Takahiro Shimada

• Professor
• Professor (Full) at Kyoto University

Atomic-scale polar topologies such as skyrmions offer important potential as technological paradigms for future electronic devices. Despite recent advances in the exploration of topological domains in complicated perovskite oxide superlattices, these exotic ferroic orders are unavoidably disrupted at the atomic scale due to intrinsic size effects....

We elucidate the transport properties and electronic structures of distorted rutile-type WO2. Electrical resistivity and Hall effect measurements of high-quality single crystals revealed the transport property characteristics of topological materials; these characteristics included an extremely large magnetoresistance of 13 200% (2 K and 9 T) and a...

Realization of ultrasmall scale electromechanical materials has been promising for advanced functional devices. Recently, single-atom devices have been proposed as the ultimate miniaturization of functional devices beyond the nanoscale; however, achieving an atomic-scale local electromechanical response is still challenging due to physical size lim...

Giant magnetic transverse thermoelectric effect, anomalous Nernst effect (ANE), was theoretically and experimentally observed in 3d-transition metal compounds. The intrinsic components of ANE can be described from the electronic structure based on the Berry phase concept. The topological electronic structure, such as the Weyl node and nodal lines,...

Understanding the nature of brittle failure in ferroelectric materials is essential, but difficult due to the complex interaction between mechanical and electrical concentrated fields near the crack tip. In this work, an extended phase-field model incorporating multiple order parameters is constructed to analyze the coupled evolution of fracture an...

Shear localization is the predominant form of failure of metals at high strain rates where amorphization often occurs. However, the mechanism and role of amorphization in shear localization are not yet clearly understood. In this work, a series of shear deformations of nanocrystals with different grain sizes at high strain rates are investigated us...

Polar topological structures such as skyrmions and merons have become an emerging research field due to their rich functionalities and promising applications in information storage. Up to now, the obtained polar topological structures are restricted to a few limited ferroelectrics with complex heterostructures, limiting their large-scale practical...

Strain engineering is a crucial approach in the engineering field to optimize various physical properties of materials by applying mechanical strain loading. However, it is extremely challenging to find out the best conditions of strain with unprecedented physical properties in the vast strain space consisting of six components. Here, we developed...

Ultimately small multiferroics with coupled ferroelectric and ferromagnetic order parameters have drawn considerable attention for their tremendous technological potential. Nevertheless, these ferroic orders inevitably disappear below the critical size of...

Atomic‐scale polar topological configurations, such as skyrmions and merons, have garnered enormous interest due to their rich emergent physical phenomena and promising applications in next‐generation electronics. Despite recent progress in the exploration of 2D ferroelectrics, isolated polar topological structures in 2D lattices have not yet been...

Van der Waals (vdW)-layered materials, such as graphite, exhibit unique mechanical properties owing to their structural and mechanical anisotropies. This study reports the development of a mechanical model that reproduces the characteristics of the nonlinear and reversible bending deformation of vdW-layered materials, while taking into account the...

Brittle fracture of a covalent material is ultimately governed by the strength of the electronic bonds. Recently, attempts have been made to alter the mechanical properties including fracture strength by excess electron/hole doping. However, the underlying mechanics/mechanism of how these doped electrons/holes interact with the bond and changes its...

The mechanical response of ferroelectric (piezoelectric) ceramics under high stress conditions has a crucial role from the perspective of engineering applications such as advanced actuator systems. In this study, we perform first-principles calculations to clarify the deformation behavior and (ideal) tensile strength for typical ferroelectric ceram...

Magnetic materials exhibit various magnetic orders and are used in many magnetic and mechanical devices. In order to control and design magnetic ordering, the stability of the magnetic phase with mechanical strain has been reported in previous studies. In this study, based on the first-principles calculation, we investigate the stain dependence of...

Topological objects with skyrmionic textures in ferroelectrics, i.e., polar skyrmions, are promising technological paradigms in next-generation electronic devices. While breakthrough discoveries of stable polar skyrmions approximately ten nanometers in size have been very recently witnessed in complex systems, such a nontrivial topological order in...

Microscopic mechanics of thermal dissipation induced by fast-moving edge dislocations are crucial for a deeper understanding of the nature of plastic deformation. Herein, we study the thermal dissipation induced by a fast-moving edge dislocation and discuss the effect of non-Schmid stress on the thermal dissipation using molecular dynamics simulati...

Low-dimensional multiferroic metals characterized by the simultaneous coexistence of ferroelectricity, conductivity, and magnetism hold tremendous potential for scientific and technological endeavors. However, the mutually exclusive mechanisms among these properties impede the discovery of multifunctional conducting multiferroics, especially at the...

This study demonstrates that bond strength can be enhanced by injecting excess electrons or holes into a material by electron beam irradiation. To determine the effect of excess electrons/holes on the interatomic bond strength, fracture toughness tests were performed on single-crystal Si micropillars under various electron-beam irradiation conditio...

Thermoelectric conversion using the anomalous Nernst effect (ANE) has excellent potential for application in energy harvesting technology. Compared to the typical thermoelectric effect known as the Seebeck effect, the thermoelectric figure of merit in ANE is about ten times smaller; material search with a large ANE response is a critical challenge...

In this study, we demonstrated that van der Waals-layered MoTe2, where two-dimensional atomic layers are closely laminated by weak interactions, has higher fracture toughness in out-of-plane cracks than that in in-plane cracks owing to structural anisotropy. In situ electron microscopy fracture toughness tests were performed on micro-sized specimen...

Understanding the dynamic behaviors of magnetic skyrmions has great potential in the application of spintronics. This is especially the case for the interaction between two skyrmions. In the literature, the attractive and repulsive skyrmion-skyrmion interactions are demonstrated as strongly related to the distance of two skyrmions and external magn...

Effective modeling of magnetization dynamics is key to understanding the nature of exotic magnetic structures and behaviors such as magnetic skyrmions and spin waves. Although the modeling of magnitude variation of magnetizations is crucial for magnets at finite temperatures (especially near the Curie temperature with the precursor effect), it is r...

Van der Waals (vdW)-stacked materials exhibit unique mechanical properties owing to their strong in-plane atomic bonds and weak interlayer vdW interactions. In this study, we experimentally demonstrate that submicron-sized graphite cantilevers can accommodate large out-of-plane deformations without fracturing and undergo self-restoration upon unloa...

The J-integral, which is crucial to the nonlinear fracture analysis within the framework of continuum fracture mechanics, fails at the atomic scale due to the atomic discreteness emerging with decreasing size. In this paper, a novel atomic J-integral calculation method is proposed for atomically sharp cracks by developing a new integral scheme to d...

Engineering the kinetic motion of magnetic skyrmions shows great potential in spintronics. Particularly, as a natural property, temperature plays a significant role in the dynamics of skyrmions. For instance, the nonlinear and the rectilinear motions of skyrmions driven by spin-transfer torque and local energy imbalance in temperature gradients, re...

Large piezoelectric effect in nonlinearly graded lead‐free ferroelectric thin films In article number 2100370, using extended phase field simulations, Le Van Lich and co‐workers demonstrate that a large piezoelectric effect can be obtained in nonlinearly graded lead‐free ferroelectric thin film. The enhancement of piezoelectric property originates...

Many experiments have shown that plastic deformation of metals generates heat, leading to temperature rise of the material. However, little is known about the underlying factors that govern the heat generation, especially in polycrystalline metals involving high-rate plastic deformation. In this work, the work-to-heat conversion during high-rate pl...

Two-dimensional (2D) materials exhibit a high strength and flexibility along with unique electrical-mechanical multiphysics properties. In this study, we experimentally demonstrated the electromechanical response of a multilayer 2D material, 2H-phase MoS 2 , by using a piezoresponse force microscope. In particular, the dominant physical quantity of...

The conversion of work into heat is one of the most significant characteristics of plastic deformation, especially in a high strain rate regime. The quantitative characterization of the fraction of plastic work converted into heat plays a crucial role in understanding the deformation process and mechanism and establishing an accurate and reasonable...

The optimal design of shape memory alloys (SMAs) with specific properties is crucial for the innovative application in advanced technologies. Herein, inspired by the recently proposed design concept of concentration modulation, we explore martensitic transformation (MT) in and design the mechanical properties of Ti-Nb nanocomposites by combining hi...

A phase field model for nonlinearly graded ferroelectric thin films is developed based on the Ginzburg–Landau theory. The developed phase field model is validated by comparing simulated results and available experiment data. Via phase field simulations, effects of gradient index on polarization field and electromechanical response are systematicall...

Discovery of non-trivial topological structures in condensed matters holds promise in novel technological paradigms. In contrast to ferromagnetics, where a variety of topological structures such as vortex, meron, and skyrmion have been discovered, only few topological structures can exist in ferroelectrics due to the lack of non-collinear interacti...

Engineering the dynamic motion of the bubble-like magnetic skyrmion exhibits great potential in the application of spintronics. Especially, as a natural property, temperature plays an important role in the skyrmion motion. For example, the non-linear motion and the random Brownian motion of skyrmions driven by temperature gradient induced spin-tran...

Material strength depends on the interatomic bond strength of a material; therefore, desired mechanical properties of the material can be designed if the bond strength can be altered. In this study, we experimentally demonstrated that the shear strength in ZnO could be continuously and reversibly altered by controlling the hole state using an elect...

Background Multilayered thermoelectric material bismuth telluride (Bi2Te3) is widely used in engineering owing to its exceptional thermoelectric performance at room temperature. However, Bi2Te3 is prone to cracks, voids, or other defects due to its multilayered structure, leading to decreased device lifetime and reliability.Objective This paper aim...

The existence of a notch in a single-atom-thick graphene nanoribbon brings about significant nonlinearity in deformation within the extremely confined stress concentration field of only a few nanometers near the notch root. However, whether the continuum-based mechanics theory is still valid for fracture due to such strongly-nonlinear and extremely...

Conventional nanoscale logic gates involve several critical challenges, such as the appearance of leakage currents, and so new approaches to logic calculation devices have been exploited. However, although polarization switching in ferroelectrics has certain advantages in this regard, including nonvolatility, these materials are not leading candida...

Stored energy plays a crucial role in dynamic recovery, recrystallization, and formation of adiabatic shear bands in metals and alloys. Here, we systematically investigate the energy storage and heat dissipation in copper single crystals with two typical orientations under shock compression and reveal their microscopic mechanisms using molecular dy...

Shock compression behaviors of copper single crystals at room and elevated temperatures are investigated by molecular dynamics simulations. The results show that when increasing the initial temperature, the kinetic energy part of stress increases while the potential energy part of stress decreases; the shear strain and the density of defects decrea...

Acquiring large electrocaloric effect close to room temperature is a prerequisite for incorporating ferroelectrics into advanced solid state cooling devices.. In this study, we report, based on phase field simulations, an approach to achieve a high electrocaloric effect near room temperature by controlling the compositional gradient along the thick...

Dynamical topology in ferroelectrics such as polarization rotation begins to attract interest because it brings new opportunities to generate properties that the static states cannot achieve. However, polarization behavior cannot be complicated under the spatially homogeneous electric fields that ferroelectrics are typically subjected to. Here, we...

Ferroic systems under considerable geometrical restrictions at nanoscale have successfully introduced novel phases such as multiferroic and topological phases. However, ferroic orders completely disappear below the critical size limit of several nanometers and the geometry cannot be relied upon to produce a variety of phases. Here, via first-princi...

The advent of modern thin-film deposition approaches has ushered in a new era of designed materials with well-controlled composition distributions, e.g., compositionally graded ferroelectric (CGFE) thin films, and thereby, it is readily accessible CGFE/FM (ferromagnetic) multilayer thin films. Being recognized this emerging class of materials, in t...

Although nanoscale multiferroics with coupled ferroelectricity and magnetism are of great technological and fundamental importance, the ferroic orders are inevitably destroyed in atomic-scale dimensions due to the intrinsic size effect or depolarization field. Here, we propose a strategy for atomic-size multiferroics by engineering the otherwise de...

We study laser shock peenings of copper single crystals using molecular dynamics simulations based on the ultrafast thermomechanical coupling model. We reveal the effects of ablation coating and confinement layer on response induced by laser shock peening and discuss the potential mechanisms. The results show that the confinement layer significantl...

Single crystals subjected to shock compression exhibit responses with distinct two-wave structures for certain crystal orientations. However, little is known to date regarding how the shock response depends on crystal orientation, and especially why the two-wave structure depends on the crystal orientation. In this work, molecular dynamics simulati...

Correction for ‘Two-dimensional polar metal of a PbTe monolayer by electrostatic doping’ by Tao Xu et al. , Nanoscale Horiz. , 2020, 5 , 1400–1406, DOI: 10.1039/D0NH00188K.

Polar metals characterized by the simultaneous coexistence of ferroelectric distortions and metallicity have attracted tremendous attention. Developing such materials at low dimensions remains challenging since both conducting electrons and reduced dimensions are supposed to quench ferroelectricity. Here, based on first-principles calculations, we...

Owing to a finite and single-atom-thick two-dimensional structure, graphene nanostructures such as nanoribbons possess outstanding physical properties and unique size-dependent characteristics due to nanoscale defects, especially for mechanical properties. Graphene nanostructures characteristically exhibit strong nonlinearity in deformation and the...

Controlling chemical reactions by mechanical loading is an interesting but challenging task. In our research, we investigated the dependence of deformation modes on an exothermic chemical reaction in Ti/Si multilayered nanofilms. Compression, shear, and compression-shear mixed modes in the stacking direction were evaluated as basic deformation mode...

The aim of this study is to experimentally seek a unification of brittle fracture by an atomic-level criterion. In situ fracture experiments are carried out in transmission electron microscope by using notched nano-cantilever specimens of single-crystal silicon. On the basis of atomic fracture mechanics, the critical atomic energy release rate GAFM...

Magnetic skyrmions with a topological particle nature are considered as potential information carriers for future spintronics memory and logic devices. The stabilization of magnetic skyrmions at zero magnetic field in nanostructured components is a prerequisite for incorporating them into advanced nonvolatile memory devices. Here, using a real-spac...

We investigate the critical size of ferroelectricity in monolayer tin telluride (SnTe) nanoribbon by using ab initio (first-principles) density functional theory calculations. The edge of SnTe nanoribbon tends to suppress the ferroelectricity and the edge effect ranges within the 4 unit cells (~1.8 nm) from the edge. In nanoribbons, ferroelectric p...

Ultrasmall ferroelectrics with nontrivial topological field textures such as polar vortices, skyrmions, and merons hold promise in technological paradigms. Such nontrivial ferroic orders and their functionalities, however, inevitably disappear below a critical size of several nanometers. Here, we propose a strategy to overcome this limitation and d...

This paper aims to propose a unified criterion for brittle fracture of general (blunt) notches and ideal (sharp) cracks. A new fracture parameter, i.e., atomic energy release rate (ERR), for which the atomic discrete nature is fully taken into account, is defined based on the concept of atomic fracture mechanics (AFM). The results show that the pro...

In ferroelectrics, domain walls have significant effects on ferroic properties. The mechanical behavior of domain walls has been investigated because of its scientific and technological importance. Numerous experiments and simulations have examined domain walls and their motion under mechanical strain. However, since nano- and micromechanical testi...

Beyond a ferroelectric critical thickness of several nanometers existed in conventional ferroelectric perovskite oxides, ferroelectricity in ultimately thin dimensions was recently discovered in SnTe monolayers. This discovery suggests the possibility that SnTe can sustain ferroelectricity during further low-dimensional miniaturization. Here, we in...

Polar metals characterized by the simultaneous coexistence of ferroelectric distortions and metallicity have attracted tremendous attention. Developing such materials at low dimensions remains challenging since both conducting electrons and reduced dimensions are supposed to quench ferroelectricity. Here, based on first-principles calculations, we...

Real-time control of material properties is challenging yet promising for material design and technological applications. Here, we experimentally demonstrated that the strength of a ZnO single crystal could be altered and restored by the rapid response to the switch-on/off of an electron beam. The effect of electron-beam irradiation on the strength...

The effect of oxygen vacancy on the ferroelectricity of the 90° domain wall structure in PbTiO3 has been investigated by ab initio (first-principles) density functional theory calculations. Based on the position of oxygen vacancies with respect to the 90° domain wall, the oxygen vacancies are classified into two types, i.e., vacancy on the PbO plan...

The control of topological defects in ferroelectrics, in particular by homogeneous electric field, has emerged as an active research direction. A polarization vortex, which is a fundamental topological defect formed in ferroelectric nanodots, has recently been demonstrated to be switchable by homogeneous electric field through the control of built-...

Ferroelectric materials exhibit novel topological polarization configurations due to geometric confinements originated from material shapes and interfaces in nanoscale. In this study, we demonstrate that those nontrivial topological ferroelectric nanostructures can be tailored in paraelectric nanoporous materials by mechanical loads. That is, in na...

A dislocation induces ferroelectricity around it in incipient ferroelectric SrTiO3 due to some reasons such as the electro-mechanical coupling and is a one-dimensional ferroelectric nanostructure. Furthermore, this microstructure is arrayed periodically in the material and dislocation structures such as a dislocation wall are formed. Due to these f...

A reversal of polarization vortexlike domains in ferroelectric nanostructures plays important roles for next generations of electronic nanodevices. However, a direct switching of the polarization vortexlike domains in ferroelectrics is a nontrivial task since the toroidal moment is conjugated to a curled electric field rather than a homogeneous one...

Ferroelectric behavior in mechanically strained SrTiO3 nanoporous is investigated using phase-field models. We find, in the paraelectric nanoporous, periodically arrayed ferroelectric nano-region is formed due to strain concentration, namely, polarization vortices emerge around each pore. Each ferroelectric region expands and connects each other wi...

For decades, silicon (Si) has been widely used for the mass production of microelectronic circuits. Recently, as the thickness has been reduced to the nanometer scale, its application has expanded to various fields, including flexible and transparent 2D semiconductors. For the reliable and reproducible operation of such large flexible and transpare...

Thermoelectric (TE) devices under thermal cycling are prone to interface defects. Based on the experimental observation, this paper proposes a generalized model to analyze the interface damage growth and predict the lifetime of thermoelectric (TE) devices under thermal cycling. The closed-form solutions of the temperature profile, interface stresse...

In contrast to the great success at the macroscale, fracture mechanics theory fails to describe the fracture at a critical size of several nanometers due to the emerging effect of atomic discreteness with decreasing material size. Here, we propose a novel formulation for brittle fracture, from macro- to even atomic scales, based on extended strain...

Deterministic switching of polarization vortices in ferroelectrics by a uniaxial mechanical load is challenging. Here, we demonstrate, by using phase field simulations and ferroelectric instability analyses , a deterministic switching of polarization vortices in a compositionally graded ferroelectric (CGFE) nanoplate under a compressive stress. The...

The coexistence of ferroelectricity, conductivity, and magnetism in a single-phase material has attracted considerable attention due to fundamental interest and tremendous technological potential. However, their mutually exclusive mechanisms hinder the discovery of multifunctional conducting multiferroics. Here, we propose a new material design app...

The present contribution reviews some recent results on the experimental characterisation of the nanoscale fracture toughness of silicon by using pre-cracked specimens and alternatively the theory of critical distances (TCD). Later, the results are discussed to provide the ultimate dimensional limit of the continuum fracture mechanics at the nanosc...

Spontaneous polarization forms a vortex form in ferroelectric nanostructures, which develops a new order parameter of toroidal moment. The toroidal moment has been believed to be applied to ultralarge capacity memories. However, ferroelectric polarization disappears when the material size is less than a few nanometers, and it is thus difficult to m...