Institute for Materials Research, Tohoku University

The inverse process of the spin-Hall effect (ISHE), conversion of a spin current into an electric current, was observed at room temperature. A pure spin current was injected into a Pt thin film using spin pumping, and it was observed to generate electromotive force transverse to the spin current. By changing the spin-current polarization direction, the magnitude of this electromotive force varies critically, consistent with the prediction of ISHE.

Room-temperature superconductivity has been a long-held dream and an area of intensive research. Recent experimental findings of superconductivity at 200 K in highly compressed hydrogen (H) sulfides have demonstrated the potential for achieving room-temperature superconductivity in compressed H-rich materials. We report first-principles structure searches for stable H-rich clathrate structures in rare earth hydrides at high pressures. The peculiarity of these structures lies in the emergence of unusual H cages with stoichiometries H_{24}, H_{29}, and H_{32}, in which H atoms are weakly covalently bonded to one another, with rare earth atoms occupying the centers of the cages. We have found that high-temperature superconductivity is closely associated with H clathrate structures, with large H-derived electronic densities of states at the Fermi level and strong electron-phonon coupling related to the stretching and rocking motions of H atoms within the cages. Strikingly, a yttrium (Y) H_{32} clathrate structure of stoichiometry YH_{10} is predicted to be a potential room-temperature superconductor with an estimated T_{c} of up to 303 K at 400 GPa, as derived by direct solution of the Eliashberg equation.

A thermodynamically stable quasicrystalline single phase with an icosahedral structure was found to be formed at an atomic composition of Al 65 Cu 20 Fe 15 in a fully annealed state as well as in a conventionally solidified state. The stable quasicrystal consisted of large grains with an average size of 0.2 mm after annealing for 48 h at 1118 K (0.98 T m ).

Recent efforts in finding materials suitable for storing hydrogen with large gravimetric density have focused attention on carbon-based nanostructures. Unfortunately, pure carbon nanotubes and fullerenes are unsuitable as hydrogen storage materials because of the weak bonding of the hydrogen molecules to the carbon frame. It has been shown very recently that coating of carbon nanostructures with isolated transition metal atoms such as Sc and Ti can increase the binding energy of hydrogen and lead to high storage capacity (up to 8 wt % hydrogen, which is 1.6 times the U.S. Department of Energy target set for 2005). This prediction has led to a great deal of excitement in the fuel cell community [see The Fuel Cell Review, http://fcr.iop.org/articles/features/2/7/4]. However, this prediction depends on the assumption that the metal atoms coated on the fullerene surface will remain isolated. Using first-principles calculations based on density functional theory, we show that Ti atoms would prefer to cluster on the C60 surface, which can significantly alter the nature of hydrogen bonding, thus affecting not only the amount of stored hydrogen but also their thermodynamics and kinetics.

We report experimental characterization of shear transformation zones (STZs) for plastic flow of bulk metallic glasses (BMGs) based on a newly developed cooperative shearing model [Johnson WL, Samwer K (2005) A universal criterion for plastic yielding of metallic glasses with a (T/T(g))(2/3) temperature dependence. Phys Rev Lett 95: 195501]. The good agreement between experimental measurements and theoretical predictions in the STZ volumes provides compelling evidence that the plastic flow of metallic glasses occurs through cooperative shearing of unstable STZs activated by shear stresses. Moreover, the ductility of BMGs was found to intrinsically correlate with their STZ volumes. The experiments presented herein pave a way to gain a quantitative insight into the atomic-scale mechanisms of BMG mechanical behavior.

The widespread enthusiasm for research on bulk metallic glasses is driven by both a fundamental interest in the structure and properties of disordered materials and their unique promise for structural and functional applications. Unlike the case for crystalline materials, the disordered and nonequilibrium nature of metallic glasses causes their underlying deformation mechanisms to be poorly known. A definite correlation between mechanical behavior and the atomic/electronic structures of metallic glasses has not been established. In this article, I focus on the micromechanisms of mechanical behavior of metallic glasses and present a brief overview of the current understanding of their strength, ductility, and plasticity at the microscopic and atomic scales. The important factors that control the mechanical behavior of metallic glasses are outlined on the basis of recent theoretical and experimental findings. The outstanding issues highlighted in this review are expected to be important for future research on the mechanical properties of metallic glasses.

Nanoporous bimetallic (Co_1−xFe_x)₂P phosphides with tuneable Co/Fe ratios exhibit versatile catalytic activities for highly efficient electrochemical water splitting.

Engineering the water dissociation sites of MoS₂ nanosheets can efficiently enhance the electrocatalytic hydrogen evolution under alkaline conditions.

The inverse spin-Hall effect (ISHE) induced by the spin pumping has been investigated systematically in simple ferromagnetic/paramagnetic bilayer systems. The spin pumping driven by ferromagnetic resonance injects a spin current into the paramagnetic layer, which gives rise to an electromotive force transverse to the spin current using the ISHE in the paramagnetic layer. In a Ni81Fe19/Pt film, we found an electromotive force perpendicular to the applied magnetic field at the ferromagnetic resonance condition. The spectral shape of the electromotive force is well reproduced using a simple Lorentz function, indicating that the electromotive force is due to the ISHE induced by the spin pumping; extrinsic magnetogalvanic effects are eliminated in this measurement. The electromotive force varies systematically by changing the microwave power, magnetic-field angle, and film size, being consistent with the prediction based on the Landau–Lifshitz–Gilbert equation combined with the models of the ISHE and spin pumping. The electromotive force was observed also in a Pt/Y3Fe4GaO12 film, in which the metallic Ni81Fe19 layer is replaced by an insulating Y3Fe4GaO12 layer, supporting that the spin-pumping-induced ISHE is responsible for the observed electromotive force.

This paper presents new developments in inorganic scintillators widely used for radiation detection. It addresses major emerging research topics outlining current needs for applications and material sciences issues with the overall aim to provide an up-to-date picture of the field. While the traditional forms of scintillators have been crystals and ceramics, new research on films, nanoparticles, and microstructured materials is discussed as these material forms can bring new functionality and therefore find applications in radiation detection. The last part of the contribution reports on the very recent evolutions of the most advanced theories, methods, and analyses to describe the scintillation mechanisms.

Atomically dispersed Ni–N_x species anchored porous carbon matrix with embedded Ni nanoparticles was synthesized for highly efficient hydrogen evolution in alkaline conditions.

As an inorganic cousin of graphene, MoS2 monolayer has attracted considerable attention. However, a full understanding of its structure and stability is still lacking due to its dependence on experimental synthesis conditions. Using first-principle calculations combined with Boltzmann transport equation, we have extensively studied the geometry, energetics, electronic structure, optical absorption, and carrier mobility of various phases of MoS2. We have not only identified the stable phases of a MoS2 monolayer, but also clarified the specific conditions under which different phases are formed. The possible pathways for transitions among different phases are also discussed.

The electrical conductivity of lithium borohydride (LiBH4) measured by the ac complex impedance method jumped by three orders of magnitude due to structural transition from orthorhombic to hexagonal at approximately 390K. The hexagonal phase exhibited a high electrical conductivity of the order of 10−3Scm−1. Furthermore, the conductivity calculated from the Nernst-Einstein equation using the correlation time obtained from Li7 nuclear magnetic resonance was in good agreement with the measured electrical conductivity. It was concluded that the electrical conductivity in the hexagonal phase is due to the Li superionic conduction.

We describe the fabrication, characterization, and applications of ultrathin, free-standing mesoporous metal membranes uniformly decorated with catalytically active nanoparticles. Platinum-plated nanoporous gold leaf (Pt-NPG) made by confining a plating reaction to occur within the pores of dealloyed silver/gold leaf is 100 nm thick and contains an extremely high, uniform dispersion of 3 nm diameter catalytic particles. This nanostructured composite holds promise as a prototypical member of a new class of fuel cell electrodes, showing good electrocatalytic performance at low platinum loading (less than 0.05 mg cm-2), while also maintaining long-term stability against coarsening and aggregation of catalytic nanoparticles.

Abstract Direct solar steam generation (DSSG) offers a promising, sustainable, and environmentally friendly solution to the energy and water crisis. In the past decades, DSSG has gained tremendous attention due to its potential applications for clean water production, desalination, wastewater treatment, and electric energy harvesting. Even though the solar–thermal conversion efficiency has approached 100% under 1 sun illumination (1 kW m −2 ) using various photothermal materials and systems, the optimization of the materials and system structure remains unclear because of the lack of evaluation methods in unity for the output efficiency. In this review, a few key concerns about different dimensional materials and systems that determine the characteristics of DSSG are explored. Quantitative analysis, including calculations and methods for the solar–thermal conversion efficiency, evaporation rate, and energy loss, is employed to evaluate the materials and systems from the point of view of ultimate utilization. This article focuses on the relationship between the system dimension and energy efficiency and notes opportunities for future system design and commercialization of DSSG.

The electric field-induced ∼180° magnetization reversal is realized for a sputtered CoFeB/MgO-based magnetic tunnel junction with perpendicular magnetic easy axis in a static external magnetic field. Application of bias voltage with nanoseconds duration results in a temporal change of magnetic easy axis in the free layer CoFeB to in-plane, which induces precessional motion of magnetization in the free layer. The magnetization reversal takes place when the bias voltage pulse duration is adjusted to a half period of the precession. We show that the back and forth magnetization reversal can be observed by using successive application of half-period voltage pulses.

A low-temperature dealloying technique was developed to tailor the characteristic length scale of nanoporous gold for advanced functional applications. By systematically investigating the kinetics of nanopore formation during free corrosion, the authors experimentally demonstrated that the dealloying process is controlled by the diffusion of gold atoms at alloy/electrolyte interfaces, which strongly relies on the reaction temperatures. Low dealloying temperatures significantly reduce the interfacial diffusivity of gold atoms and result in an ultrafine nanoporous structure that has been proved to be useful with improved chemical and physical properties.

We show that an annular detector placed within the bright field cone in scanning transmission electron microscopy allows direct imaging of light elements in crystals. In contrast to common high angle annular dark field imaging, both light and heavy atom columns are visible simultaneously. In contrast to common bright field imaging, the images are directly and robustly interpretable over a large range of thicknesses. We demonstrate this through systematic simulations and present a simple physical model to obtain some insight into the scattering dynamics.

The authors report the surface enhanced Raman scattering (SERS) of nanoporous gold with nanopore sizes ranging from 5to700nm. Their comprehensive investigations prove that the strongest SERS enhancement of nanoporous gold takes place from the samples with an ultrafine nanopore size of ∼5–10nm. Both the enhancement factor and detection limit of the ultrafine nanoporous substrate are one to two orders of magnitude better than those of coarsened nonporous gold with smooth surfaces. Moreover, careful microstructure characterization reveals that the anomalous SERS enhancement of the annealed nanoporous gold arises from roughsurfaces with characteristic surface irregularities.

Quantum anomalous Hall effect (QAHE), which generates dissipation-less edge current without external magnetic field, is observed in magnetic-ion doped topological insulators (TIs) such as Cr- and V-doped (Bi,Sb)2Te3. The QAHE emerges when the Fermi level is inside the magnetically induced gap around the original Dirac point of the TI surface state. Although the size of gap is reported to be about 50 meV, the observable temperature of QAHE has been limited below 300 mK. We attempt magnetic-Cr modulation doping into topological insulator (Bi,Sb)2Te3 films to increase the observable temperature of QAHE. By introducing the rich-Cr-doped thin (1 nm) layers at the vicinity of both the surfaces based on non-Cr-doped (Bi,Sb)2Te3 films, we have succeeded in observing the QAHE up to 2 K. The improvement in the observable temperature achieved by this modulation-doping appears to be originating from the suppression of the disorder in the surface state interacting with the rich magnetic moments. Such a superlattice designing of the stabilized QAHE may pave a way to dissipation-less electronics based on the higher-temperature and zero magnetic-field quantum conduction.