arXiv subjects

Yoshichika Ōnuki

Publications and source records attributed to Yoshichika Ōnuki.

At least 19 recordsLinked to original sources

arXiv2026

Helical-to-Fan Transitions under Magnetic Fields in the Noncentrosymmetric Tetragonal Magnet EuRhGe$_3$

The magnetic structure of EuRhGe$_3$, a noncentrosymmetric body-centered tetragonal magnet with the space group $I4mm$, has been investigated by resonant X-ray diffraction. Below $T_{\text{N}}=12$ K, EuRhGe$_3$ undergoes a helical magnetic ordering with an incommensurate propagation vector $q=(0, 0, 0.809)$, in which the magnetic moments lie in the $ab$ plane and rotate by a constant turn angle of $145.8^{\circ}$ between adjacent layers. When a magnetic field is applied along the $a$ axis at 2 K, a second-harmonic $2q$ peak develops, indicating that the circular helix is gradually distorted into a helimagnetic soliton-lattice state, which eventually undergoes a lock-in transition to the commensurate structure with $q=0.8$ at 3.8 T. Above the subsequent phase boundary at 5 T, the helicity is lost, and a spin-flop $xyz$-fan (elliptic conical) state is realized, in which the moments oscillate predominantly along the $b$ axis but are accompanied by a small $c$-axis component. At higher fields, the system enters a conventional planar $xy$-fan phase without a $c$-axis component. EuRhGe$_3$ provides a prototypical example of a helimagnet that exhibits a full sequence of field-induced structures, evolving from a circular helix to a spin-flop $xyz$-fan (elliptic conical), and finally to a planar $xy$-fan structure, which has been theoretically predicted.

cond-mat.str-el↗

arXiv2026

Magnetic Order of Dresselhaus-type Antiferromagnet EuIr$_4$In$_2$Ge$_4$ Studied by Single Crystal Neutron Diffraction

The magnetic order of EuIr$_4$In$_2$Ge$_4$, which crystallizes in a Dresselhaus-type noncentrosymmetric tetragonal structure, was investigated using two complementary single-crystal neutron diffraction approaches. Time-of-flight single-crystal diffraction reveals antiferromagnetic Bragg reflections with propagation vector $q = (1, 0, 0)$ below the Néel temperature $T_{\rm N}$ = 2.5 K, indicating a breaking of body-centered translational symmetry. Polarized neutron diffraction on a triple-axis spectrometer demonstrates that the ordered Eu$^{2+}$ $4f$ moments lie within the basal plane and form a collinear antiferromagnetic structure with antiparallel alignment between corner and body-center sites. Despite the Dresselhaus-type spin splitting in the conduction bands, the magnetic order remains simple, implying weak coupling between localized moments and itinerant electrons.

cond-mat.str-el↗

arXiv2025

Metastability of the Topological Magnetic Orders in the Chiral Antiferromagnet EuPtSi

We report resistivity and Hall effect measurements in the chiral antiferromagnet EuPtSi. Depending on the magnetic field orientation with respect to the crystallographic axes, EuPtSi presents different topological magnetic phases below the Néel temperature $T_N=4.05$K. In particular, for a field $H \parallel $ [111], it exhibits the well known skyrmion lattice A-phase inside the conical phase between $T=0.45$K and $T_N$ in the field range from 0.8T to 1.4T. Remarkably, the skyrmion lattice state in EuPtSi, composed of nanoscale skyrmions, can be extended down to very low temperature (lower than 0.1K) through field-cooling regardless of the cooling rate and of the magnetic history. Similarly the metastability of the A'- and B-phases ($H \parallel $ [100]) at low temperature is evidenced by our measurements. These results suggest that EuPtSi is a peculiar example where the competition between the topological stability and the thermal agitation can lead to metastable quantum skyrmion state.

cond-mat.str-el↗

arXiv2025

Successive orthorhombic distortions in kagome metals by molecular orbital formation

The kagome lattice, with its inherent frustration, hosts a plethora of exotic phenomena, including the emergence of $3\mathbf{q}$ charge density wave order. The high rotational symmetry, required to realize such an unconventional charge order, is broken in many kagome materials by orthorhombic distortions at high temperature, the origin of which is much less discussed despite their ubiquity. In this study, synchrotron X-ray diffraction reveals a structural phase transition from a parent hexagonal phase to an orthorhombic ground state, mediated by a critical regime of diffuse scattering in the prototypical kagome metals $R$Ru$_3$Si$_2$ ($R$=rare-earth). Structural analysis uncovers an interlayer dimerization of kagome atoms in the low-temperature phase. Accordingly, a dimer model with one-dimensional disorder on kagome layers successfully reproduces the diffuse scattering. The observations point to molecular orbital formation between kagome $4d_{z^2}$ orbitals as the driving force behind the transition, consistent with \textit{ab initio} calculations. A framework based on electronegativity and atomic radii is proposed to evaluate the stability of the hexagonal phase in kagome metals, guiding the design of highly symmetric materials.

cond-mat.str-el↗

arXiv2025

Triangular lattice magnet GdGa$_2$ with short-period spin cycloids and possible skyrmion phases

The two-dimensional triangular lattice (TAL) is a model system of magnetic frustration and competing interactions, where skyrmion spin vortices can be induced by a vertical magnetic field $B$. We target the binary compound GdGa$_2$ with an undistorted TAL of Gd$^{3+}$ Heisenberg moments. At higher temperature ($T > 5$ K, $B = 0$, phase II), we reveal the cycloidal spin textures in GdGa$_2$ via resonant elastic X-ray scattering (REXS). Further, a transition with strong magneto-elastic response occurs when cooling into the zero-field ground state ($T < 5$ K, phase I). We also report the thermodynamic phase boundaries of $B$-induced magnetic A-phases, which are suppressed by an in-plane magnetic field and which have enhanced resistivity due to the partial opening of a charge gap. In analogy to Gd$_2$PdSi$_3$ and GdRu$_2$Si$_2$, these phases may represent a superposition of various cycloids, possibly a Néel skyrmion lattice. Our work lays the basis for further studies of the magnetic phase diagram of GdGa$_2$.

cond-mat.str-el↗

arXiv2024

Giant Hall effect in a highly conductive frustrated magnet GdCu$_2$

The Hall effect is one of the most fundamental but elusive phenomena in condensed matter physics due to the rich variety of underlying mechanisms. Here we report an exceptionally large Hall effect in a frustrated magnet GdCu$_2$ with high conductivity. The Hall conductivity at the base temperature is as high as 4 x 10$^4$ $Ω^{-1}$cm$^{-1}$ and shows abrupt sign changes under magnetic fields. Remarkably, the giant Hall effect is rapidly suppressed as the longitudinal conductivity is lowered upon increasing temperature or introducing tiny amount of quenched disorder. Our systematic transport measurements together with neutron scattering measurements and ab initio band calculations indicate that the unusual Hall effect can be understood in terms of spin-splitting induced emergence/disappearance of Fermi pockets as well as skew scattering from spin-chiral cluster fluctuations in a field-polarized state. The present study demonstrates complex interplay among magnetization, spin-dependent electronic structure, and spin fluctuations in producing the giant Hall effect in highly conductive frustrated magnets.

cond-mat.str-el↗

arXiv2024

Single helicity of the triple-$q$ triangular skyrmion lattice state in cubic chiral helimagnet EuPtSi

We investigated the magnetic helicity of the triple-$q$ magnetic structure of the triangular skyrmion lattice in the ``A-phase" of EuPtSi for a magnetic field along the [111] axis by resonant x-ray diffraction using a circularly polarized beam. We show that all three Fourier components of the triple-$q$ structure are perpendicular to the respective $q$ vectors and have the same helicity. They are connected by the rotation operations about the [111] axis. The helicity is the same as that of the single-$q$ helimagnetic phase at low fields, suggesting that the antisymmetric exchange interaction inherent in the chiral structure supports the formation of the triangular skyrmion lattice. We also observe that the helical plane in the helimagnetic phase is tilted to the magnetic field to form a conical structure before the first-order transition to the skyrmion lattice phase.

cond-mat.str-el↗

arXiv2023

Anomalously large spin-dependent electron correlation in nearly half-metallic ferromagnet CoS$_2$

The spin-dependent band structure of CoS$_2$ which is a candidate for a half-metallic ferromagnet was investigated by both spin- and angle-resolved photoemission spectroscopy and theoretical calculations, in order to reappraise the half-metallicity and electronic correlations. We determined the three-dimensional Fermi surface and the spin-dependent band structure. As a result, we found that a part of the minority spin bands is on the occupied side in the vicinity of the Fermi level, providing spectroscopic evidence that CoS$_2$ is not but very close to a half-metal. Band calculations using density functional theory with generalized gradient approximation showed a good agreement with the observed majority spin $e_g$ bands, while it could not explain the observed band width of the minority-spin eg bands. On the other hand, theoretical calculations using dynamical mean field theory could better reproduce the strong mass renormalization in the minority-spin $e_g$ bands. All those results strongly suggest the presence of anomalously enhanced spin-dependent electron correlation effects on the electronic structure in the vicinity of the half-metallic state. We also report the temperature dependence of the electronic structure across the Curie temperature and discuss the mechanism of the thermal demagnetization. Our discovery of the anomalously large spin-dependent electronic correlations not only demonstrates a key factor in understanding the electronic structure of half-metals but also provides a motivation to improve theoretical calculations on spin-polarized strongly correlated systems.

cond-mat.str-el↗

arXiv2023

Nature of the Antiferromagnetic Order in GdCu$_2$

A magnetic order in orthorhombic GdCu$_2$ was investigated via a single crystal neutron diffraction with thermal neutron. Magnetic peaks were observed at incommensurate positions described by the ordering vector $q$=($δ$,~1,~0) with $δ$=0.678 at 4.6~K. This ordering vector is close to the commensurate one with $q_{\rm c}$=(2/3,~1,~0) reported earlier, but clearly deviates. The incommensurate nature of the magnetic order in GdCu$_2$ is further corroborated by the peak shift with temperature below $T_{\rm N}$.

cond-mat.str-el↗

arXiv2023

Distorted triangular skyrmion lattice in a noncentrosymmetric tetragonal magnet

Magnetic skyrmions are particle-like spin-swirling objects ubiquitously realized in magnets. They are topologically stable chiral kinks composed of multiple modulation waves of spiral spin structures, where the helicity of each spiral is usually selected by antisymmetric exchange interactions in noncentrosymmetric crystals. We report an experimental observation of a distorted triangular lattice of skyrmions in the polar tetragonal magnet EuNiGe$_3$, reflecting a strong coupling with the lattice. Moreover, through resonant x-ray diffraction, we find that the magnetic helicity of the original spiral at zero field is reversed when the skyrmion lattice is formed in a magnetic field. This means that the energy gain provided by the skyrmion lattice formation is larger than the antisymmetric exchange interaction. Our findings will lead us to a further understanding of emergent magnetic states.

cond-mat.str-el↗

arXiv2023

Helicity Selection of the Cycloidal Order in Noncentrosymmetric EuIrGe$_3$

The magnetic helicities of the cycloidal ordering in EuIrGe$_3$, with a noncentrosymmetric tetragonal structure, have been studied by circularly polarized resonant X-ray diffraction. It is shown that the helicity of each cycloidal domain is uniquely determined and satisfies the symmetry relations of the $C_{4v}$ point group of the crystal structure. The result shows that the cycloidal helicity is determined by the Dzyaloshinskii-Moriya type antisymmetric exchange interaction. The domain selection and the phase transition by the external magnetic field along [100] and [110] have also been studied. It is shown that the cycloidal plane prefers to be perpendicular to the field and the transverse conical state is realized.

cond-mat.str-el↗

arXiv2023

Rhombic skyrmion lattice coupled with orthorhombic structural distortion in EuAl$_{4}$

The centrosymmetric tetragonal itinerant magnet EuAl$_{4}$ exhibits an intricate magnetic phase diagram including rhombic and square skyrmion-lattice (SkL) phases in the external magnetic field. Here, we report a multi-axis dilatometric investigation of EuAl$_{4}$ by means of a newly designed fiber-Bragg-grating technique complemented by a resonant x-ray scattering experiment, revealing anisotropic magnetostriction and magnetovolume effect associated with successive phase transitions. The rhombic and square SkL phases are found to possess $\sim$0.10% and $\sim$0.03% orthorhombic structural distortion within the $ab$ plane, respectively. We propose that the coupling between the spin system and the lattice deformation should be essential for the structural instability in EuAl$_{4}$, yielding a rich variety of topological spin textures with spontaneous rotational-symmetry breaking as well as a potential controllability of the SkL phases by uniaxial stress or pressure.

cond-mat.str-el↗

arXiv2022

Cycloidal Magnetic Ordering in Noncentrosymmetric EuIrGe$_3$

Successive magnetic phase transitions at $T_{\text{N}}$=12.2 K, $T_{\text{N}}^{\;\prime}$=7.0 K, and $T_{\text{N}}^{\;*}$=5.0 K in EuIrGe$_3$, an intermetallic compound with a body centered tetragonal lattice belonging to a polar space group $I4mm$, has been investigated by neutron diffraction and resonant X-ray diffraction. It is shown that EuIrGe$_3$ exhibits an incommensurate longitudinal sinusoidal order with $q\sim (0, 0, 0.792)$ and $m_{q} \parallel c\text{-axis}$ in the high temperature phase ($T_{\text{N}}^{\;\prime}< T < T_{\text{N}}$), which changes to a cycloidal order with $q=(δ', 0, 0.8)$ ($δ'\sim 0.017$) and $m_{q} \parallel ac\text{-plane}$ in the intermediate phase ($T_{\text{N}}^{\;*} < T < T_{\text{N}}^{\;\prime}$). In the low temperature phase ($T < T_{\text{N}}^{\;*}$), the cycloidal plane rotates by $45^{\circ}$ to have $q=(δ, δ, 0.8)$ ($δ\sim 0.012$). It is also pointed out that the X-ray scattering amplitude from odd-parity magnetic quadrupole due to the polar environment interfere with that from normal even-parity magnetic dipole in the magnetic ordered phase.

cond-mat.str-el↗

arXiv2021

Charge-Density-Wave Order and Multiple Magnetic Transitions in Divalent Europium Compound EuAl$_4$

Multiple transition phenomena in divalent Eu compound EuAl$_4$ with the tetragonal structure were investigated via the single-crystal time-of-flight neutron Laue technique. At 30.0 K below a charge-density-wave (CDW) transition temperature of $T_{\rm CDW}$ = 140 K, superlattice peaks emerge near nuclear Bragg peaks described by an ordering vector $q_{\rm CDW}$=(0 0 $δ_c$) with $δ_c{\sim}$0.19. In contrast, magnetic peaks appear at $q_2 = (δ_2 δ_2 0)$ with $δ_2$ = 0.085 in a magnetic-ordered phase at 13.5 K below $T_{\rm N1}$ = 15.4 K. By further cooling to below $T_{\rm N3}$ = 12.2 K, the magnetic ordering vector changes into $q_1 = (δ_1 0 0)$ with $δ_1$ = 0.17 at 11.5 K and slightly shifts to $δ_1$ = 0.194 at 4.3 K. No distinct change in the magnetic Bragg peak was detected at $T_{\rm N2}$=13.2 K and $T_{\rm N4}$=10.0 K. The structural modulation below $T_{\rm CDW}$ with $q_{\rm CDW}$ is characterized by the absence of the superlattice peak in the (0 0 $l$) axis. As a similar CDW transition was observed in SrAl$_4$, the structural modulation with $q_{\rm CDW}$ could be mainly ascribed to the displacement of Al ions within the tetragonal $ab$-plane. Complex magnetic transitions are in stark contrast to a simple collinear magnetic structure in isovalent EuGa$_4$. This could stem from different electronic structures with the CDW transition between two compounds.

cond-mat.str-el↗

arXiv2018

Unique Helical Magnetic Order and Field-Induced Phase in Trillium Lattice Antiferromagnet EuPtSi

Magnetic transition phenomena in cubic chiral antiferromagnet EuPtSi with $T_{\rm N}$=4.0~K were investigated by means of single crystal neutron diffraction. At 0.3~K in the ground state, magnetic peaks emerge at positions represented by an ordering vector ${q}_{1}$=$(0.2, 0.3, 0)$ and its cyclic permutation. Upon heating, an additional magnetic peak splitting with hysteresis was uncovered at around $T^*_{\rm N}{\sim}$2.5~K, indicating the presence of a first-order commensurate-incommensurate transition with ${q}^*_{1}$=$(0.2, 0.3, δ)$ ($δ_{\rm max}{\simeq}$0.04) at $T^*_{\rm N}$. A half-polarized neutron scattering experiment for polarization parallel to the scattering vector revealed that polarization antiparallel to the scattering vector has stronger intensity in both magnetic phases. This feature clarifies the single chiral character of the helical structure with moments lying perpendicular to the ordering vector in both ordered states. Under a vertical magnetic field of 1.2~T for ${B}{\parallel}$[1,1,1] at 1.9~K entering into the so-called $A$ phase, magnetic peaks form characteristic hexagonal patterns in the equatorial scattering plane around nuclear peaks. An ordering vector ${q}_{A}{\simeq}({\pm}0.09, {\pm}0.20, {\mp}0.28)$ of the $A$-phase has similar periodic length as $q_{1}$, and could be the hallmark of a formation of skyrmion lattice in EuPtSi.

cond-mat.str-el↗

arXiv2018

Linear magnetoresistance in the low-field limit in density-wave materials

The magnetoresistance (MR) of a material is typically insensitive to reversing the applied field direction and varies quadratically with magnetic field in the low-field limit. Quantum effects [1], unusual topological band structures [2], and inhomogeneities that lead to wandering current paths [3, 4] can induce a crossover from quadratic to linear magnetoresistance with increasing magnetic field. Here we explore a series of metallic charge- and spin-density-wave systems that exhibit extremely large positive linear magnetoresistance. By contrast to other linear MR mechanisms, this effect remains robust down to miniscule magnetic fields of tens of Oersted at low temperature. We frame an explanation of this phenomenon in a semi-classical narrative for a broad category of materials with partially-gapped Fermi surfaces due to density waves.

cond-mat.mtrl-sci↗

arXiv2018

Microscopic Observation of Heavy Quasiparticle Formation in the Intermediate Valence Compound EuNi$_2$P$_2$: $^{31}$P NMR Study

We report $^{31}$P NMR measurements under various magnetic fields up to 7 T for the intermediate valence compound EuNi$_2$P$_2$, which shows heavy electronic states at low temperatures. In the high-temperature region above 40 K, the Knight shift followed the Curie--Weiss law reflecting localized $4f$ states. In addition, the behavior corresponding to the temperature variation of the average valence of Eu was observed in the nuclear spin-lattice relaxation rate $1/T_1$. With the occurrence of the Kondo effect, $1/T_1$ was clearly reduced below 40 K, and the Knight shift becomes almost constant at low temperatures. From these results, the formation of heavy quasiparticles by the hybridization of Eu $4f$ electrons and conduction electrons was clarified from microscopic viewpoints. Furthermore, a characteristic spin fluctuation was observed at low temperatures, which would be associated with valence fluctuations caused by the intermediate valence state of EuNi$_2$P$_2$.

cond-mat.str-el↗

arXiv2017

Electronic structures of UX$_3$ (X=Al, Ga, and In) studied by photoelectron spectroscopy

The electronic structures of UX$_3$ (X=Al, Ga, and In) were studied by photoelectron spectroscopy to understand the relationship between their electronic structures and magnetic properties. The band structures and Fermi surfaces of UAl$_3$ and UGa$_3$ were revealed experimentally by angle-resolved photoelectron spectroscopy (ARPES), and they were compared with the result of band-structure calculations. The topologies of the Fermi surfaces and the band structures of UAl$_3$ and UGa$_3$ were explained reasonably well by the calculation, although bands near the Fermi level ($E_\mathrm{F}$) were renormalized owing to the finite electron correlation effect. The topologies of the Fermi surfaces of UAl$_3$ and UGa$_3$ are very similar to each other, except for some minor differences. Such minor differences in their Fermi surface or electron correlation effect might take an essential role in their different magnetic properties. No significant changes were observed between the ARPES spectra of UGa$_3$ in the paramagnetic and antiferromagnetic phases, suggesting that UGa$_3$ is an itinerant weak antiferromagnet. The effect of chemical pressure on the electronic structures of UX$_3$ compounds was also studied by utilizing the smaller lattice constants of UAl$_3$ and UGa$_3$ than that of UIn$_3$. The valence band spectrum of UIn$_3$ is accompanied by a satellite-like structure on the high-binding-energy side. The core-level spectrum of UIn$_3$ is also qualitatively different from those of UAl$_3$ and UGa$_3$. These findings suggest that the U~$5f$ states in UIn$_3$ are more localized than those in UAl$_3$ and UGa$_3$.

cond-mat.str-el↗