A State of High T _c Superconductivity: There are strong indications that high-temperature superconductivity in the cuprates is formed amid competing orders, but only two have been observed unambiguously. The so-called stripe order has been observed in a Lanthanum-based cuprate family and consists of coexisting charge-and-spin modulations and occurs at a characteristic dopant concentration in which the critical temperature T _c has a dip. Now, Ghiringhelli et al. (p. 821 , published online 12 July; see the Perspective by Tranquada ) have used resonant inelastic x-ray scattering to uncover a related but apparently two-dimensional charge order in the much cleaner YBCO cuprate family. The charge fluctuations were not commensurate with the lattice and did not originate in the characteristic oxygen chains of YBCO. The order appeared only in a narrow interval of dopant concentrations and competed with superconductivity, which provides a natural explanation for a plateau in T _c observed in the same range.

Recently, charge density wave (CDW) order in the CuO2 planes of underdoped YBa2Cu3O6+$\delta${} was detected using resonant soft x-ray scattering. An important question remains: is the chain layer responsible for this charge ordering? Here, we explore the energy and polarization dependence of the resonant scattering intensity in a detwinned sample of YBa2Cu3O6.75 with ortho-III oxygen ordering in the chain layer. We show that the ortho-III CDW order in the chains is distinct from the CDW order in the planes. The ortho-III structure gives rise to a commensurate superlattice reflection at Q=[0.33\,0\,L] whose energy and polarization dependence agrees with expectations for oxygen ordering and a spatial modulation of the Cu valence in the chains. Incommensurate peaks at [0.30 0 L] and [0 0.30 L] from the CDW order in the planes are shown to be distinct in Q as well as their temperature, energy, and polarization dependence, and are thus unrelated to the structure of the chain layer. Moreover, the energy dependence of the CDW order in the planes is shown to result from a spatial modulation of energies of the Cu 2p to 3dx2-y2 transition, similar to stripe-ordered 214 cuprates.

We probe the collective magnetic modes of ${\mathrm{La}}_2{\mathrm{CuO}}_4$ and underdoped ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$ (LSCO) by momentum resolved resonant inelastic x-ray scattering (RIXS) at the Cu $L_3$ edge. For the undoped antiferromagnetic sample, we show that the single magnon dispersion measured with RIXS coincides with the one determined by inelastic neutron scattering, thus demonstrating that x rays are an alternative to neutrons in this field. In the spin dynamics of LSCO, we find a branch dispersing up to $\sim 400\mathrm{meV}$ coexisting with one at lower energy. The high-energy branch has never been seen before. It indicates that underdoped LSCO is in a dynamic inhomogeneous spin state.

We present a $5\mathrm{m}$ long spectrometer for soft x rays to be used at a synchrotron radiation beamline for resonant x-ray emission spectroscopy and resonant inelastic x-ray scattering in the $400–1600\mathrm{eV}$ energy range. It is based on a variable line spacing spherical grating (average groove density of $3200{\mathrm{mm}}^{-1}$ , $R=58.55\mathrm{m}$ ) and a charge coupled device two dimensional detector. With an x-ray spot on the sample of $10\mu \mathrm{m}$ , the targeted resolving power is higher than 10 000 at all energies below $1100\mathrm{eV}$ and better than 7000 at $1500\mathrm{eV}$ . The off-line tests made with Al and $\mathrm{Mg}K{\alpha }_{1,2}$ fluorescence emissions indicate that the spectrometer can actually work at 12 000 and 17 000 resolving power at the $L_3$ edges of Cu $\left(930\mathrm{eV}\right)$ and of Ti $\left(470\mathrm{eV}\right)$ , respectively. SAXES (superadvanced x-ray emission spectrometer) is mounted on a rotating platform allowing to vary the scattering angle from 25° to 130°. The spectrometer will be operational at the ADRESS (advanced resonant spectroscopies) beamline of the Swiss Light Source from 2007.