To explore the doping dependence of the recently discovered charge density wave (CDW) order in YBa2Cu3Oy, we present a bulk-sensitive high-energy x-ray study for several oxygen concentrations, including strongly underdoped YBa2Cu3O6.44. Combined with previous data around the so-called 1/8 doping, we show that bulk CDW order exists at least for hole concentrations (p) in the CuO2 planes of 0.078132. This implies that CDW order exists in close vicinity to the quantum critical point for spin density wave (SDW) order. In contrast to the pseudogap temperature T * , the onset temperature of CDW order decreases with underdoping to TCDW ∼ 90 K in YBa2Cu3O6.44. Together with a weakened order parameter this suggests a competition between CDW and SDW orders. In addition, the CDW order in YBa2Cu3O6.44 shows the same type of competition with superconductivity as a function of temperature and magnetic field as samples closer to p = 1/8. At low p the CDW incommensurability continues the previously reported linear increasing trend with underdoping. In the entire doping range the in-plane correlation length of the CDW order in b axis direction depends only very weakly on the hole concentration, and appears independent of the type and correlation length of the oxygen-chain order. The onset temperature of the CDW order is remarkably close to a temperature T † that marks the maximum of 1/(T1T ) in planar 63 Cu NQR/NMR experiments, potentially indicating a response of the spin dynamics to the formation of the CDW. Our discussion of these findings includes a detailed comparison to the charge stripe order in La2−x Bax CuO4.

X-ray diffraction measurements show that the high-temperature superconductor YBa2Cu3O6.54, with ortho-II oxygen order, has charge density wave order (CDW) in the absence of an applied magnetic field. The dominant wavevector of the CDW is qCDW = (0, 0.328(2), 0.5), with the in-plane component parallel to the b-axis (chain direction). It has a similar incommensurability to that observed in ortho-VIII and ortho-III samples, which have different dopings and oxygen orderings. Our results for ortho-II contrast with recent high-field NMR measurements, which suggest a commensurate wavevector along the a-axis. We discuss the relationship between spin and charge correlations in YBa2Cu3Oy, and recent high-field quantum oscillation, NMR and ultrasound experiments.

Understanding the interaction between surfaces and their surroundings is crucial in many materials-science fields, such as catalysis, corrosion, and thin-film electronics, but existing characterization methods have not been capable of fully determining the structure of surfaces during dynamic processes, such as catalytic reactions, in a reasonable time frame. We demonstrate an x-ray-diffraction-based characterization method that uses high-energy photons (85 kiloelectron volts) to provide unexpected gains in data acquisition speed by several orders of magnitude and enables structural determinations of surfaces on time scales suitable for in situ studies. We illustrate the potential of high-energy surface x-ray diffraction by determining the structure of a palladium surface in situ during catalytic carbon monoxide oxidation and follow dynamic restructuring of the surface with subsecond time resolution.

The magnetic excitations in the cuprate superconductors might be essential for an understanding of high-temperature superconductivity. In these cuprate superconductors the magnetic excitation spectrum resembles an hour-glass and certain resonant magnetic excitations within are believed to be connected to the pairing mechanism, which is corroborated by the observation of a universal linear scaling of superconducting gap and magnetic resonance energy. So far, charge stripes are widely believed to be involved in the physics of hour-glass spectra. Here we study an isostructural cobaltate that also exhibits an hour-glass magnetic spectrum. Instead of the expected charge stripe order we observe nano phase separation and unravel a microscopically split origin of hour-glass spectra on the nano scale pointing to a connection between the magnetic resonance peak and the spin gap originating in islands of the antiferromagnetic parent insulator. Our findings open new ways to theories of magnetic excitations and superconductivity in cuprate superconductors.

In situ high-energy surface X-ray diffraction was employed to determine the surface structure dynamics of a Pd(100) single crystal surface acting as a model catalyst to promote CO oxidation. The measurements were performed under semirealistic conditions, i.e., 100 mbar total gas pressure and 600 K sample temperature. The surface structure was studied in detail both in a steady gas flow and in a gradually changing gas composition with a time resolution of 0.5 s. The experimental technique allows for rapid reciprocal space mapping providing the complete information on structural changes of a surface with unprecedented time resolution in harsh conditions. Our results show that the (√5 × √5)R27°-PdO(101) surface oxide forms in a close to stoichiometric O 2 and CO gas mixture as the mass spectrometry indicates a transition to a highly active state with the reaction rate limited by the CO mass transfer to the Pd(100) surface. Using a low excess of O 2 in the gas stoichiometry, islands of bulk oxide grow epitaxially in the same (101) crystallographic orientation of the bulk PdO unit cell according to a Stranski−Krastanov type of growth. The morphology of the islands is analyzed quantitatively. Upon further increase of the O 2 partial pressure a polycrystalline Pd oxide forms on the surface. ■ INTRODUCTIONFor more than a century heterogeneous catalysis has been extensively exploited by the industry, and as a consequence it has been intensively studied. 1 One of the most prominent examples is the CO oxidation reaction, CO + 1 / 2 O 2 → CO 2 . This process transforms highly toxic carbon monoxide, formed e.g. as a byproduct during incomplete combustion of the fuel in internal combustion engines, to less harmful carbon dioxide gas. However, the reaction is very slow under the operational conditions in the gas phase and requires thus the presence of a solid catalysts to proceed at a sufficiently high rate. Because of its importance and relatively simple mechanism, this reaction has become the subject of numerous studies aiming to resolve the atomic-scale processes that occur on the surface of catalysts. 2 Supported nanoparticles of late transition metals represent a well-known and efficient type of oxidation catalyst and are currently widely used in catalytic converters. 3,4 Hence, a deep understanding of the fundamental processes proceeding in such systems is important for improvement of existing catalyst-based solutions and development of new potential approaches. For this purpose, studies of atomic-scale surface structure and determination of the active phase of catalysts under working conditions are essential. However, the complexity of such systems and the inability of many experimental techniques to work under realistic pressuresthe challenges known as material and pressure gapssignificantly narrow the selection of available methods for structural determination and necessitate the use of model systems. One of the commonly used approaches is to study single crystals with different surface crystallographic orientation...