Experimental evidence on high-Tc cuprates reveals ubiquitous charge density wave (CDW) modulations 1-10 , which coexist with superconductivity. Although the CDW had been predicted by theory 11-13 , important questions remain about the extent to which the CDW influences lattice and charge degrees of freedom and its characteristics as functions of doping and temperature. These questions are intimately connected to the origin of the CDW and its relation to the mysterious cuprate pseudogap 10,14 . Here, we use ultrahigh resolution resonant inelastic x-ray scattering (RIXS) to reveal new CDW character in underdoped Bi 2 Sr 2 CaCu 2 O 8+δ . At low temperature, we observe dispersive excitations from an incommensurate CDW that induces anomalously enhanced phonon intensity, unseen using other techniques. Near the pseudogap temperature T*, the CDW persists, but the associated excitations significantly weaken and the CDW wavevector shifts, becoming nearly commensurate with a periodicity of four lattice constants. The dispersive CDW excitations, phonon anomaly, and temperature dependent commensuration provide a comprehensive momentum space picture of complex CDW behavior and point to a closer relationship with the pseudogap state.With sufficient energy resolution, RIXS can be an ideal probe for revealing the CDW excitations in cuprates. By tuning the incident photon energy to the Cu L 3 -edge ( Fig. 1a), the resonant absorption and emission processes can leave the system in excited final states, which couple to a variety of excitations arising from orbital, spin, charge, and lattice degrees of freedom 15 . Thus, information of these elementary excitations in energy and momentum space can be deduced from analyzing the RIXS spectra as functions of the energy loss and the momentum transfer of the photons (Fig. 1a). This is highlighted

Previously we have presented evidence for stripe order of holes and spins in La1.6−xNd0.4SrxCuO4 with x = 0.12. Here we show, via neutron diffraction measurements of magnetic scattering, that similar order occurs in crystals with x = 0.15 and 0.20. Zero-field-cooled magnetization measurements show that all 3 compositions are also superconducting, with the superconducting transition temperature increasing as the low-temperature staggered magnetization decreases. 74.72.Dn,71.45.Lr,75.50.Ee,75.70.Kw Neutron scattering studies [1,2,3,4,5,6,7] of dynamical magnetic correlations in superconducting La 2−x Sr x CuO 4 have provided important clues to the nature of electronic correlations within the doped CuO 2 planes. The lowenergy magnetic scattering, which is characterized by the two-dimensional antiferromagnetic wavevector Q AF = ( [7]. In one common interpretation [8,9,10,11], the incommensurate peaks are viewed as the dynamical response of a spatially uniform electron liquid with a nearly-nested Fermi surface. From a rather different perspective, the Q-dependent structure is taken as evidence for spatial inhomogeneity associated with charge segregation [12,13,14] or charge-density-wave correlations [15,16,17,18]. Evidence for the latter picture is provided by our recent discovery [19,20] of incommensurate charge and spin order in La 1.6−x Nd 0.4 Sr x CuO 4 with x = 0.12; however, given the claim [21] that bulk superconductivity is absent at this composition, one might choose to argue that these results are not directly relevant to the case of superconducting samples.To test the relationship between charge-stripe order and superconductivity, we have now investigated two other Sr concentrations, x = 0.15 and 0.20. Our neutron diffraction measurements on single-crystal samples reveal elastic incommensurate magnetic peaks for both compositions, thus demonstrating the presence of chargestripe order. Since the x = 0.20 crystal was known to be superconducting from previous work [22], we decided to check the x = 0.12 and 0.15 crystals for superconductivity as well. To our surprise, zero-field-cooled susceptibility measurements exhibit a bulk shielding signal for all three compositions. Since both the incommensurate peak splitting, ǫ, and the superconducting transition temperature vary with x, the results strongly suggest a local coexistence of superconductivity and stripe order. The fact that T c decreases as the staggered magnetization increases indicates that these two types of order compete with one another [23]. Furthermore, since the variation of ǫ with x in the Nd-doped crystals is essentially identical to that obtained from recent inelastic measurements [7] on crystals of La 2−x Sr x CuO 4 , it seems inescapable that dynamical charge-stripe correlations are present in the optimally doped material.The crystals studied in this work were grown at the University of Tokyo using the traveling-solvent floatingzone method. The transport properties of the x = 0.12 and 0.20 compositions were reported several years ago [22...

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La2-xBa x Cu04 has been found to undergo the following sequence of transformations upon cooling: tetr.(/4/mmm)-• ovi\\oXBmab) -• telvXP^i/ncm), over a range of composition 0.05 < x < 0.20. The newly discovered low-temperature tetragonal phase can be thought of as a coherent superposition of the twin-related Bmab structures. The system can be modeled as an A'F-spin system with temperaturedependent quartic anisotropy, v(T). Slight differences between orthorhombic and tetragonal structures appear to have large effects upon the superconductivity.