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Author

Shigeo Mori

Other affiliations: Aoyama Gakuin University, Waseda University, Alcatel-Lucent  ...read more
Bio: Shigeo Mori is an academic researcher from Osaka Prefecture University. The author has contributed to research in topics: Ferroelectricity & Charge ordering. The author has an hindex of 34, co-authored 288 publications receiving 7242 citations. Previous affiliations of Shigeo Mori include Aoyama Gakuin University & Waseda University.


Papers
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Journal ArticleDOI
10 Jun 1999-Nature
TL;DR: In this article, it was shown that the magnetoresistive response increases dramatically when the Curie temperature (T C) is reduced, and that the massive magnetoresistance in low-T C systems can be explained by percolative transport through the ferromagnetic domains; this depends sensitively on the relative spin orientation of adjacent magnetoric domains which can be controlled by applied magnetic fields.
Abstract: Colossal magnetoresistance1—an unusually large change of resistivity observed in certain materials following application of magnetic field—has been extensively researched in ferromagnetic perovskite manganites. But it remains unclear why the magnetoresistive response increases dramatically when the Curie temperature (T C) is reduced. In these materials, T C varies sensitively with changing chemical pressure; this can be achieved by introducing trivalent rare-earth ions of differing size into the perovskite structure2,3,4, without affecting the valency of the Mn ions. The chemical pressure modifies local structural parameters such as the Mn–O bond distance and Mn–O–Mn bond angle, which directly influence the case of electron hopping between Mn ions (that is, the electronic bandwidth). But these effects cannot satisfactorily explain the dependence of magnetoresistance on T C. Here we demonstrate, using electron microscopy data, that the prototypical (La,Pr,Ca)MnO3 system is electronically phase-separated into a sub-micrometre-scale mixture of insulating regions (with a particular type of charge-ordering) and metallic, ferromagnetic domains. We find that the colossal magnetoresistive effect in low-T C systems can be explained by percolative transport through the ferromagnetic domains; this depends sensitively on the relative spin orientation of adjacent ferromagnetic domains which can be controlled by applied magnetic fields.

1,417 citations

Journal ArticleDOI
25 Aug 2005-Nature
TL;DR: Experimental evidence for ferroelectricity arising from electron correlations in the triangular mixed valence oxide, LuFe2O4 is reported, and resonant X-ray scattering measurements are used to determine the ordering of the Fe2+ and Fe3+ ions.
Abstract: Ferroelectric materials have a spontaneous electric polarization and are widely used in electronic devices including memories. Conventional ferroelectricity arises from ionic displacement, but an alternative mechanism has been proposed based on ordered electrons. Evidence for the latter behaviour has now been found in a mixed valence oxide. This type of ferroelectricity may offer potential for devices with enhanced controllability. Ferroelectric materials are widely used in modern electric devices such as memory elements, filtering devices and high-performance insulators. Ferroelectric crystals have a spontaneous electric polarization arising from the coherent arrangement of electric dipoles1 (specifically, a polar displacement of anions and cations). First-principles calculations2,3 and electron density analysis4 of ferroelectric materials have revealed that the covalent bond between the anions and cations, or the orbital hybridization of electrons on both ions, plays a key role in establishing the dipolar arrangement. However, an alternative model—electronic ferroelectricity5—has been proposed in which the electric dipole depends on electron correlations, rather than the covalency. This would offer the attractive possibility of ferroelectric materials that could be controlled by the charge, spin and orbital degrees of freedom of the electron. Here we report experimental evidence for ferroelectricity arising from electron correlations in the triangular mixed valence oxide, LuFe2O4. Using resonant X-ray scattering measurements, we determine the ordering of the Fe2+ and Fe3+ ions. They form a superstructure that supports an electric polarization consisting of distributed electrons of polar symmetry. The polar ordering arises from the repulsive property of electrons—electron correlations—acting on a frustrated geometry.

842 citations

Journal ArticleDOI
02 Apr 1998-Nature
TL;DR: In this article, a different pattern of charge localization in the charge-ordered phase of the manganese oxide La1−xCaxMnO3 (x ⩾ 0.5) was reported.
Abstract: The propensity of systems of charge and spin to form, under certain conditions, ‘stripe’ phases has recently attracted much attention, as it has been suggested that dynamically fluctuating stripe phases may be of central importance for an understanding of the physics of high-temperature superconductors1,2,3,4,5. A related phenomenon — static charge stripes — characterizes6 the insulating antiferromagnetic ground state of the manganese oxides, a class of materials which (like the copper oxide superconductors) have a perovskite structure, and are notable for their extraordinary electronic and magnetic properties, such as colossal magnetoresistance and charge ordering7,8. Here we report a different pattern of charge localization in the charge-ordered phase of the manganese oxide La1−xCaxMnO3 (x ⩾ 0.5). This pattern takes the form of extremely stable pairs of Mn3+O6 stripes, with associated large lattice contractions (due to the Jahn–Teller effect), separated periodically by stripes of non-distorted Mn4+O6 octahedra. These periodicities, which adopt integer values between 2 and 5 times the lattice parameter of the orthorhombic unit cell, correspond to the commensurate carrier concentrations (x = 1/2, 2/3, 3/4 and 4/5): for other values of x, the pattern of charge ordering is a mixture of the two adjacent commensurate configurations. These paired Jahn–Teller stripes appear therefore to be the fundamental building blocks of the charge-ordered state in the manganese oxides, and so may be expected to have profound implications for the magnetic and transport properties of these materials.

474 citations

Journal ArticleDOI
TL;DR: Using Lorenz microscopy and small-angle electron diffraction, it is directly present that the chiral magnetic soliton lattice continuously evolves from a chiral helimagnetic structure in small magnetic fields in Cr(1/3)NbS2.
Abstract: Using Lorenz microscopy and small-angle electron diffraction, we directly present that the chiral magnetic soliton lattice (CSL) continuously evolves from a chiral helimagnetic structure in small magnetic fields in ${\mathrm{Cr}}_{1/3}{\mathrm{NbS}}_{2}$. An incommensurate CSL undergoes a phase transition to a commensurate ferromagnetic state at the critical field strength. The period of a CSL, which exerts an effective potential for itinerant spins, is tuned by simply changing the field strength. Chiral magnetic orders observed do not exhibit any structural dislocation, indicating their high stability and robustness in ${\mathrm{Cr}}_{1/3}{\mathrm{NbS}}_{2}$.

390 citations

Journal ArticleDOI
TL;DR: It is reported that the Fe4+-based quadruple perovskite CaCu3Fe4O12 has high activity, which is comparable to or exceeding those of state-of-the-art catalysts such as Ba 0.5Sr0.5Co0.8Fe0.2O3−δ and the gold standard RuO2.
Abstract: The oxygen evolution reaction that occurs during water oxidation is of considerable importance as an essential energy conversion reaction for rechargeable metal-air batteries and direct solar water splitting. Cost-efficient ABO3 perovskites have been studied extensively because of their high activity for the oxygen evolution reaction; however, they lack stability, and an effective solution to this problem has not yet been demonstrated. Here we report that the Fe(4+)-based quadruple perovskite CaCu3Fe4O12 has high activity, which is comparable to or exceeding those of state-of-the-art catalysts such as Ba(0.5)Sr(0.5)Co(0.8)Fe(0.2)O(3-δ) and the gold standard RuO2. The covalent bonding network incorporating multiple Cu(2+) and Fe(4+) transition metal ions significantly enhances the structural stability of CaCu3Fe4O12, which is key to achieving highly active long-life catalysts.

347 citations


Cited by
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Journal ArticleDOI
21 Mar 2014-ACS Nano
TL;DR: In this paper, the 2D counterpart of layered black phosphorus, which is called phosphorene, is introduced as an unexplored p-type semiconducting material and the authors find that the band gap is direct, depends on the number of layers and the in-layer strain, and significantly larger than the bulk value of 0.31-0.36 eV.
Abstract: We introduce the 2D counterpart of layered black phosphorus, which we call phosphorene, as an unexplored p-type semiconducting material. Same as graphene and MoS2, single-layer phosphorene is flexible and can be mechanically exfoliated. We find phosphorene to be stable and, unlike graphene, to have an inherent, direct, and appreciable band gap. Our ab initio calculations indicate that the band gap is direct, depends on the number of layers and the in-layer strain, and is significantly larger than the bulk value of 0.31–0.36 eV. The observed photoluminescence peak of single-layer phosphorene in the visible optical range confirms that the band gap is larger than that of the bulk system. Our transport studies indicate a hole mobility that reflects the structural anisotropy of phosphorene and complements n-type MoS2. At room temperature, our few-layer phosphorene field-effect transistors with 1.0 μm channel length display a high on-current of 194 mA/mm, a high hole field-effect mobility of 286 cm2/V·s, and an...

5,233 citations

Journal ArticleDOI
TL;DR: This review acquaints some materials for performing OER activity, in which the metal oxide materials build the basis of OER mechanism while non-oxide materials exhibit greatly promising performance toward overall water-splitting.
Abstract: There is still an ongoing effort to search for sustainable, clean and highly efficient energy generation to satisfy the energy needs of modern society. Among various advanced technologies, electrocatalysis for the oxygen evolution reaction (OER) plays a key role and numerous new electrocatalysts have been developed to improve the efficiency of gas evolution. Along the way, enormous effort has been devoted to finding high-performance electrocatalysts, which has also stimulated the invention of new techniques to investigate the properties of materials or the fundamental mechanism of the OER. This accumulated knowledge not only establishes the foundation of the mechanism of the OER, but also points out the important criteria for a good electrocatalyst based on a variety of studies. Even though it may be difficult to include all cases, the aim of this review is to inspect the current progress and offer a comprehensive insight toward the OER. This review begins with examining the theoretical principles of electrode kinetics and some measurement criteria for achieving a fair evaluation among the catalysts. The second part of this review acquaints some materials for performing OER activity, in which the metal oxide materials build the basis of OER mechanism while non-oxide materials exhibit greatly promising performance toward overall water-splitting. Attention of this review is also paid to in situ approaches to electrocatalytic behavior during OER, and this information is crucial and can provide efficient strategies to design perfect electrocatalysts for OER. Finally, the OER mechanism from the perspective of both recent experimental and theoretical investigations is discussed, as well as probable strategies for improving OER performance with regards to future developments.

3,976 citations

Journal ArticleDOI
Han Liu, Adam T. Neal, Zhen Zhu, David Tománek, Peide D. Ye1 
TL;DR: In this article, a few-layer phosphorene has been introduced as a 2D p-type material for electronic applications, which has an inherent, direct and appreciable band gap that depends on the number of layers.
Abstract: Preceding the current interest in layered materials for electronic applications, research in the 1960's found that black phosphorus combines high carrier mobility with a fundamental band gap. We introduce its counterpart, dubbed few-layer phosphorene, as a new 2D p-type material. Same as graphene and MoS2, phosphorene is flexible and can be mechanically exfoliated. We find phosphorene to be stable and, unlike graphene, to have an inherent, direct and appreciable band-gap that depends on the number of layers. Our transport studies indicate a carrier mobility that reflects its structural anisotropy and is superior to MoS2. At room temperature, our phosphorene field-effect transistors with 1.0 um channel length display a high on-current of 194 mA/mm, a high hole field-effect mobility of 286 cm2/Vs, and an on/off ratio up to 1E4. We demonstrate the possibility of phosphorene integration by constructing the first 2D CMOS inverter of phosphorene PMOS and MoS2 NMOS transistors.

3,846 citations

Journal ArticleDOI
TL;DR: It is found that even a weak magnetoelectric interaction can lead to spectacular cross-coupling effects when it induces electric polarization in a magnetically ordered state.
Abstract: Magnetism and ferroelectricity are essential to many forms of current technology, and the quest for multiferroic materials, where these two phenomena are intimately coupled, is of great technological and fundamental importance. Ferroelectricity and magnetism tend to be mutually exclusive and interact weakly with each other when they coexist. The exciting new development is the discovery that even a weak magnetoelectric interaction can lead to spectacular cross-coupling effects when it induces electric polarization in a magnetically ordered state. Such magnetic ferroelectricity, showing an unprecedented sensitivity to ap plied magnetic fields, occurs in 'frustrated magnets' with competing interactions between spins and complex magnetic orders. We summarize key experimental findings and the current theoretical understanding of these phenomena, which have great potential for tuneable multifunctional devices.

3,683 citations