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.

Heisenberg interactions are ubiquitous in magnetic materials and play a central role in modelling and designing quantum magnets. Bond-directional interactions^1,2,3 offer a novel alternative to Heisenberg exchange and provide the building blocks of the Kitaev model⁴, which has a quantum spin liquid as its exact ground state. Honeycomb iridates, A₂IrO₃ (A = Na, Li), offer potential realizations of the Kitaev magnetic exchange coupling, and their reported magnetic behaviour may be interpreted within the Kitaev framework. However, the extent of their relevance to the Kitaev model remains unclear, as evidence for bond-directional interactions has so far been indirect. Here we present direct evidence for dominant bond-directional interactions in antiferromagnetic Na₂IrO₃ and show that they lead to strong magnetic frustration. Diffuse magnetic X-ray scattering reveals broken spin-rotational symmetry even above the Néel temperature, with the three spin components exhibiting short-range correlations along distinct crystallographic directions. This spin- and real-space entanglement directly uncovers the bond-directional nature of these interactions, thus providing a direct connection between honeycomb iridates and Kitaev physics.

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 show that in resonant inelastic x-ray scattering (RIXS) at the copper $L$ and $M$ edge direct spin-flip scattering is in principle allowed. We demonstrate how this possibility can be exploited to probe the dispersion of magnetic excitations, for instance magnons, of cuprates such as the high $T_c$ superconductors. We compute the relevant local and momentum dependent magnetic scattering amplitudes, which we compare to the elastic and $dd$-excitation scattering intensities. For cuprates these theoretical results put RIXS as a technique on the same footing as neutron scattering.