to Weyl orbits based on the Weyl semimetal band structure, so far experimentally reported results are neither consistent with each other nor with theoretically expected behavior, leaving the origin of the oscillations in SrRuO$_3$ films still elusive. In this report, we have carefully evaluated the quantum oscillations observed in three-dimensional thick SrRuO$_3$ films with a high residual resistivity ratio of RRR = 82. We reveal the coexistence of two oscillation components both derived from two-dimensional electronic states and with slightly different masses, suggesting the involvement of the surface Fermi arc states formed between different Weyl point pairs.

cond-mat.str-el↗

arXiv2024

Observation of in-plane anomalous Hall effect associated with orbital magnetization

For over a century, the Hall effect, a transverse effect under out-of-plane magnetic field or magnetization, has been a cornerstone for magnetotransport studies and applications. Modern theoretical formulation based on the Berry curvature has revealed the potential that even in-plane magnetic field can induce anomalous Hall effect, but its experimental demonstration has remained difficult due to its potentially small magnitude and strict symmetry requirements. Here we report observation of the in-plane anomalous Hall effect by measuring low-carrier density films of magnetic Weyl semimetal EuCd$_2$Sb$_2$. Anomalous Hall resistance exhibits distinct three-fold rotational symmetry for changes in the in-plane field component, and this can be understood in terms of out-of-plane Weyl points splitting or orbital magnetization induced by in-plane field, as also confirmed by model calculation. Our findings demonstrate the importance of in-plane field to control the Hall effect, accelerating materials development and further exploration of various in-plane field induced phenomena.

cond-mat.mes-hall↗

arXiv2024

Magnetic structure of EuCd$_2$Sb$_2$ single-crystal thin-film

We investigate the magnetic order in single crystalline EuCd$_2$Sb$_2$ thin films using a combined theoretical and experimental approach. Resonant elastic x-ray scattering experiments reveal a sharp magnetic peak at $q = (0, 0, \frac{1}{2})$ below $T_N = 7.2$ K, indicative of interlayer antiferromagnetic ordering. Additionally, we observe a weak diffuse magnetic signal centered at $q = (0, 0, 1)$ that persists above $T_N$, up to $T_C \sim 11$ K. Our Monte-Carlo simulations of a classical spin model approximation of the Eu magnetic sublattice demonstrate that the diffuse signal can arise from ferromagnetic coupling in the top few layers due to surface oxidation. On the other hand, the bulk of the sample exhibits antiferromagnetic coupling between layers. Finally, our fit of the model parameters to the magnetic ordering temperatures, shed light on the exchange couplings that are key in stabilizing the observed composite magnetic order.

cond-mat.mtrl-sci↗

arXiv2024

Ferromagnetic state with large magnetic moments realized in epitaxially strained Sr3Ru2O7 films

Technical advancement of oxide molecular beam epitaxy (MBE) has opened new avenues for studying various quantum transport phenomena in correlated transition-metal oxides, as exemplified by the exotic superconductivity of Sr$_2$RuO$_4$ and quantum oscillations of SrRuO$_3$. On the other hand, film research of another Ruddlesden-Popper strontium ruthenate Sr$_3$Ru$_2$O$_7$ which exhibits a unique quantum phase related to metamagnetism in bulk systems did not progress well. Here we report the fabrication of high-quality Sr$_3$Ru$_2$O$_7$ thin films by oxide MBE and the observation of a strain-induced ferromagnetic ground state. The change in magnetic exchange coupling evaluated by first-principles calculations indicates a systematic relation between the compression of the $c$-axis length and induced ferromagnetism. Giant epitaxial strain in high-quality films will be a key to a comprehensive understanding of the magnetism in Ruddlesden-Popper strontium ruthenates Sr$_{n+1}$Ru$_n$O$_{3n+1}$, which sensitively depends on the ratio of in-plane to out-of-plane Ru-Ru distances.

cond-mat.str-el↗

arXiv2024

Berry curvature derived negative magnetoconductivity observed in type-II magnetic Weyl semimetal films

Here we study nonmonotonic features which appear both in magnetoresistivity and anomalous Hall resistivity during the simple magnetization process, by systematically measuring type-II magnetic Weyl semimetal EuCd$_2$Sb$_2$ films over a wide carrier density range. We find that a positive magnetoresistivity hump can be explained as manifestation of a field-linear term in the generalized magnetoconductivity formula including the Berry curvature. As also confirmed by model calculation, the term can be negative and pronounced near the Weyl point energy in the case that the Weyl cones are heavily tilted. Our findings demonstrate extensive effects of the Berry curvature on various magnetotransport in magnetic Weyl semimetals beyond the anomalous Hall effect.

cond-mat.mtrl-sci↗

arXiv2024

Intrinsic insulating transport characteristics in low-carrier density EuCd2As2 films

Searching for an ideal magnetic Weyl semimetal hosting only a single pair of Weyl points has been a focal point for systematic clarification of its unique magnetotransport derived from the interplay between topology and magnetization. Among the candidates, triangular-lattice antiferromagnet EuCd$_2$As$_2$ has been attracting special attention due to the prediction of the ideal Weyl semimetal phase in the ferromagnetic state, however, transport properties of low-carrier density samples have remained elusive. Here we report molecular beam epitaxy growth of EuCd$_2$As$_2$ films, achieving low-hole density in the range of $10^{15}$-$10^{16}$ cm$^{-3}$ at low temperature. Transport measurements of such low-carrier density films reveal an insulating behavior with an activation gap of about 200 meV, which persists even in the field-induced ferromagnetic state. Our work provides an important experimental clue that EuCd$_2$As$_2$ is intrinsically insulating, contrary to the previous prediction.

cond-mat.mtrl-sci↗

arXiv2023

Edge and bulk states in Weyl-orbit quantum Hall effect as studied by Corbino measurements

We investigate edge and bulk states in Weyl-orbit based quantum Hall effect by measuring a Corbino-type device fabricated from a topological Dirac semimetal (Cd1-xZnx)3As2 film. Clear quantum Hall plateaus are observed when measuring one-sided terminals of the Corbino-type device. This indicates that edge states of the Weyl-orbit quantum Hall effect form closed trajectories consisting of Fermi arcs and chiral zero modes independently on inner and outer sides. On the other hand, the bulk resistance does not diverge at fields where the quantum Hall plateau appears, suggesting that the Weyl orbits in the bulk region are not completely localized when applying electric current through the bulk region.

cond-mat.mes-hall↗

arXiv2022

Strain-modulated anisotropic electronic structure in superconducting RuO$_2$ films

The binary ruthenate, RuO$_2$, has been the subject of intense interest due to its itinerant antiferromagnetism and strain-induced superconductivity. The strain mechanism and its effect on the microscopic electronic states leading to the normal and superconducting state, however, remain undisclosed. Here, we investigate highly-strained epitaxial (110) RuO$_2$ films using polarization-dependent oxygen K-edge X-ray absorption spectroscopy (XAS). Through the detection of pre-edge peaks, arising from O:$2p$ - Ru:$4d$ hybridization, we uncover the effects of epitaxial strain on the orbital/electronic structure near the Fermi level. Our data show robust strain-induced shifts of orbital levels and a reduction of hybridization strength. Furthermore, we reveal a pronounced in-plane anisotropy of the electronic structure along the $[110]/[1\bar{1}0]$ directions naturally stemming from the symmetry-breaking epitaxial strain of the substrate. The $B_{2g}$ symmetry component of the epitaxially-enforced strain breaks a sublattice degeneracy, resulting in an increase of the density of states at the Fermi level ($E_F$), possibly paving the way to superconductivity. These results underscore the importance of the effective reduction from tetragonal to orthorhombic lattice symmetry in (110) RuO$_2$ films and its relevance towards the superconducting and magnetic properties.

cond-mat.supr-con↗

arXiv2022

Maximizing intrinsic anomalous Hall effect by controlling the Fermi level in simple Weyl semimetal films

Large intrinsic anomalous Hall effect (AHE) originating in the Berry curvature has attracted growing attention for potential applications. Recently proposed magnetic Weyl semimetal EuCd$_2$Sb$_{\mathrm{2}}$ provides an excellent platform for controlling the intrinsic AHE because it only hosts a Weyl-points related band structure near the Fermi energy. Here we report the fabrication of EuCd$_2$Sb$_{\mathrm{2}}$ single-crystalline films and control of their anomalous Hall effect by film technique. As also analyzed by first-principles calculations of energy-dependent intrinsic anomalous Hall conductivity, the obtained anomalous Hall effect shows a sharp peak as a function of carrier density, demonstrating clear energy dependence of the intrinsic AHE.

cond-mat.str-el↗