Tokyo Institute of Technology

About: Tokyo Institute of Technology is a education organization based out in Tokyo, Tôkyô, Japan. It is known for research contribution in the topics: Catalysis & Thin film. The organization has 46775 authors who have published 101656 publications receiving 2357893 citations. The organization is also known as: Tokyo Tech & Tokodai.

PWM control and experiment of modular multilevel converters

Abstract: A modular multilevel converter (MMC) is one of the next-generation multilevel PWM converters intended for high- or medium-voltage power conversion without transformers. The MMC consists of cascade connection of multiple bidirectional PWM chopper-cells and floating dc capacitors per leg, thus requiring voltage-balancing control of their chopper-cells. However, no paper has been discussed explicitly on voltage-balancing control with theoretical and experimental verifications. This paper deals with two types of modular multilevel PWM converters with focus on their circuit configurations and voltage-balancing control. Combination of averaging and balancing controls enables the MMCs to achieve voltage balancing without any external circuit. The viability of the MMCs as well as the effectiveness of the PWM control method is confirmed by simulation and experiment.

Gold nanobridge stabilized by surface structure

Abstract: We formed gold nanowires using electron-beam irradiation in an ultrahigh vacuum electron microscope. The dimensions of these nanowires were 0.8--3 nm in thickness and 5--10 nm in length. The nanowires showed lasting stability. We propose a structure model for a nanowire with a thickness of 2 nm. The model is represented by a hexagonal prism; the surface layer of the prism has a hexagonal-close-packed (hcp) lattice, and the core of the prism has a face-centered-cubic structure. The nanowire is considered to be stabilized by the hcp lattices of the surfaces.

Time-reversal symmetry breaking and spontaneous Hall effect without magnetic dipole order

Abstract: Spin liquids are magnetically frustrated systems, in which spins are prevented from ordering or freezing, owing to quantum or thermal fluctuations among degenerate states induced by the frustration. Chiral spin liquids are a hypothetical class of spin liquids in which the time-reversal symmetry is macroscopically broken in the absence of an applied magnetic field or any magnetic dipole long-range order. Even though such chiral spin-liquid states were proposed more than two decades ago, an experimental realization and observation of such states has remained a challenge. One of the characteristic order parameters in such systems is a macroscopic average of the scalar spin chirality, a solid angle subtended by three nearby spins. In previous experimental reports, however, the spin chirality was only parasitic to the non-coplanar spin structure associated with a magnetic dipole long-range order or induced by the applied magnetic field, and thus the chiral spin-liquid state has never been found. Here, we report empirical evidence that the time-reversal symmetry can be broken spontaneously on a macroscopic scale in the absence of magnetic dipole long-range order. In particular, we employ the anomalous Hall effect to directly probe the broken time-reversal symmetry for the metallic frustrated magnet Pr(2)Ir(2)O(7). An onset of the Hall effect is observed at zero field in the absence of uniform magnetization, within the experimental accuracy, suggesting an emergence of a chiral spin liquid. The origin of this spontaneous Hall effect is ascribed to chiral spin textures, which are inferred from the magnetic measurements indicating the spin ice-rule formation.

Energetics and electronic structures of encapsulated C60 in a carbon nanotube.

Abstract: We report total-energy electronic structure calculations that provide energetics of encapsulation of ${\mathrm{C}}_{60}$ in the carbon nanotube and electronic structures of the resulting carbon peapods. We find that the encapsulating process is exothermic for the $(10,10)$ nanotube, whereas the processes are endothermic for the $(8,8)$ and $(9,9)$ nanotubes, indicative that the minimum radius of the nanotube for the encapsulation is 6.4 \AA{}. We also find that the ${\mathrm{C}}_{60}@(10,10)$ is a metal with multicarriers each of which distributes either along the nanotube or on the ${\mathrm{C}}_{60}$ chain. This unusual feature is due to the nearly free electron state that is inherent to hierarchical solids with sufficient space inside.