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J. Appl. Cryst.の発刊に際して

The magnetoelectric effect--the induction of magnetization by means of an electric field and induction of polarization by means of a magnetic field--was first presumed to exist by Pierre Curie, and subsequently attracted a great deal of interest in the 1960s and 1970s (refs 2-4). More recently, related studies on magnetic ferroelectrics have signalled a revival of interest in this phenomenon. From a technological point of view, the mutual control of electric and magnetic properties is an attractive possibility, but the number of candidate materials is limited and the effects are typically too small to be useful in applications. Here we report the discovery of ferroelectricity in a perovskite manganite, TbMnO3, where the effect of spin frustration causes sinusoidal antiferromagnetic ordering. The modulated magnetic structure is accompanied by a magnetoelastically induced lattice modulation, and with the emergence of a spontaneous polarization. In the magnetic ferroelectric TbMnO3, we found gigantic magnetoelectric and magnetocapacitance effects, which can be attributed to switching of the electric polarization induced by magnetic fields. Frustrated spin systems therefore provide a new area to search for magnetoelectric media.

Magnetic control of ferroelectric polarization

Graphene is very popular because of its many fascinating properties, but its lack of an electronic bandgap has stimulated the search for 2D materials with semiconducting character. Transition metal dichalcogenides (TMDCs), which are semiconductors of the type MX2, where M is a transition metal atom (such as Mo or W) and X is a chalcogen atom (such as S, Se or Te), provide a promising alternative. Because of its robustness, MoS2 is the most studied material in this family. TMDCs exhibit a unique combination of atomic-scale thickness, direct bandgap, strong spin–orbit coupling and favourable electronic and mechanical properties, which make them interesting for fundamental studies and for applications in high-end electronics, spintronics, optoelectronics, energy harvesting, flexible electronics, DNA sequencing and personalized medicine. In this Review, the methods used to synthesize TMDCs are examined and their properties are discussed, with particular attention to their charge density wave, superconductive and topological phases. The use of TMCDs in nanoelectronic devices is also explored, along with strategies to improve charge carrier mobility, high frequency operation and the use of strain engineering to tailor their properties. Two-dimensional transition metal dichalcogenides (TMDCs) exhibit attractive electronic and mechanical properties. In this Review, the charge density wave, superconductive and topological phases of TMCDs are discussed, along with their synthesis and applications in devices with enhanced mobility and with the use of strain engineering to improve their properties.

2D transition metal dichalcogenides

Bond-orientational order in molecular-dynamics simulations of supercooled liquids and in models of metallic glasses is studied. Quadratic and third-order invariants formed from bond spherical harmonics allow quantitative measures of cluster symmetries in these systems. A state with short-range translational order, but extended correlations in the orientations of particle clusters, starts to develop about 10% below the equilibrium melting temperature in a supercooled Lennard-Jones liquid. The order is predominantly icosahedral, although there is also a cubic component which we attribute to the periodic boundary conditions. Results are obtained for liquids cooled in an icosahedral pair potential as well. Only a modest amount of orientational order appears in a relaxed Finney dense-random-packing model. In contrast, we find essentially perfect icosahedral bond correlations in alternative "amorphon" cluster models of glass structure.

Bond-orientational order in liquids and glasses

Preface. 1. Synchrotron X-ray scattering studies of Langmuir films (J. Als-Nielsen and H. Mowald). 2. Coronary angiography project at the Photon Factory using a large monochromatic beam (K. Hyodo, K. Nishimura and M. Ando). 3. Optical and dielectric properties of materials relevant to biological research (M.W. Williams, E.T. Arakawa and T. Inagaki). 4. Crystal-structure analysis of biological macromolecules by synchrotron-radiation diffraction (H.D. Bartunik). 5. Fibre diffraction: collagen (A. Bigi and N. Roveri). 6. Scattering from non-crystalline systems (M.H.J. Koch). 7. Synchrotron radiation sources (W. Brefeld and P. Gurtler). 8. A Bragg reflection monochromator for synchrotron radiation angiography (G.S. Brown). 9. Trace element analysis by X-ray fluorescence (A. Iida and Y. Gohshi). 10. Medical applications of synchrotron radiation (J.C. Giacomini and H.J. Gordon). 11. Microradiography and microtomography (W. Graeff and K. Engelke). 12. NIKOS - non-invasive angiography at HASYLAB (W. Graeff and W.-R. Dix). 13. Radiobiological experiments in the X-ray region with synchrotron radiation (K. Hieda and T. Ito). 14. Synchrotron radiation for angiography, an overview (E. Rubenstein and R. Hofstadter). 15. X-ray microscopy (G. Schmahl and P.-C. Cheng). 16. UV spectroscopy (E.S. Stevens). 17. Anomalous X-ray scattering (H.B. Stuhrmann, G. Goerigk and B. Munk). 18. Detectors for angiography (A. Thompson). 19. Progress in X-ray synchrotron diffraction studies of muscle contraction (K. Wakabayashi and Y. Amemiya). 20A. Imaging apparatus and techniques for synchrotron radiation (H.D. Zeman). 20B. The processing of intravenous coronary angiograms produced by synchrotron radiation (H.D. Zeman). Author index. Subject index.

Handbook on synchrotron radiation

Powder neutron diffraction studies of undoped ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4\mathrm{\ensuremath{-}}\mathrm{y}}$ have revealed new superlattice peaks below \ensuremath{\sim}220 K. The absence of corresponding x-ray superlattice lines and an observed susceptibility anomaly near 220 K suggest the occurrence of antiferromagnetism. From the magnetic peak intensities we deduce a structure consisting of ferromagnetic sheets of Cu spins alternating along the [100] orthorhombic axis, with the spins aligned along the [001] orthorhombic axis, The low-temperature magnetic moment is approximately 0.5${\mathrm{\ensuremath{\mu}}}_{\mathrm{B}}$/(Cu atom). The tetragonal-orthorhombic transition at 505 K has also been studied.

Antiferromagnetism in La 2 CuO 4-y

Neutron-diffraction studies of ${\mathrm{TiSe}}_{2}$ show that a second-order structural phase transition occurs at ${T}_{0}=202$ K involving transverse atomic displacements with wave vector $\stackrel{\ensuremath{\rightarrow}}{\mathrm{q}}=(\frac{1}{2},0,\frac{1}{2})$. The electronic transport properties of the most nearly stoichiometric crystals show that ${\mathrm{TiSe}}_{2}$ is a semimetal with ${n}_{e}={n}_{h}={10}^{20}$/${\mathrm{cm}}^{3}$. Small impurity concentrations or deviations from stoichiometry reduce ${n}_{h}$, increase ${n}_{e}$, and suppress the phase transition. This reduction together with the observed displacement pattern lead us to speculate that the transition is driven by an electron-hole coupling.

Electronic properties and superlattice formation in the semimetal TiSe 2

Neutron-scattering studies of $2H$-Nb${\mathrm{Se}}_{2}$ and $2H$-Ta${\mathrm{Se}}_{2}$ reveal the strong Kohn anomalies and incommensurate superlattice characteristic of charge-density-wave instabilities. Whereas the Nb${\mathrm{Se}}_{2}$ superlattice (${T}_{0}=33.5$ K) remains incommensurate to 5 K, Ta${\mathrm{Se}}_{2}$ (${T}_{0}=122.3$ K) locks to a commensurate $3a$ superlattice at 90 K. The temperature dependence of the superlattice wave vector and the lockin behavior are understood using a free energy involving terms third order in atomic displacement. A secondary lattice distortion required by this model is observed.

Study of Superlattice Formation in 2 H -Nb Se 2 and 2 H -Ta Se 2 by Neutron Scattering

We have used the triple-axis neutron-scattering technique to study $2H\ensuremath{-}\mathrm{Ta}{\mathrm{Se}}_{2}$ and $2H\ensuremath{-}\mathrm{Nb}{\mathrm{Se}}_{2}$ which undergo charge-density wave transitions at ${T}_{0}=122.3$ K and ${T}_{0}=33.5$ K, respectively. The transitions in both compounds appear to be second-order and involve atomic displacements of ${\ensuremath{\Sigma}}_{1}$ symmetry. At inception, the superlattices in both compounds have nearly identical incommensurate wave vectors with magnitude ${q}_{\ensuremath{\delta}}=\frac{1}{3(1\ensuremath{-}\ensuremath{\delta}){a}^{*}}$, with $\ensuremath{\delta}\ensuremath{\sim}0.02$. The Nb${\mathrm{Se}}_{2}$ superlattice remains incommensurate to 5 K but Ta${\mathrm{Se}}_{2}$ undergoes a first-order lock-in transition where $\ensuremath{\delta}\ensuremath{\rightarrow}0$ at 90 K. The temperature dependence of the superlattice wave vector $\stackrel{\ensuremath{\rightarrow}}{\mathrm{q}}$ in the incommensurate phase and the lock-in transition are discussed using a free energy involving third-order "umklapp" terms and a secondary order parameter. The secondary lattice distortion which is predicted in this model is observed experimentally. Most phonon branches having energies less than 10 meV with propagation vectors in the [$\ensuremath{\zeta}00$] and [$00\ensuremath{\zeta}$] directions have been measured at 300 K. Strong anomalies are found in the ${\ensuremath{\Sigma}}_{1}[\ensuremath{\zeta}00]$ phonon branches in both materials near the wave vector ${\stackrel{\ensuremath{\rightarrow}}{\mathrm{q}}}_{c}=(\frac{1}{3},0,0)$ characteristic of the low-temperature superlattices. Substantial softening of this phonon is observed as the transition is approached. In addition, the spectral profile exhibits a central peak which is not measurably inelastic.

David E. Moncton

Papers

Handbook on synchrotron radiation

Antiferromagnetism in La 2 CuO 4-y

Electronic properties and superlattice formation in the semimetal TiSe 2

Study of Superlattice Formation in 2 H -Nb Se 2 and 2 H -Ta Se 2 by Neutron Scattering

Neutron scattering study of the charge-density wave transitions in 2 H − Ta Se 2 and 2 H − Nb Se 2