Theory of Collective Excitations in the Quadruple-Q Magnetic Hedgehog Lattices.

Hedgehog and antihedgehog spin textures in magnets behave as emergent monopoles and antimonopoles, which give rise to astonishing transport and electromagnetic phenomena. Using the Kondo-lattice model in three dimensions, we theoretically study collective spin-wave excitation modes of magnetic hedgehog lattices which have recently been discovered in itinerant magnets such as MnSi_{1-x}Ge_{x} and SrFeO_{3}. It is revealed that the spin-wave modes, which appear in the subterahertz regime, have dominant amplitudes localized at Dirac strings connecting hedgehog-antihedgehog pairs and are characterized by their translational oscillations. It is found that their spectral features sensitively depend on the number and configuration of the Dirac strings and, thus, can be exploited for identifying the topological phase transitions associated with the monopole-antimonopole pair annihilations.

Magnon-Skyrmion Hybrid Quantum Systems: Tailoring Interactions via Magnons.

Coherent and dissipative interactions between different quantum systems are essential for the construction of hybrid quantum systems and the investigation of novel quantum phenomena. Here, we propose and analyze a magnon-skyrmion hybrid quantum system, consisting of a micromagnet and nearby magnetic skyrmions. We predict a strong-coupling mechanism between the magnonic mode of the micromagnet and the quantized helicity degree of freedom of the skyrmion. We show that with this hybrid setup it is possible to induce magnon-mediated nonreciprocal interactions and responses between distant skyrmion qubits or between skyrmion qubits and other quantum systems like superconducting qubits. This work provides a quantum platform for the investigation of diverse quantum effects and quantum information processing with magnetic microstructures.

Spontaneous Vortex-Antivortex Pairs and Their Topological Transitions in a Chiral-Lattice Magnet.

The spontaneous formation and topological transitions of vortex-antivortex pairs have implications for a broad range of emergent phenomena, for example, from superconductivity to quantum computing. Unlike magnets exhibiting collinear spin textures, helimagnets with noncollinear spin textures provide unique opportunities to manipulate topological forms such as (anti)merons and (anti)skyrmions. However, it is challenging to achieve multiple topological states and their interconversion in a single helimagnet due to the topological protection for each state. Here, the on-demand creation of multiple topological states in a helimagnet Fe0.5 Co0.5 Ge, including a spontaneous vortex pair of meron with topological charge N = -1/2 and antimeron with N = 1/2, and a vortex-antivortex bundle, that is, a bimeron (meron pair) with N = -1 is reported. The mutual transformation between skyrmions and bimerons with respect to the competitive effects of magnetic field and magnetic shape anisotropy is demonstrated. It is shown that electric currents drive the individual bimerons to form their connecting assembly and then into a skyrmion lattice. These findings signify the feasibility of designing topological states and offer new insights into the manipulation of noncollinear spin textures for potential applications in various fields.

Chiral Skyrmions Interacting with Chiral Flowers.

The chiral nature of active matter plays an important role in the dynamics of active matter interacting with chiral structures. Skyrmions are chiral objects, and their interactions with chiral nanostructures can lead to intriguing phenomena. Here, we explore the random-walk dynamics of a thermally activated chiral skyrmion interacting with a chiral flower-like obstacle in a ferromagnetic layer, which could create topology-dependent outcomes. It is a spontaneous mesoscopic order-from-disorder phenomenon driven by the thermal fluctuations and topological nature of skyrmions that exists only in ferromagnetic and ferrimagnetic systems. The interactions between the skyrmions and chiral flowers at finite temperatures can be utilized to control the skyrmion position and distribution without applying any external driving force or temperature gradient. The phenomenon that thermally activated skyrmions are dynamically coupled to chiral flowers may provide a new way to design topological sorting devices.

Handwritten digit recognition by spin waves in a Skyrmion reservoir.

By performing numerical simulations for the handwritten digit recognition task, we demonstrate that a magnetic skyrmion lattice confined in a thin-plate magnet possesses high capability of reservoir computing. We obtain a high recognition rate of more than 88%, higher by about 10% than a baseline taken as the echo state network model. We find that this excellent performance arises from enhanced nonlinearity in the transformation which maps the input data onto an information space with higher dimensions, carried by interferences of spin waves in the skyrmion lattice. Because the skyrmions require only application of static magnetic field instead of nanofabrication for their creation in contrast to other spintronics reservoirs, our result consolidates the high potential of skyrmions for application to reservoir computing devices.

Dynamical Phase Transitions in the Photodriven Charge-Ordered Dirac-Electron System.

We study photoinduced phase transitions and charge dynamics in the interacting Dirac-electron system with a charge-ordered ground state theoretically by taking an organic salt α-(BEDT-TTF)_{2}I_{3}. By analyzing the extended Hubbard model for this compound using a combined method of numerical simulations based on the time-dependent Schrödinger equation and the Floquet theory, we observe successive dynamical phase transitions from the charge-ordered insulator to a gapless Dirac semimetal and, eventually, to a Chern insulator phase under irradiation with circularly polarized light. These phase transitions occur as a consequence of two major effects of circularly polarized light, i.e., closing of the charge gap through melting the charge order and opening of the topological gap by breaking the time-reversal symmetry at the Dirac points. We demonstrate that these photoinduced phenomena are governed by charge dynamics of driven correlated Dirac electrons.

Low-Energy Excitations of Skyrmion Crystals in a Centrosymmetric Kondo-Lattice Magnet: Decoupled Spin-Charge Excitations and Nonreciprocity.

We theoretically study spin and charge excitations of skyrmion crystals stabilized by conduction-electron-mediated magnetic interactions via spin-charge coupling in a centrosymmetric Kondo-lattice model by large-scale spin-dynamics simulations combined with the kernel polynomial method. We reveal clear segregation of spin and charge excitation channels and nonreciprocal nature of the spin excitations governed by the Fermi-surface geometry, which are unique to the skyrmion crystals in centrosymmetric itinerant hosts and can be a source of novel physical phenomena.

Electrically driven spin torque and dynamical Dzyaloshinskii-Moriya interaction in magnetic bilayer systems.

Efficient control of magnetism with electric means is a central issue of current spintronics research, which opens an opportunity to design integrated spintronic devices. However, recent well-studied methods are mostly based on electric-current injection, and they are inevitably accompanied by considerable energy losses through Joule heating. Here we theoretically propose a way to exert spin torques into magnetic bilayer systems by application of electric voltages through taking advantage of the Rashba spin-orbit interaction. The torques resemble the well-known electric-current-induced torques, providing similar controllability of magnetism but without Joule-heating energy losses. The torques also turn out to work as an interfacial Dzyaloshinskii-Moriya interaction which enables us to activate and create noncollinear magnetism like skyrmions by electric-voltage application. Our proposal offers an efficient technique to manipulate magnetizations in spintronics devices without Joule-heating energy losses.

Raman cell imaging with boron cluster molecules conjugated with biomolecules.

Raman spectroscopic measurements and theoretical calculation revealed that the Raman bands corresponding to the B-H stretching vibrations of two types of simple icosahedral boron clusters, ortho-carborane 3 and closo-dodecaborate 4 appeared at approximately 2450-2700 cm-1, and did not overlap with those of cellular components. Although ortho-carborane 3 possesses a possible property as a Raman probe, it was difficult to measure Raman imaging in the cell due to its poor water solubility. In fact, ortho-carborane derivative 6, which internally has an alkyne moiety, exhibited very weak Raman signals of the C[triple bond, length as m-dash]C stretching and the B-H stretching vibrations were barely detected at a 400 ppm boron concentration in HeLa cells. In contrast, closo-dodecaborate derivatives such as BSH (5) were found to be a potential Raman imaging probe cluster for target molecules in the cell. BSH-conjugated cholesterol 7 (BSH-Chol) was synthesized and used in Raman imaging in cells. Raman imaging and spectral analysis revealed that BSH-based Raman tags provide a versatile platform for quantitative Raman imaging.

Damage-free tip-enhanced Raman spectroscopy for heat-sensitive materials.

We report a method to establish experimental conditions for tip-enhanced Raman spectroscopy (TERS) with low thermal and mechanical damage to samples. In this method, we monitor the thermal desorption of thiol molecules from a gold-coated probe of an atomic force microscope (AFM) via TERS spectra. Temperatures for desorption of thiol molecules (60-100 °C) from gold surfaces cover the temperature range for degradation of heat-sensitive biomaterials (e.g. proteins). By monitoring the desorption of the thiols on the probe, we can estimate the power of an excitation laser for the samples to reach their critical temperatures for thermal degradation. Furthermore, we also found that an active oscillation of AFM cantilevers significantly promotes the heat transfer from the probe to the surrounding medium. This enables us to employ a higher power density of the excitation laser, resulting in a stronger Raman signal compared with the signal obtained with a contact mode. We propose that this combinatory method is effective in acquiring strong TERS signals while suppressing thermal and mechanical damage to soft and heat-sensitive samples.

Laser Scanning-Assisted Tip-Enhanced Optical Microscopy for Robust Optical Nanospectroscopy.

Laser-scanning-assisted tip-enhanced optical microscopy was developed for robust optical nanospectroscopy. The laser-scanning system was utilized to automatically set and keep the center of a tight laser-focusing spot in the proximity of a metallic tip with around 10 nm precision. This enabled us to efficiently and stably induce plasmon-coupled field enhancement at the apex of the metallic probe tip. The laser-scanning technique was also applied to tracking and compensating the thermal drift of the metallic tip in the spot. This technique is usable for long-term tip-enhanced optical spectroscopy without any optical degradation.

Correlating single-molecule and ensemble-average measurements of peptide adsorption onto different inorganic materials.

The coating of solid-binding peptides (SBPs) on inorganic material surfaces holds significant potential for improved surface functionalization at nano-bio interfaces. In most related studies, the goal has been to engineer peptides with selective and high binding affinity for a target material. The role of the material substrate itself in modulating the adsorption behavior of a peptide molecule remains less explored and there are few studies that compare the interaction of one peptide with different inorganic substrates. Herein, using a combination of two experimental techniques, we investigated the adsorption of a 16 amino acid-long random coil peptide to various inorganic substrates - gold, silicon oxide, titanium oxide and aluminum oxide. Quartz crystal microbalance-dissipation (QCM-D) experiments were performed in order to measure the peptide binding affinity for inorganic solid supports at the ensemble average level, and atomic force microscopy (AFM) experiments were conducted in order to determine the adhesion force of a single peptide molecule. A positive trend was observed between the total mass uptake of attached peptide and the single-molecule adhesion force on each substrate. Peptide affinity for gold was appreciably greater than for the oxide substrates. Collectively, the results obtained in this study offer insight into the ways in which inorganic materials can differentially influence and modulate the adhesion of SBPs.

Dynamical magnetoelectric phenomena of multiferroic skyrmions.

Magnetic skyrmions, vortex-like swirling spin textures characterized by a quantized topological invariant, realized in chiral-lattice magnets are currently attracting intense research interest. In particular, their dynamics under external fields is an issue of vital importance both for fundamental science and for technical application. Whereas observations of magnetic skyrmions has been limited to metallic magnets so far, their realization was also discovered in a chiral-lattice insulating magnet Cu2OSeO3 in 2012. Skyrmions in the insulator turned out to exhibit multiferroic nature with spin-induced ferroelectricity. Strong magnetoelectric coupling between noncollinear skyrmion spins and electric polarizations mediated by relativistic spin-orbit interaction enables us to drive motion and oscillation of magnetic skyrmions by application of electric fields instead of injection of electric currents. Insulating materials also provide an environment suitable for detection of pure spin dynamics through spectroscopic measurements owing to the absence of appreciable charge excitations. In this article, we review recent theoretical and experimental studies on multiferroic properties and dynamical magnetoelectric phenomena of magnetic skyrmions in insulators. We argue that multiferroic skyrmions show unique coupled oscillation modes of magnetizations and polarizations, so-called electromagnon excitations, which are both magnetically and electrically active, and interference between the electric and magnetic activation processes leads to peculiar magnetoelectric effects in a microwave frequency regime.

Quantitative Evaluation of Peptide-Material Interactions by a Force Mapping Method: Guidelines for Surface Modification.

Peptide coatings on material surfaces have demonstrated wide application across materials science and biotechnology, facilitating the development of nanobio interfaces through surface modification. A guiding motivation in the field is to engineer peptides with a high and selective binding affinity to target materials. Herein, we introduce a quantitative force mapping method in order to evaluate the binding affinity of peptides to various hydrophilic oxide materials by atomic force microscopy (AFM). Statistical analysis of adhesion forces and probabilities obtained on substrates with a materials contrast enabled us to simultaneously compare the peptide binding affinity to different materials. On the basis of the experimental results and corresponding theoretical analysis, we discuss the role of various interfacial forces in modulating the strength of peptide attachment to hydrophilic oxide solid supports as well as to gold. The results emphasize the precision and robustness of our approach to evaluating the adhesion strength of peptides to solid supports, thereby offering guidelines to improve the design and fabrication of peptide-coated materials.

Multiferroics. Magnetoelectric domain control in multiferroic TbMnO3.

The manipulation of domains by external fields in ferroic materials is of major interest for applications. In multiferroics with strongly coupled magnetic and electric order, however, the magnetoelectric coupling on the level of the domains is largely unexplored. We investigated the field-induced domain dynamics of TbMnO3 in the multiferroic ground state and across a first-order spin-flop transition. In spite of the discontinuous nature of this transition, the reorientation of the order parameters is deterministic and preserves the multiferroic domain pattern. Landau-Lifshitz-Gilbert simulations reveal that this behavior is intrinsic. Such magnetoelectric correlations in spin-driven ferroelectrics may lead to domain wall-based nanoelectronics devices.

Microwave Magnetochiral Effect in Cu_{2}OSeO_{3}.

We theoretically find that in the multiferroic chiral magnet Cu_{2}OSeO_{3} resonant magnetic excitations are coupled to the collective oscillation of the electric polarization, and thereby attain simultaneous activity to the ac magnetic field and ac electric field. Because of the interference between these magnetic and electric activation processes, this material hosts a gigantic magnetochiral dichroism for microwaves, that is, a directional dichroism at gigahertz frequencies in the Faraday geometry. The absorption intensity of a microwave differs by as much as ~30% depending on whether its propagation direction is parallel or antiparallel to the external magnetic field.

Current-induced skyrmion dynamics in constricted geometries.

Magnetic skyrmions--vortex-like swirling spin structures with a quantized topological number that are observed in chiral magnets--are appealing for potential applications in spintronics because it is possible to control their motion with ultralow current density. To realize skyrmion-based spintronic devices, it is essential to understand skyrmion motions in confined geometries. Here we show by micromagnetic simulations that the current-induced motion of skyrmions in the presence of geometrical boundaries is very different from that in an infinite plane. In a channel of finite width, transverse confinement results in steady-state characteristics of the skyrmion velocity as a function of current that are similar to those of domain walls in ferromagnets, whereas the transient behaviour depends on the initial distance of the skyrmion from the boundary. Furthermore, we show that a single skyrmion can be created by an electric current in a simple constricted geometry comprising a plate-shaped specimen of suitable size and geometry. These findings could guide the design of skyrmion-based devices in which skyrmions are used as information carriers.

Universal current-velocity relation of skyrmion motion in chiral magnets.

Current-driven motion of the magnetic domain wall in ferromagnets is attracting intense attention because of potential applications such as racetrack memory. There, the critical current density to drive the motion is ~10(9)-10(12) A m(-2). The skyrmions recently discovered in chiral magnets have much smaller critical current density of ~10(5)-10(6) A m(-2), but the microscopic mechanism is not yet explored. Here we present a numerical simulation of Landau-Lifshitz-Gilbert equation, which reveals a remarkably robust and universal current-velocity relation of the skyrmion motion driven by the spin-transfer-torque unaffected by either impurities or nonadiabatic effect in sharp contrast to the case of domain wall or spin helix. Simulation results are analysed using a theory based on Thiele's equation, and it is concluded that this behaviour is due to the Magnus force and flexible shape-deformation of individual skyrmions and skyrmion crystal, which enable them to avoid pinning centres.

Long-Term Stability of Residual Stress Improvement by Water Jet Peening Considering Working Processes.

To prevent primary water stress corrosion cracking (PWSCC), water jet peening (WJP) has been used on the welds of Ni-based alloys in pressurized water reactors (PWRs). Before WJP, the welds are machined and buffed in order to conduct a penetrant test (PT) to verify the weld qualities to access, and microstructure evolution takes place in the target area due to the severe plastic deformation. The compressive residual stresses induced by WJP might be unstable under elevated temperatures because of the high dislocation density in the compressive stress layer. Therefore, the stability of the compressive residual stresses caused by WJP was investigated during long-term operation by considering the microstructure evolution due to the working processes. The following conclusions were made: The compressive residual stresses were slightly relaxed in the surface layers of the thermally aged specimens. There were no differences in the magnitude of the relaxation based on temperature or time. The compressive residual stresses induced by WJP were confirmed to remain stable under elevated temperatures. The stress relaxation at the surface followed the Johnson-Mehl equation, which states that stress relaxation can occur due to the recovery of severe plastic strain, since the estimated activation energy agrees very well with the self-diffusion energy for Ni. By utilizing the additivity rule, it was indicated that stress relaxation due to recovery is completed during the startup process. It was proposed that the long-term stability of WJP under elevated temperatures must be assessed based on compressive stresses with respect to the yield stress. Thermal elastic-plastic creep analysis was performed to predict the effect of creep strain. After 100 yr of simulated continuous operation at 80% capacity, there was little change in the WJP compressive stresses under an actual operating temperature of 623 K. Therefore, the long-term stability of WJP during actual operation was analytically predicted.

Spin-wave modes and their intense excitation effects in Skyrmion crystals.

We theoretically study spin-wave modes and their intense excitations activated by microwave magnetic fields in the Skyrmion-crystal phase of insulating magnets by numerically analyzing a two-dimensional spin model using the Landau-Lifshitz-Gilbert equation. Two peaks of spin-wave resonances with frequencies of ∼1 GHz are found for in-plane ac magnetic field where distribution of the out-of-plane spin components circulates around each Skyrmion core. Directions of the circulations are opposite between these two modes, and hence the spectra exhibit a salient dependence on the circular polarization of irradiating microwave. A breathing-type mode is also found for an out-of-plane ac magnetic field. By intensively exciting these collective modes, melting of the Skyrmion crystal accompanied by a redshift of the resonant frequency is achieved within nanoseconds.