48 research outputs found
Oxygen superstructures throughout the phase diagram of
Short-range lattice superstructures have been studied with high-energy x-ray
diffuse scattering in underdoped, optimally doped, and overdoped . A new four-unit-cell superstructure was observed in
compounds with . Its temperature, doping, and material dependence
was used to attribute its origin to short-range oxygen vacancy ordering, rather
than electronic instabilities in the layers. No significant diffuse
scattering is observed in YBaCuO. The oxygen superstructures must
be taken into account when interpreting spectral anomalies in
Orbital ordering transition in CaRuO observed with resonant x-ray diffraction
Resonant x-ray diffraction performed at the and
absorption edges of Ru has been used to investigate the magnetic and orbital
ordering in CaRuO single crystals. A large resonant enhancement due to
electric dipole transitions is observed at the wave-vector
characteristic of antiferromagnetic ordering. Besides the previously known
antiferromagnetic phase transition at K, an additional phase
transition, between two paramagnetic phases, is observed around 260 K. Based on
the polarization and azimuthal angle dependence of the diffraction signal, this
transition can be attributed to orbital ordering of the Ru electrons.
The propagation vector of the orbital order is inconsistent with some
theoretical predictions for the orbital state of CaRuO.Comment: to appear in PR
Crystallographic Orientation Dependence of Surface Segregation and Alloying on PdCu Catalysts for CO<sub>2</sub> Hydrogenation
The influence of the crystallographic orientation on surface segregation and alloy formation in model PdCu methanol synthesis catalysts was investigated in situ using near-ambient pressure X-ray photoelectron spectroscopy under CO2 hydrogenation conditions. Combined with scanning tunneling microscopy and density functional theory calculations, the study showed that submonolayers of Pd undergo spontaneous alloy formation on Cu(110) and Cu(100) surfaces in vacuum, whereas they do not form an alloy on Cu(111). Upon heating in H2, inward diffusion of Pd into the Cu lattice is favored, facilitating alloying on all Cu surfaces. Under CO2 hydrogenation reaction conditions, the alloying trend becomes stronger, promoted by the reaction intermediate HCOO*, especially on Pd/Cu(110). This work demonstrates that surface alloying may be a key factor in the enhancement of the catalytic activity of PdCu catalysts as compared to their monometallic counterparts. Furthermore, it sheds light on the hydrogen activation mechanism during catalytic hydrogenation on copper-based catalysts
Lattice dynamical signature of charge density wave formation in underdoped YBa2Cu3O6+x
We report a detailed Raman scattering study of the lattice dynamics in
detwinned single crystals of the underdoped high temperature superconductor
YBa2Cu3O6+x (x=0.75, 0.6, 0.55 and 0.45). Whereas at room temperature the
phonon spectra of these compounds are similar to that of optimally doped
YBa2Cu3O6.99, additional Raman-active modes appear upon cooling below ~170-200
K in underdoped crystals. The temperature dependence of these new features
indicates that they are associated with the incommensurate charge density wave
state recently discovered using synchrotron x-ray scattering techniques on the
same single crystals. Raman scattering has thus the potential to explore the
evolution of this state under extreme conditions.Comment: 12 pages, 11 figure
Magnetic field induced transitions in multiferroic TbMnO3 probed by resonant and non-resonant X-ray diffraction
Multiferroic TbMnO3 is investigated using x-ray diffraction in high magnetic
fields. Measurements on first and second harmonic structural reflections due to
modulations induced by the Mn and Tb magnetic order are presented as function
of temperature and field oriented along the a and b-directions of the crystal.
The relation to changes in ordering of the rare earth moments in applied field
is discussed. Observations below T_N(Tb) without and with applied magnetic
field point to a strong interaction of the rare earth order, the Mn moments and
the lattice. Also, the incommensurate to commensurate transition of the wave
vector at the critical fields is discussed with respect to the Tb and Mn
magnetic order and a phase diagram on basis of these observations for magnetic
fields H||a and H||b is presented. The observations point to a complicated and
delicate magneto-elastic interaction as function of temperature and field.Comment: 12 pages, 15 figures, 2 references adde
Optical response of ferromagnetic YTiO_3 studied by spectral ellipsometry
We have studied the temperature dependence of spectroscopic ellipsometry
spectra of an electrically insulating, nearly stoichiometric YTiO_3 single
crystal with ferromagnetic Curie temperature T_C = 30 K. The optical response
exhibits a weak but noticeable anisotropy. Using a classical dispersion
analysis, we identify three low-energy optical bands at 2.0, 2.9, and 3.7 eV.
Although the optical conductivity spectra are only weakly temperature dependent
below 300 K, we are able to distinguish high- and low-temperature regimes with
a distinct crossover point around 100 K. The low-temperature regime in the
optical response coincides with the temperature range in which significant
deviations from Curie-Weiss mean field behavior are observed in the
magnetization. Using an analysis based on a simple superexchange model, the
spectral weight rearrangement can be attributed to intersite d_i^1d_j^1
\longrightarrow d_i^2d_j^0 optical transitions. In particular, Kramers-Kronig
consistent changes in optical spectra around 2.9 eV can be associated with the
high-spin-state (^3T_1) optical transition. This indicates that other
mechanisms, such as weakly dipole-allowed p-d transitions and/or
exciton-polaron excitations, can contribute significantly to the optical band
at 2 eV. The recorded optical spectral weight gain of 2.9 eV optical band is
significantly suppressed and anisotropic, which we associate with complex
spin-orbit-lattice phenomena near ferromagnetic ordering temperature in YTiO_3
Surface Segregation in CuNi Nanoparticle Catalysts During CO<sub>2</sub> Hydrogenation: The Role of CO in the Reactant Mixture
Surface segregation and restructuring in size-selected CuNi nanoparticles were investigated via near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) at various temperatures in different gas environments. Particularly in focus were structural and morphological changes occurring under CO2 hydrogenation conditions in the presence of carbon monoxide (CO) in the reactant gas mixture. Nickel surface segregation was observed when only CO was present as adsorbate. The segregation trend is inverted in a reaction gas mixture consisting of CO2, H2, and CO, resulting in an increase of copper concentration on the surface. Density functional theory calculations attributed the inversion of the segregation trend to the formation of a stable intermediate on the nanocatalyst surface (CH3O) in the CO-containing reactant mixture, which modifies the nickel segregation energy, thus driving copper to the surface. The promoting role of CO for the synthesis of methanol was demonstrated by catalytic characterization measurements of silica-supported CuNi NPs in a fixed-bed reactor, revealing high methanol selectivity (over 85%) at moderate pressures (20 bar). The results underline the important role of intermediate reaction species in determining the surface composition of bimetallic nanocatalysts and help understand the effect of CO cofeed on the properties of CO2 hydrogenation catalysts
Electronic superlattice revealed by resonant scattering from random impurities in Sr3Ru2O7
Resonant elastic x-ray scattering (REXS) is an exquisite element-sensitive
tool for the study of subtle charge, orbital, and spin superlattice orders
driven by the valence electrons, which therefore escape detection in
conventional x-ray diffraction (XRD). Although the power of REXS has been
demonstrated by numerous studies of complex oxides performed in the soft x-ray
regime, the cross section and photon wavelength of the material-specific
elemental absorption edges ultimately set the limit to the smallest
superlattice amplitude and periodicity one can probe. Here we show -- with
simulations and REXS on Mn-substituted SrRuO -- that these
limitations can be overcome by performing resonant scattering experiments at
the absorption edge of a suitably-chosen, dilute impurity. This establishes
that -- in analogy with impurity-based methods used in electron-spin-resonance,
nuclear-magnetic resonance, and M\"ossbauer spectroscopy -- randomly
distributed impurities can serve as a non-invasive, but now momentum-dependent
probe, greatly extending the applicability of resonant x-ray scattering
techniques