arXiv subjects

S. Seki

Publications and source records attributed to S. Seki.

At least 37 records · Page 2Linked to original sources

arXiv2018

Phonon Magnetochiral Effect

Magnetochiral effect (MChE) of phonons, a nonreciprocal acoustic property arising due to the symmetry principles, is demonstrated in a chiral-lattice ferrimagnet Cu$_2$OSeO$_3$. Our high-resolution ultrasound experiments reveal that the sound velocity differs for parallel and antiparallel propagation with respect to the external magnetic field.The sign of the nonreciprocity depends on the chirality of the crystal in accordance with the selection rule of the MChE. The nonreciprocity is enhanced below the magnetic ordering temperature and at higher ultrasound frequencies, which is quantitatively explained by a proposed magnon-phonon hybridization mechanism.

cond-mat.mtrl-sci↗

arXiv2018

Emergent topological spin structures in a centrosymmetric cubic perovskite

The skyrmion crystal (SkX) characterized by a multiple-q helical spin modulation has been reported as a unique topological state that competes with the single-q helimagnetic order in non-centrosymmetric materials. Here we report the discovery of a rich variety of multiple-q helimagnetic spin structures in the centrosymmetric cubic perovskite SrFeO3. On the basis of neutron diffraction measurements, we have identified two types of robust multiple-q topological spin structures that appear in the absence of external magnetic fields: an anisotropic double-q spin spiral and an isotropic quadruple-q spiral hosting a three-dimensional lattice of hedgehog singularities. The present system not only diversifies the family of SkX host materials, but furthermore provides an experimental missing link between centrosymmetric lattices and topological helimagnetic order. It also offers perspectives for integration of SkXs into oxide electronic devices.

cond-mat.str-el↗

arXiv2017

Spin-wave spectroscopy on Dzyaloshinskii-Moriya interaction in room-temperature chiral magnets hosting skyrmions

Propagation character of spin wave was investigated for chiral magnets FeGe and Co-Zn-Mn alloys, which can host magnetic skyrmions near room temperature. On the basis of the frequency shift between counter-propagating spin waves, the magnitude and sign of Dzyaloshinskii-Moriya (DM) interaction were directly evaluated. The obtained magnetic parameters quantitatively account for the size and helicity of skyrmions as well as their materials variation, proving that the DM interaction plays a decisive role in the skyrmion formation in this class of room-temperature chiral magnets. The propagating spin-wave spectroscopy can thus be an efficient tool to study DM interaction in bulk single-phase compounds. Our results also demonstrate a function of spin-wave diode based on chiral crystal structures at room temperature.

cond-mat.mtrl-sci↗

arXiv2016

A new search for the $K_{L} \to π^0 ν\overlineν$ and $K_{L} \to π^{0} X^{0}$ decays

We searched for the $CP$-violating rare decay of neutral kaon, $K_{L} \to π^0 ν\overlineν$, in data from the first 100 hours of physics running in 2013 of the J-PARC KOTO experiment. One candidate event was observed while $0.34\pm0.16$ background events were expected. We set an upper limit of $5.1\times10^{-8}$ for the branching fraction at the 90\% confidence level (C.L.). An upper limit of $3.7\times10^{-8}$ at the 90\% C.L. for the $K_{L} \to π^{0} X^{0}$decay was also set for the first time, where $X^{0}$ is an invisible particle with a mass of 135 MeV/$c^{2}$.

hep-ex↗

arXiv2016

Thermal generation of spin current in a helimagnetic multiferroic hexaferrite

We report the experimental observation of longitudinal spin Seebeck effect in a multiferroic helimagnet Ba0.5Sr1.5Zn2Fe12O22. Temperature gradient applied normal to Ba0.5Sr1.5Zn2Fe12O22/Pt interface generates inverse spin Hall voltage of spin current origin in Pt, whose magnitude was found to be proportional to bulk magnetization of Ba0.5Sr1.5Zn2Fe12O22 even through the successive magnetic transitions among various helimagnetic and ferrimagnetic phases. This finding demonstrates that the helimagnetic spin wave can be an effective carrier of spin current. By controlling the population ratio of spin-helicity domains characterized by clockwise/counter-clockwise manner of spin rotation with use of poling electric field in the ferroelectric helimagnetic phase, we found that spin-helicity domain distribution does not affect the magnitude of spin current injected into Pt. The results suggest that the spin-wave spin current is rather robust against the spin-helicity domain wall, unlike the case with the conventional ferromagnetic domain wall.

cond-mat.str-el↗

arXiv2016

Long-lived neutral-kaon flux measurement for the KOTO experiment

The KOTO ($K^0$ at Tokai) experiment aims to observe the CP-violating rare decay $K_L \rightarrow π^0 ν\barν$ by using a long-lived neutral-kaon beam produced by the 30 GeV proton beam at the Japan Proton Accelerator Research Complex. The $K_L$ flux is an essential parameter for the measurement of the branching fraction. Three $K_L$ neutral decay modes, $K_L \rightarrow 3π^0$, $K_L \rightarrow 2π^0$, and $K_L \rightarrow 2γ$ were used to measure the $K_L$ flux in the beam line in the 2013 KOTO engineering run. A Monte Carlo simulation was used to estimate the detector acceptance for these decays. Agreement was found between the simulation model and the experimental data, and the remaining systematic uncertainty was estimated at the 1.4\% level. The $K_L$ flux was measured as $(4.183 \pm 0.017_{\mathrm{stat.}} \pm 0.059_{\mathrm{sys.}}) \times 10^7$ $K_L$ per $2\times 10^{14}$ protons on a 66-mm-long Au target.

physics.ins-det↗

arXiv2015

Development of a low-mass and high-efficiency charged particle detector

We developed a low-mass and high-efficiency charged particle detector for an experimental study of the rare decay $K_L \rightarrow π^0 ν\barν$. The detector is important to suppress the background with charged particles to the level below the signal branching ratio predicted by the Standard Model (O(10$^{-11}$)). The detector consists of two layers of 3-mm-thick plastic scintillators with wavelength shifting fibers embedded and Multi Pixel Photon Counters for readout. We manufactured the counter and evaluated the performance such as light yield, timing resolution, and efficiency. With this design, we achieved the inefficiency per layer against penetrating charged particles to be less than $1.5 \times 10^{-5}$, which satisfies the requirement of the KOTO experiment determined from simulation studies.

physics.ins-det↗

arXiv2015

Thermal generation of spin current in an antiferromagnet

Longitudinal spin Seebeck effect has been investigated for an uniaxial antiferromagnetic insulator Cr2O3, characterized by a spin-flop transition under magnetic field along the c-axis. We have found that temperature gradient applied normal to Cr2O3/Pt interface induces inverse spin Hall voltage of spin current origin in Pt, whose magnitude turns out to be always proportional to magnetization in Cr2O3. The observed voltage shows significant enhancement for the lower temperature region, which can be ascribed to the phonon-drag effect on the relevant spin excitations. The above results establish that antiferromagnetic spin waves with high frequency above 100 GHz can be an effective carrier of spin current.

cond-mat.str-el↗

arXiv2015

Nonreciprocal spin wave propagation in chiral-lattice ferromagnets

Spin current, i.e. the flow of spin angular momentum or magnetic moment, has recently attracted much attention as the promising alternative for charge current with better energy efficiency. Genuine spin current is generally carried by the spin wave (propagating spin precession) in insulating ferromagnets, and should hold the chiral symmetry when it propagates along the spin direction. Here, we experimentally demonstrate that such a spin wave spin current (SWSC) shows nonreciprocal propagation characters in a chiral-lattice ferromagnet. This phenomenon originates from the interference of chirality between the SWSC and crystal-lattice, which is mediated by the relativistic spin-orbit interaction. The present finding enables the design of perfect spin current diode, and highlights the importance of the chiral aspect in SWSC.

cond-mat.str-el↗

arXiv2015

Thermally Driven Ratchet Motion of Skyrmion Microcrystal and Topological Magnon Hall Effect

Spontaneously emergent chirality is an issue of fundamental importance across the natural sciences. It has been argued that a unidirectional (chiral) rotation of a mechanical ratchet is forbidden in thermal equilibrium, but becomes possible in systems out of equilibrium. Here we report our finding that a topologically nontrivial spin texture known as a skyrmion - a particle-like object in which spins point in all directions to wrap a sphere - constitutes such a ratchet. By means of Lorentz transmission electron microscopy we show that micron-sized crystals of skyrmions in thin films of Cu2OSeO3 and MnSi display a unidirectional rotation motion. Our numerical simulations based on a stochastic Landau-Lifshitz-Gilbert equation suggest that this rotation is driven solely by thermal fluctuations in the presence of a temperature gradient, whereas in thermal equilibrium it is forbidden by the Bohr-van Leeuwen theorem. We show that the rotational flow of magnons driven by the effective magnetic field of skyrmions gives rise to the skyrmion rotation, therefore suggesting that magnons can be used to control the motion of these spin textures.

cond-mat.str-el↗

arXiv2014

An aerogel Cherenkov detector for multi-GeV photon detection with low sensitivity to neutrons

We describe a novel photon detector which operates under an intense flux of neutrons. It is composed of lead-aerogel sandwich counter modules. Its salient features are high photon detection efficiency and blindness to neutrons. As a result of Monte Carlo (MC) simulations, the efficiency for photons with the energy larger than 1 GeV is expected to be higher than 99.5% and that for 2 GeV/$c$ neutrons less than 1%. The performance on the photon detection under such a large flux of neutrons was measured for a part of the detector. It was confirmed that the efficiency to photons with the energy $>$1 GeV was consistent with the MC expectation within 8.2% uncertainty.

physics.ins-det↗

arXiv2014

Coupled skyrmion sublattices in Cu2OSeO3

We report the observation of the skyrmion lattice in the chiral multiferroic insulator Cu2OSeO3 using Cu L3-edge resonant soft x-ray diffraction. We observe the unexpected existence of two distinct skyrmion sublattices that arise from inequivalent Cu sites with chemically identical coordination numbers but different magnetically active orbitals. The skyrmion sublattices are rotated with respect to each other implying a long wavelength modulation of the lattice. The modulation vector could be controlled with an applied magnetic field, associating this Moire-like phase with a continuous phase transition. Our findings will open a new class of science involving manipulation of quantum topological states.

cond-mat.str-el↗

arXiv2012

Superconductivity in Cu$_{x}$IrTe$_{2}$ induced by interlayer hybridization

The change in the electronic structure of layered Cu$_{x}$IrTe$_{2}$ has been characterized by transport and spectroscopic measurements, combined with first-principles calculations. The Cu-intercalation suppresses the monoclinic distortion, giving rise to the stabilization of the trigonal phase with superconductivity. Thermopower and Hall resistivity measurements suggest the multiband nature with hole and electron carriers for this system, which is masked by the predominance of the hole carriers enhanced by the interlayer hybridization in the trigonal phase. Rather than the instability of Ir $d$ band, a subtle balance between the interlayer and intralayer Te-Te hybridizations is proposed as a main factor dominating the structural transition and the superconductivity.

cond-mat.supr-con↗

arXiv2012

Formation and rotation of skyrmion crystal in the chiral-lattice insulator Cu2OSeO3

Small angle neutron scattering experiments were performed on a bulk single crystal of chiral-lattice multiferroic insulator Cu$_2$OSeO$_3$. In the absence of an external magnetic field, helical spin order with magnetic modulation vector $q \parallel <001>$ was identified. When a magnetic field is applied, a triple-$q$ magnetic structure emerges normal to the field in the A-phase just below the magnetic ordering temperature $T_c$, which suggests the formation of a triangular lattice of skyrmions. Notably, the favorable $q$-direction in the A-phase changes from $q \parallel <110>$ to $q \parallel <001>$ upon approaching $T_c$. Near the phase boundary between these two states, the external magnetic field induces a 30$^\circ$-rotation of the skyrmion lattice. This suggests a delicate balance between the magnetic anisotropy and the spin texture near $T_c$, such that even a small perturbation significantly affects the ordering pattern of the skyrmions.

cond-mat.str-el↗

arXiv2012

Magnetoelectric nature of skyrmions in a chiral magnetic insulator Cu2OSeO3

Dielectric properties were investigated under various magnitudes and directions of magnetic field (H) for a chiral magnetic insulator Cu2OSeO3. We found that the skyrmion crystal induces electric polarization (P) along either in-plane or out-of-plane direction of the spin vortices depending on the applied H-direction. The observed H-dependence of P in ferrimagnetic, helimagnetic, and skyrmion crystal state can be consistently described by the d-p hybridization model, highlighting an important role of relativistic spin-orbit interaction in the magnetoelectric coupling in Cu2OSeO3. Our analysis suggests that each skyrmion particle can locally carry electric dipole or quadrupole, which implies that the dynamics of skyrmions are controllable by the external electric field.

cond-mat.str-el↗

arXiv2012

Observation of magnetic excitations of skyrmion crystal in a helimagnetic insulator

The skyrmion is a topologically stable spin texture, in which the spin direction wraps a sphere. The topological nature gives rise to emergent electromagnetic phenomena such as topological Hall effect. Recently, the crystallization of nanoscale skyrmions was observed in chiral helimagnets with use of the neutron diffraction and Lorentz transmission electron microscopy. The skyrmions are quite mobile under electric current density as low as $\sim$ 10$^6$ A/m$^2$, and, in some cases, stable up to near room temperature. These features suggest that the skyrmions may work as a magnetic stable-variable similar to the bubble memory but equipped with topological functionalities. Here, we investigate the low-energy excitations of the skyrmion crystal in a helimagnetic insulator Cu$_2$OSeO$_3$ in terms of microwave response. We have observed two distinct excitations of the skyrmion with different polarization characteristics; the counter-clockwise circulating mode at 1 GHz and the breathing mode at 1.5 GHz. These modes may play a crucial role in the low energy dynamics of skyrmions and hence in their manipulation via external stimuli.

cond-mat.mtrl-sci↗

arXiv2011

Magnetic-field induced competition of two multiferroic orders in a triangular-lattice helimagnet MnI2

Magnetic and dielectric properties with varying magnitude and direction of magnetic field H have been investigated for a triangular lattice helimagnet MnI2. The in-plane electric polarization P emerges in the proper screw magnetic ground state below 3.5 K, showing the rearrangement of six possible multiferroic domains as controlled by the in-plane H. With every 60-degree rotation of H around the [001]-axis, discontinuous 120-degree flop of P-vector is observed as a result of the flop of magnetic modulation vector q. With increasing the in-plane H above 3 T, however, the stable q-direction changes from q||<1-10> to q||<110>, leading to a change of P-flop patterns under rotating H. At the critical field region (~3 T), due to the phase competition and resultant enhanced q-flexibility, P-vector smoothly rotates clockwise twice while H-vector rotates counter-clockwise once.

cond-mat.str-el↗

arXiv2010

Cupric chloride CuCl2 as an S=1/2 chain multiferroic

Magnetoelectric properties were investigated for an S=1/2 chain antiferromagnet CuCl2, which turns out to be the first example of non-chalcogen based spiral-spin induced multiferroics. Upon the onset of helimagnetic order propagating along the b-axis under zero magnetic field (H), we found emergence of ferroelectric polarization along the c-axis. Application of H along the b-axis leads to spin-flop transition coupled with drastic suppression of ferroelectricity, and rotation of H around the b-axis induces the rotation of spin-spiral plane and associated polarization direction. These behaviors are explained well within the framework of the inverse Dzyaloshinskii-Moriya model, suggesting the robustness of this magnetoelectric coupling mechanism even under the strong quantum fluctuation.

cond-mat.str-el↗