ISODISPLACEis a new internet-server tool for exploring structural phase transitions. Given parent-phase structural information, it generates atomic displacement patterns induced by irreducible representations of the parent space-group symmetry and allows a user to visualize and manipulate the amplitude of each distortion mode interactively.ISODISPLACEis freely accessible at http://stokes.byu.edu/isodisplace.htmlviacommon internet browsers.

High-transition-temperature superconductivity arises in copper oxides when holes or electrons are doped into the CuO 2 planes of their insulating parent compounds.While hole-doping quickly induces metallic behavior and superconductivity in many cuprates, electron-doping alone is insufficient in materials such as R 2 CuO 4 (R is Nd, Pr, La, Ce, etc.), where it is necessary to anneal an as-grown sample in a low-oxygen environment to remove a tiny amount of oxygen in order to induce superconductivity. Here we show that the microscopic process of oxygen reduction repairs Cu deficiencies in the as-grown materials and creates oxygen vacancies in the stoichiometric CuO 2 planes, effectively reducing disorder and providing itinerant carriers for superconductivity. The resolution of this long-standing materials issue suggests that the fundamental mechanism for superconductivity is the same for electron-and hole-doped copper oxides.The parent compounds of the high-transition-temperature (high-T c ) copper-oxide superconductors are antiferromagnetic (AF) Mott insulators composed of twodimensional CuO 2 planes separated by charge reservoir layers [1][2][3] . When holes are doped into these planes, the static long-range AF order is quickly destroyed and the lamellar copper-oxide materials become metallic and superconducting over a wide hole-doping range. In the case of electron-doped materials such as the T'-structured R 2 CuO 4 (R is Nd, Pr, La, Ce, etc.), electron-doping alone is insufficient, and annealing the as-grown sample in a low oxygen environment to remove a tiny amount of oxygen is necessary to induce superconductivity 2,3 . Previous work [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23] suggests that oxygen reduction may influence mobile carrier concentrations 7 , decrease disorder/impurity scattering 8,10,11,23 , or suppress the long-range AF order 16,17,22 . However, the microscopic process of oxygen reduction, its effect on the large electron-hole phase diagram asymmetry and mechanism of superconductivity 2,3 are still unknown. Here we use x-ray and neutron scattering data, combined with chemical and thermo-gravimetric analysis measurements in the electron-doped Pr 0.88 LaCe 0.12 CuO 4 to show that the microscopic process of oxygen reduction is to repair Cu deficiencies in the as-grown materials 12,13 and to create oxygen vacancies in the stoichiometric CuO 2 (refs. 16,17,22), effectively repairing disorder in the CuO 2 planes and providing itinerant carriers for superconductivity.The role of the reduction process in the superconductivity of electron-doped high-T c copper oxides has been a long-standing unsolved problem. For the hole-doped cuprates, low doping levels (e.g. 5%) entirely suppress AF order and superconductivity appears over a wide range of hole concentrations (from 6% to 30%). In the case of T' structured electron-doped superconductors, doping alone by substituting the trivalent ions R 3+ in R 2 CuO 4 with tetravalent Ce 4+ is insufficient to induce superconductivity a...

Charge-lattice fluctuations are observed in the layered perovskite manganite LaSr 2 Mn 2 O 7 by Raman spectroscopy as high as 340 K and with decreasing temperature they become static and form a charge ordered (CO) phase below T CO =210 K. In the static regime, superlattice reflections are observed through neutron and x-ray diffraction with a propagation vector (h+1/4,k-1/4,l). Crystallographic analysis of the CO state demonstrates that the degree of charge and orbital ordering in this manganite is weaker than the charge ordering in three dimensional perovskite manganites. A T N =170K a type-A antiferromagnetism (AF) develops and competes with the charge ordering, that eventually melts below T*=100K. High resolution diffraction measurements suggest that that CO-and AF-states do not coincide within the same region in the material but rather co-exist as separate phases. The transition to type-A antiferromagnetism at lower temperatures is characterized by the competition between these two phases.

A complete table of (3 + 1)D, (3 + 2)D and (3 + 3)D superspace groups (SSGs) has been enumerated that corrects omissions and duplicate entries in previous tables of superspace groups and Bravais classes. The theoretical methods employed are not new, though the implementation is both novel and robust. The paper also describes conventions for assigning a unique one-line symbol for each group in the table. Finally, a new online data repository is introduced that delivers more complete information about each SSG than has been presented previously.

Crystalline solids consisting of three-dimensional networks of interconnected rigid units are ubiquitous amongst functional materials. In many cases, application-critical properties are sensitive to rigid-unit rotations at low temperature, high pressure or specific stoichiometry. The shared atoms that connect rigid units impose severe constraints on any rotational degrees of freedom, which must then be cooperative throughout the entire network. Successful efforts to identify cooperative-rotational rigid-unit modes (RUMs) in crystals have employed split-atom harmonic potentials, exhaustive testing of the rotational symmetry modes allowed by group representation theory, and even simple geometric considerations. This article presents a purely algebraic approach to RUM identification wherein the conditions of connectedness are used to construct a linear system of equations in the rotational symmetry-mode amplitudes.