Takahiro Shimada 교수

Materials fail by the nucleation and propagation of a crack, the critical condition of which is quantitatively described by fracture mechanics that uses an intensity of singular stress field characteristically formed near the crack-tip. However, the continuum assumption basing fracture mechanics obscures the prediction of failure of materials at the nanoscale due to discreteness of atoms. Here, we demonstrate the ultimate dimensional limit of fracture mechanics at the nanoscale, where only a sma

The nature of intrinsic point defects, i.e., energetic, ferroelectric, and electronic properties of vacancies in ferroelectric PbTiO${}_{3}$, is studied using first-principles calculations based on the hybrid Hartree-Fock density functional, which correctly reproduces the band gap and thus provides the accurate defect electronic states. The oxygen vacancies are found to behave as double shallow donors and are thermodynamically stable over a wide range of Fermi levels under oxygen-poor conditions

The crucial role of intrinsic localized modes (ILMs) in the atomic scale as a trigger of defect nucleation was studied using molecular-dynamics simulations for a (5,5) armchair carbon nanotube (CNT) under axial tension. A localized vibration at a pair of neighboring atoms was found to be the ILM, which simultaneously produces an intense concentration of kinetic energy, even in the structurally homogeneous CNT. The excited ILM was gradually amplified by the nonlinearity of C-C interaction. The am

The possible origin of ferromagnetism in PbTiO3 containing vacancies is investigated by performing first-principles calculations. We demonstrate that O and Ti vacancies both induce ferromagnetism but by different mechanisms: the ferromagnetism driven by the O vacancy originates from the spin-polarized eg state of the nearest Ti atom, whereas that driven by the Ti vacancy is due to the half-metallic px state of the nearest O atom. The results presented here provide fundamental insights into the d

A processing element and a structure element of data flow computer SIGMA-1 for scientific computations is now operational. The elements are evaluated for several benchmark programs. For efficient execution of loop constructs, the sticky token mechanism which holds loop invariants is evaluated and exhibits a remarkable effect. From the standpoint that performance of a single processor of a data flow computer must be comparable to that of a Von Neumann computer, comparison of both computers is dis

Ab initio density-functional theory calculations within the local density approximation were conducted to elucidate whether critical thickness for ferroelectricity intrinsically exists in free-standing polydomain ${\text{PbTiO}}_{3}$ ultrathin films, where there was no screening effect of electrodes. The ferroelectric polydomain state was found to be energetically favorable over the paraelectric state even in the thinnest film one unit-cell thick, indicating no intrinsic critical thickness exist

We investigated the atomistic and electronic structure of the 90\ifmmode^\circ\else\textdegree\fi{} domain wall in $\mathrm{Pb}\mathrm{Ti}{\mathrm{O}}_{3}$ and the fundamental mechanism of domain switching induced by shear stress using first-principles density functional theory calculations within the local density approximation. Under strain-free condition, the magnitude of polarization at the center of the domain wall decreased by 20% from that of the bulk, and the direction rotated within the

To achieve a fundamental understanding of the multiferroic behavior and electronic properties of intrinsic vacancies in $\mathrm{BiFe}{\mathrm{O}}_{3}$, here we performed first-principles calculations based on hybrid Hartree-Fock density functional theories, which can accurately describe defect electronic structures. Oxygen vacancies, which behave as deep donors with high concentrations under oxygen-poor conditions, reduce the magnetic moments at neighboring Fe ions in the neutral state, while c

The atomistic and electronic structures of ${\text{PbTiO}}_{3}$ nanowires with characteristic edges consisting of (100) and (010) surfaces and the crucial role of axial tensile strain on ferroelectricity have been investigated by means of ab initio density-functional theory calculations. Ferroelectricity is enhanced at the edge of the PbO-terminated nanowire because the Pb-O covalent bond that predominates ferroelectric distortions is locally strengthened. On the other hand, a considerable suppr

We have developed an efficient interatomic potential for PbTiO3 in the framework of the shell model by fitting its parameters to reproduce both the mechanical and ferroelectric properties derived from ab initio density functional theory calculations. The optimized potential successfully yields the crystal structures, elastic properties and phonon dispersion curves, whereas the spontaneous polarization and effective charges are slightly underestimated. It reproduces well characteristic ferroelect

We simulate from first principles the energetic, structural, and electronic properties of ferroelectric (FE) ultrathin PbTiO3 nanotubes. The nanotube possesses spontaneous polarization despite their sidewalls being thinner than the critical thickness at which the thin films lose ferroelectricity; this indicates the absence of an intrinsic critical size for ferroelectricity. The ground state of the nanotube is not purely FE since it primarily involves antiferrodistortive (AFD) rotations of oxygen

Tailoring materials to obtain unique, or significantly enhanced material properties through rationally designed structures rather than chemical constituents is principle of metamaterial concept, which leads to the realization of remarkable optical and mechanical properties. Inspired by the recent progress in electromagnetic and mechanical metamaterials, here we introduce the concept of ferroelectric nano-metamaterials, and demonstrate through an experiment in silico with hierarchical nanostructu

Ultrathin multiferroics with coupled ferroelectric and ferromagnetic order parameters hold promise for novel technological paradigms, such as extremely thin magnetoelectric memories. However, these ferroic orders and their functions inevitably disappear below a fundamental size limit of several nanometers. Herein, we propose a novel design strategy for nanoscale multiferroics smaller than the critical size limit by engineering the dislocations in nonmagnetic ferroelectrics, even though these lat