Safety and Potential Usability of Immersive Virtual Reality for Brain Rehabilitation: A Pilot Study.

Objective: This study was conducted to demonstrate the safety and usability of an immersive virtual reality (VR) game as a rehabilitative training by assessing adverse events (AEs), adherence, and satisfaction in patients with brain injury who had free optional opportunities. Materials and Methods: The results were analyzed retrospectively. Seventy-eight patients with brain injury, undergoing rehabilitation treatment for motor impairment, were recruited. Among them, 51 were available for postintervention survey. The immersive type of VR training was programmed to facilitate use of the paralyzed upper extremity through a fishing simulation game. The Oculus Rift was used as head-mounted display device. Patients were observed for any AEs as defined in the Common Terminology Criteria for AEs during and after each VR training session. A postintervention telephone survey was done to investigate adherence-related factors and safety. Results: The results were analyzed after dividing the patients into nonadherence (patients participated <3 times) and high-adherence (≥3 times) groups. No serious AEs were reported during and after the VR training, and several patients reported other AEs, predominantly dizziness, with one case requiring cessation of VR training. Overall, the satisfaction rate was 54%. Compared with the nonadherence group, the high-adherence group expressed higher satisfaction with VR training, regarded it as effective for recovery from upper limb paralysis, accepted VR as comprehensible, and considered the level of difficulty to be appropriate (P < 0.05). Conclusion: Immersive VR training appeared to be safe for patients with brain injury.

Targeted Writing and Deleting of Magnetic Skyrmions in Two-Terminal Nanowire Devices.

Controllable writing and deleting of nanoscale magnetic skyrmions are key requirements for their use as information carriers for next-generation memory and computing technologies. While several schemes have been proposed, they require complex fabrication techniques or precisely tailored electrical inputs, which limits their long-term scalability. Here, we demonstrate an alternative approach for writing and deleting skyrmions using conventional electrical pulses within a simple, two-terminal wire geometry. X-ray microscopy experiments and micromagnetic simulations establish the observed skyrmion creation and annihilation as arising from Joule heating and Oersted field effects of the current pulses, respectively. The unique characteristics of these writing and deleting schemes, such as spatial and temporal selectivity, together with the simplicity of the two-terminal device architecture, provide a flexible and scalable route to the viable applications of skyrmions.

Direct Demonstration of Topological Stability of Magnetic Skyrmions via Topology Manipulation.

Topological protection precludes a continuous deformation between topologically inequivalent configurations in a continuum. Motivated by this concept, magnetic skyrmions, topologically nontrivial spin textures, are expected to exhibit topological stability, thereby offering a prospect as a nanometer-scale nonvolatile information carrier. In real materials, however, atomic spins are configured as not continuous but discrete distributions, which raises a fundamental question if the topological stability is indeed preserved for real magnetic skyrmions. Answering this question necessitates a direct comparison between topologically nontrivial and trivial spin textures, but the direct comparison in one sample under the same magnetic fields has been challenging. Here we report how to selectively achieve either a skyrmion state or a topologically trivial bubble state in a single specimen and thereby experimentally show how robust the skyrmion structure is in comparison with the bubbles. We demonstrate that topologically nontrivial magnetic skyrmions show longer lifetimes than trivial bubble structures, evidencing the topological stability in a real discrete system. Our work corroborates the physical importance of the topology in the magnetic materials, which has hitherto been suggested by mathematical arguments, providing an important step toward ever-dense and more-stable magnetic devices.

Dynamics of the Bloch point in an asymmetric permalloy disk.

A Bloch point (BP) is a topological defect in a ferromagnet at which the local magnetization vanishes. With the difficulty of generating a stable BP in magnetic nanostructures, the intrinsic nature of a BP and its dynamic behaviour has not been verified experimentally. We report a realization of steady-state BPs embedded in deformed magnetic vortex cores in asymmetrically shaped Ni80Fe20 nanodisks. Time-resolved nanoscale magnetic X-ray imaging combined with micromagnetic simulation shows detailed dynamic character of BPs, revealing rigid and limited lateral movements under magnetic field pulses as well as its crucial role in vortex-core dynamics. Direct visualizations of magnetic structures disclose the unique dynamical feature of a BP as an atomic scale discrete spin texture and allude its influence on the neighbouring spin structures such as magnetic vortices.

ThunderPunch: A bare-hand, gesture-based, large interactive display interface with upper-body-part detection in a top view.

We present a new bare-hand gesture interface for large-screen interaction in which multiple users can participate simultaneously and interact with virtual content directly. To better reflect the intent of our new interface, we have created a new type of hardware system with a large hybrid display, named ThunderPunch. Unlike the conventional method, which involves positioning the camera in front, the cameras are mounted on the ceiling so that they avoid covering the large screen. To achieve bare-hand interaction in this hardware structure, we propose real-time algorithms that detect multiple body poses and recognize punching and touching gestures from top-view depth images. A pointing and touching test shows that the proposed algorithm is usable and that it outperforms other algorithms. In addition, we created a game to make the best use of the proposed system.

Solution-Processed Ferrimagnetic Insulator Thin Film for the Microelectronic Spin Seebeck Energy Conversion.

The longitudinal spin Seebeck effects with a ferro- or ferrimagnetic insulator provide a new architecture of a thermoelectric device that could significantly improve the energy conversion efficiency. Until now, epitaxial yttrium iron garnet (YIG) films grown on gadolinium gallium garnet (GGG) substrates by a pulsed laser deposition have been most widely used for spin thermoelectric energy conversion studies. In this work, we developed a simple route to obtain a highly uniform solution-processed YIG film and used it for the on-chip microelectronic spin Seebeck characterization. We improved the film roughness down to ∼0.2 nm because the extraction of thermally induced spin voltage relies on the interfacial quality. The on-chip microelectronic device has a dimension of 200 µm long and 20 µm wide. The solution-processed 20 nm thick YIG film with a 10 nm Pt film was used for the spin Seebeck energy converter. For a temperature difference of Δ T ≈ 0.036 K applied on the thin YIG film, the obtained Δ V ≈ 28 µV, which is equivalent to SLSSE ≈ 80.4 nV/K, is close to the typical reported values for thick epitaxial YIG films. The temperature and magnetic field-dependent behaviors of spin Seebeck effects in our YIG films suggest active magnon excitations through the noncoherent precession channel. The effective SSE generation with the solution-processed thin YIG film provides versatile applications of the spin thermoelectric energy conversion.

Imaging the magnetic structures of artificial quasicrystal magnets using resonant coherent diffraction of circularly polarized X-rays.

Unraveling nanoscale spin structures has long been an important activity addressing various scientific interests, that are also readily adaptable to technological applications. This has invigorated the development of versatile nanoprobes suitable for imaging specimens under native conditions. Here we have demonstrated the resonant coherent diffraction of an artificial quasicrystal magnet with circularly polarized X-rays. The nanoscale magnetic structure was revealed from X-ray speckle patterns by comparing with micromagnetic simulations, as a step toward understanding the intricate relationship between the chemical and spin structures in an aperiodic quasicrystal lattice. Femtosecond X-ray pulses from free electron lasers are expected to immediately extend the current work to nanoscale structure investigations of ultrafast spin dynamics, surpassing the present spatio-temporal resolution.

Inertia-driven resonant excitation of a magnetic skyrmion.

Topological spin structures such as magnetic domain walls, vortices, and skyrmions, have been receiving great interest because of their high potential application in various spintronic devices. To utilize them in the future spintronic devices, it is first necessary to understand the dynamics of the topological spin structures. Since inertial effect plays a crucial role in the dynamics of a particle, understanding the inertial effect of topological spin structures is an important task. Here, we report that a strong inertial effect appears steadily when a skyrmion is driven by an oscillating spin-Hall-spin-torque (SHST). We find that the skyrmion exhibits an inertia-driven hypocycloid-type trajectory when it is excited by the oscillating SHST. This motion has not been achieved by an oscillating magnetic field, which only excites the breathing mode without the inertial effect. The distinct inertial effect can be explained in terms of a spin wave excitation in the skyrmion boundary which is induced by the non-uniform SHST. Furthermore, the inertia-driven resonant excitation provides a way of experimentally estimating the inertial mass of the skyrmion. Our results therefore pave the way for the development of skyrmion-based device applications.

Magnetism of the hypo-oxide state at the diffuse interface between the ferromagnet and antiferromagnet phases.

At the interface between ferromagnetic and antiferromagnetic phases, various spin configurations with a higher degrees of complexity than in the bulk states can be derived due to the diverse possible interface atomic structures, where coupling interactions among the constituting atoms can form in consistence with altered atomic configurations. The interface magnetic properties then depend on the collective behavior of such spin structures. In the present work, an extended interfacial configuration of a hypo-oxide state was prepared by establishing the gradient of oxygen concentration across the spatially diffuse interface region between ferromagnetic metallic and antiferromagnetic oxide phases at the nanometer scale. With these mixed ferromagnetic and antiferromagnetic couplings among the atoms in the interfacial hypo- or sub-oxide state, novel magnetic behavior can be induced. We report here, for the first time, a significant increase of saturation magnetization with temperature over a broad temperature range, which is against the conventional expectation for any generally known magnetic materials. And the unusual temperature dependent behavior can be understood as the combined effects of competing ferromagnetic and antiferromagnetic couplings acting on atoms in and near the interface region.

Mechanically Robust, Stretchable Solar Absorbers with Submicron-Thick Multilayer Sheets for Wearable and Energy Applications.

A facile method to fabricate a mechanically robust, stretchable solar absorber for stretchable heat generation and an enhanced thermoelectric generator (TEG) is demonstrated. This strategy is very simple: it uses a multilayer film made of titanium and magnesium fluoride optimized by a two-dimensional finite element frequency-domain simulation, followed by the application of mechanical stresses such as bending and stretching to the film. This process produces many microsized sheets with submicron thickness (∼500 nm), showing great adhesion to any substrates such as fabrics and polydimethylsiloxane. It exhibits a quite high light absorption of approximately 85% over a wavelength range of 0.2-4.0 µm. Under 1 sun illumination, the solar absorber on various stretchable substrates increased the substrate temperature to approximately 60 °C, irrespective of various mechanical stresses such as bending, stretching, rubbing, and even washing. The TEG with the absorber on the top surface also showed an enhanced output power of 60%, compared with that without the absorber. With an incident solar radiation flux of 38.3 kW/m2, the output power significantly increased to 24 mW/cm2 because of the increase in the surface temperature to 141 °C.

Spin-orbit torque-driven skyrmion dynamics revealed by time-resolved X-ray microscopy.

Magnetic skyrmions are topologically protected spin textures with attractive properties suitable for high-density and low-power spintronic device applications. Much effort has been dedicated to understanding the dynamical behaviours of the magnetic skyrmions. However, experimental observation of the ultrafast dynamics of this chiral magnetic texture in real space, which is the hallmark of its quasiparticle nature, has so far remained elusive. Here, we report nanosecond-dynamics of a 100nm-diameter magnetic skyrmion during a current pulse application, using a time-resolved pump-probe soft X-ray imaging technique. We demonstrate that distinct dynamic excitation states of magnetic skyrmions, triggered by current-induced spin-orbit torques, can be reliably tuned by changing the magnitude of spin-orbit torques. Our findings show that the dynamics of magnetic skyrmions can be controlled by the spin-orbit torque on the nanosecond time scale, which points to exciting opportunities for ultrafast and novel skyrmionic applications in the future.

Integrated arrays of air-dielectric graphene transistors as transparent active-matrix pressure sensors for wide pressure ranges.

Integrated electronic circuitries with pressure sensors have been extensively researched as a key component for emerging electronics applications such as electronic skins and health-monitoring devices. Although existing pressure sensors display high sensitivities, they can only be used for specific purposes due to the narrow range of detectable pressure (under tens of kPa) and the difficulty of forming highly integrated arrays. However, it is essential to develop tactile pressure sensors with a wide pressure range in order to use them for diverse application areas including medical diagnosis, robotics or automotive electronics. Here we report an unconventional approach for fabricating fully integrated active-matrix arrays of pressure-sensitive graphene transistors with air-dielectric layers simply formed by folding two opposing panels. Furthermore, this realizes a wide tactile pressure sensing range from 250 Pa to ∼3 MPa. Additionally, fabrication of pressure sensor arrays and transparent pressure sensors are demonstrated, suggesting their substantial promise as next-generation electronics.

Effect of the solvent used for fabrication of perovskite films by solvent dropping on performance of perovskite light-emitting diodes.

Organic-inorganic hybrid perovskites have emerged as a next-generation candidate for light-emitting device applications due to their excellent optical and electrical properties with narrow band emission compared to organic emitters. The morphological control of perovskite films with full surface coverage and few defect sites is essential for achieving highly efficient perovskite light-emitting diodes (PeLEDs). Here, we obtain a highly uniform perovskite film with a remarkably reduced number of defect sites in a perovskite crystal using chlorobenzene dropping. This effort leads to the enhanced performance of PeLEDs with a CH3NH3PbBr3 film using chlorobenzene dropping with a maximum luminance of 14 460 cd m-2 (at 3.8 V) and a maximum external quantum efficiency (EQE) of 0.71% (at 2.8 V). This research confirms that the role of the solvent in the solvent dropping method is to fabricate a dense and uniform perovskite film and to passivate the defect sites of the perovskite crystal films.

Optical design of ZnO-based antireflective layers for enhanced GaAs solar cell performance.

A series of hierarchical ZnO-based antireflection coatings with different nanostructures (nanowires and nanosheets) is prepared hydrothermally, followed by means of RF sputtering of MgF2 layers for coaxial nanostructures. Structural analysis showed that both ZnO had a highly preferred orientation along the 〈0001〉 direction with a highly crystalline MgF2 shell coated uniformly. However, a small amount of Al was present in nanosheets, originating from Al diffusion from the Al seed layer, resulting in an increase of the optical bandgap. Compared with the nanosheet-based antireflection coatings, the nanowire-based ones exhibited a significantly lower reflectance (∼2%) in ultraviolet and visible light wavelength regions. In particular, they showed perfect light absorption at wavelength less than approximately 400 nm. However, a GaAs single junction solar cell with nanosheet-based antireflection coatings showed the largest enhancement (43.9%) in power conversion efficiency. These results show that the increase of the optical bandgap of the nanosheets by the incorporation of Al atoms allows more photons enter the active region of the solar cell, improving the performance.

Thermoelectric Signal Enhancement by Reconciling the Spin Seebeck and Anomalous Nernst Effects in Ferromagnet/Non-magnet Multilayers.

The utilization of ferromagnetic (FM) materials in thermoelectric devices allows one to have a simpler structure and/or independent control of electric and thermal conductivities, which may further remove obstacles for this technology to be realized. The thermoelectricity in FM/non-magnet (NM) heterostructures using an optical heating source is studied as a function of NM materials and a number of multilayers. It is observed that the overall thermoelectric signal in those structures which is contributed by spin Seebeck effect and anomalous Nernst effect (ANE) is enhanced by a proper selection of NM materials with a spin Hall angle that matches to the sign of the ANE. Moreover, by an increase of the number of multilayer, the thermoelectric voltage is enlarged further and the device resistance is reduced, simultaneously. The experimental observation of the improvement of thermoelectric properties may pave the way for the realization of magnetic-(or spin-) based thermoelectric devices.

Stochastic formation of magnetic vortex structures in asymmetric disks triggered by chaotic dynamics.

The non-trivial spin configuration in a magnetic vortex is a prototype for fundamental studies of nanoscale spin behaviour with potential applications in magnetic information technologies. Arrays of magnetic vortices interfacing with perpendicular thin films have recently been proposed as enabler for skyrmionic structures at room temperature, which has opened exciting perspectives on practical applications of skyrmions. An important milestone for achieving not only such skyrmion materials but also general applications of magnetic vortices is a reliable control of vortex structures. However, controlling magnetic processes is hampered by stochastic behaviour, which is associated with thermal fluctuations in general. Here we show that the dynamics in the initial stages of vortex formation on an ultrafast timescale plays a dominating role for the stochastic behaviour observed at steady state. Our results show that the intrinsic stochastic nature of vortex creation can be controlled by adjusting the interdisk distance in asymmetric disk arrays.

Wave modes of collective vortex gyration in dipolar-coupled-dot-array magnonic crystals.

Lattice vibration modes are collective excitations in periodic arrays of atoms or molecules. These modes determine novel transport properties in solid crystals. Analogously, in periodical arrangements of magnetic vortex-state disks, collective vortex motions have been predicted. Here, we experimentally observe wave modes of collective vortex gyration in one-dimensional (1D) periodic arrays of magnetic disks using time-resolved scanning transmission x-ray microscopy. The observed modes are interpreted based on micromagnetic simulation and numerical calculation of coupled Thiele equations. Dispersion of the modes is found to be strongly affected by both vortex polarization and chirality ordering, as revealed by the explicit analytical form of 1D infinite arrays. A thorough understanding thereof is fundamental both for lattice vibrations and vortex dynamics, which we demonstrate for 1D magnonic crystals. Such magnetic disk arrays with vortex-state ordering, referred to as magnetic metastructure, offer potential implementation into information processing devices.

Resonant amplification of vortex-core oscillations by coherent magnetic-field pulses.

Vortex structures in soft magnetic nanodisks are highly attractive due to their scientific beauty and potential technological applications. Here, we experimentally demonstrated the resonant amplification of vortex oscillations by application of simple coherent field pulses tuned to optimal width and time intervals. In order to investigate vortex excitations on the sub-ns time scale, we employed state-of-the-art time-resolved full-field soft X-ray microscopy of 70 ps temporal and 25 nm lateral resolution. We found that, due to the resonant enhancement of the vortex gyration motion, the signal input power can be significantly reduced to ~ 1 Oe in field strength, while increasing signal gains, by increasing the number of the optimal field pulses. We identified the origin of this behavior as the forced resonant amplification of vortex gyration. This work represents an important milestone towards the potential implementation of vortex oscillations in future magnetic vortex devices.

Logic operations based on magnetic-vortex-state networks.

Logic operations based on coupled magnetic vortices were experimentally demonstrated. We utilized a simple chain structure consisting of three physically separated but dipolar-coupled vortex-state Permalloy disks as well as two electrodes for application of the logical inputs. We directly monitored the vortex gyrations in the middle disk, as the logical output, by time-resolved full-field soft X-ray microscopy measurements. By manipulating the relative polarization configurations of both end disks, two different logic operations are programmable: the XOR operation for the parallel polarization and the OR operation for the antiparallel polarization. This work paves the way for new-type programmable logic gates based on the coupled vortex-gyration dynamics achievable in vortex-state networks. The advantages are as follows: a low-power input signal by means of resonant vortex excitation, low-energy dissipation during signal transportation by selection of low-damping materials, and a simple patterned-array structure.