Satellite Meeting 5 (Kensington Suite)
New Frontiers in X-ray Science
Organisers:
Sarnjeet Dhesi (Diamond Light Source)
Gerrit van der Laan (Daresbury Laboratory)
The UK is entering a new and exciting phase in synchrotron radiation (SR) science with the advent of the Diamond Light Source, allowing innovative and ground breaking scientific research which makes it timely to look to future possibilities. This workshop is designed to give a general audience an idea of the wide ranging capabilities of third generation synchrotrons with talks on interfaces, coherence, spectroscopy and microscopy. The invited speakers, all working at the cutting edge of synchrotron radiation research in the physical sciences, will cover surface probes, such as surface x-ray diffraction and photoemission electron microscopy (PEEM). In addition, novel and exotic materials (spintronics, correlated systems, orbital ordering, superconductivity) and novel SR spectroscopic techniques (dynamics, x-ray coherent scattering, phase retrieval, inelastic x-ray scattering, and high energy photoemission) will be covered.
Tuesday 13th (afternoon)
- Introduction
- Dr. S.S. Dhesi (Diamond Light Source)
- Session 1 Chair - Prof. G.T. Materlik(Diamond Light Source)
- 15:00 Talk 1 Prof. J.F. van der Veen (SLS, Switzerland)
- Coherent X rays - Their use for imaging
and nanofocusing
ABSTRACT: Hard X rays are only weakly absorbed by matter. Hence, most objects are 'seen' by X rays as phase objects. For 'visibility' it is required that the X ray wave fronts be coherent over the object's area to be viewed. We describe some recent advances in wave front sensing, phase contrast imaging and holography.
Coherency is also exploited in focusing devices such as Fresnel zone plates and tapered waveguides. Which is the smallest beam size that such devices could ultimately deliver? - 15:45 Talk 2 Prof. S. Ferrer (ALBA, Spain)
- Present
and future perspectives of surface x-ray diffraction
ABSTRACT: The present satus of surface x-ray diffraction will be illustrated with several examples from the ESRF: chemical reactions at atmospheric pressures on single crystal surfaces, determination of the strain field on the surface of a stepped crystal and the surface oxidation of the tip of a Fe whisker. From the above experiments, future trends on the applications will be discussed.
Coherency is also exploited in focusing devices such as Fresnel zone plates and tapered waveguides. Which is the smallest beam size that such devices could ultimately deliver? - Session 2 Chair - Dr. N.B. Brookes (ESRF, Grenoble)
- 16:30 Talk 3 Dr. S. Heun (TASC, INFM-CNR Laboratory, Trieste, Italy)
- Low-dimensional systems studied by SPELEEM
ABSTRACT: The rapid progress of experimental techniques with access to chemical composition, electronic structure, magnetization, and fluctuations in these properties at the sub-micron level has been driven by the demand imposed by the continuous miniaturization and increasing complexity of nanostructured materials used in modern technology. X-ray photoelectron spectroscopy in combination with photoemission electron microscopy (PEEM) is among the techniques which can provide this information.
In my talk, after a basic introduction to the PEEM technique in general and the spectroscopic photoemission and low energy electron microscope (SPELEEM) in particular, I will discuss the properties of selected low-dimensional systems with possible device applications which will illustrate the potential of this method in a particular way. I will mainly focus on our investigations of the chemical composition of semiconductor quantum dots (InAs/GaAs and Ge/Si). Concentration maps across individual islands were obtained by photoelectron spectroscopy with high lateral resolution, which provide insight in the intermixing process which occurs during the growth of such islands. - 17:00 Talk 4 Dr. G. Panaccione (TASC, INFM-CNR Laboratory, Trieste, Italy)
- Volume sensitive photoemission from solids with Hard X-Rays: results
and perspectives
ABSTRACT: PhotoEmission Spectroscopy (PES) is one of the most powerful tools to investigate solid state properties. A strong surface sensitivity, i.e. a few atomic layers, can be achieved by tuning the kinetic energy of photoelectron to the minimum of the so called universal curve of escape depth. The state of the art in high quality surface science experiments can be summarized by an energy resolution of 5 - 30 meV for valence band spectra with hv < 40 eV, and of 30-200 meV at hv > 100 eV for core level spectra. On the other hand, there is a lack of corresponding experimental information on truly bulk properties in important materials such as high temperature superconductors and heavy fermions systems as well as in the case of low dimensional artificial solids. We report of PES experiments performed at ESRF (Beamline ID16), within the frame of the European VOLPE (VOLume PhotoEmission from solids) project, where we were able to achieve high energy resolution, namely 71 meV at hv =5933 eV as measured from the Au fermi level, as well as good statistics spectra up to 6-8 keV. We demonstrate that, at these kinetic energies, one may obtain information from >100 Angstrom of depth, corresponding to a maximum surface contribution < 3-5 %. We present recent results obtained on transition metals and highly correlated systems, and we discuss the relevant technical and scientific aspects of this new field of research. - 17:30Talk 5 Dr. G. Ghiringhelli (Politecnico di Milano, Italy)
- Resonant inelastic SOFT x-ray scattering
towards high resolution measurements
ABSTRACT: Resonant inelastic x-ray scattering (RIXS) can be effectively used to measure the energy and symmetry of neutral electronic excitations in solids. In the soft x-ray range it is particularly powerful for strongly correlated electron systems based on 3d transition metals, because at the L 2,3 edges the excitation and de-excitation processes, upon which RIXS is based, directly involve the 3d states.
The reduced intensity and the instrumental energy resolution usually hinder the detection very detailed spectral features, but the recent progress in the energy resolving power of our RIXS apparatus has opened new exciting possibilities in the 400-1000 eV range.
We have measured the dd excitations in cuprates, manganites, cobaltates and other interesting systems where electronic correlation plays a central role. We present some examples of high resolution RIXS at the L 2,3 edges of Cu, Ni, Co, Mn and Ti: the rich spectral features can be straightforwardly interpreted using atomic models (crystal field, cluster, impurity). All the spectra were measured using the AXES spectrometer, permanently installed at the beam line ID08 of the ESRF in Grenoble. The combined resolution ranges from 240 meV at the Ti to 650 meV at the Cu. These promising results have motivated the construction of a new very high resolution RIXS spectrometer, allowing also the k dependence, to be installed soon at the Swiss Light Source. - 18:00 Close Day 1
- Continue discussions at the bar
Wednesday 14th (morning)
- Session 3 Chair - Prof. W.G. Stirling (ESRF, Grenoble)
- 09:00Talk 6 Dr. J. Hill (BNL , USA)
- Orbital Dynamics: From Femtoseconds to Minutes
ABSTRACT: A full understanding of a given system requires knowledge of both the ground state and its associated dynamics. Here we report resonant inelastic x-ray scattering experiments of the electronic excitation spectrum in a series of doped manganites. Temperature dependent changes correlated with the magnetic order were observed as high as 10 eV. We associate these changes with spin-dependent intersite d-d excitations. On a very different time scale, the orbital order in the half-doped manganites is short ranged - an apparently glass-like state. We report coherent soft x-ray scattering experiments searching for the very slow dynamics (minutes) of this orbital glass. - 09:45Talk 7 Prof. P. Hatton (University of Durham, UK)
- Resonant Soft X-ray Diffraction studies
of Magnetic and Orbital Ordering in Manganites
ABSTRACT:Hole doped manganites are examples of strongly correlated electron systems that display a rich variety of complex structural phases involving charge, magnetic and orbital ordering. We will describe how resonant soft x-ray diffraction close to the manganese L2 and L3 edges can provide direct band-specific spectroscopic information on the magnetic and orbital ordering in manganites.
In the single layer manganite La0.5Sr1.5MnO4 we have been able to measure both the magnetic and orbital ordering reflections separately. Systematic modeling with atomic multiplet crystal field calculations was used to extract meaningful information regarding the interplay of spin, orbital, and Jahn-Teller order. These calculations provide a good general agreement with the observed energy dependence of the scattered intensity for a dominant orbital ordering of the dx2-z2 /dy2-z2 type.
In the bilayer manganite LaSr2Mn2O7 within the A-type antiferromagnetic phase we found strong intensity at the orbital and magnetic reflections. This shows that even in the absence of strong Jahn-Teller distortions, this compound is strongly orbitally ordered. The fit to the energy dependence of the orbital order reflection shows the absence of the Jahn-Teller distortion. Fitting of the Mn L-edge resonance spectra demonstrates the presence of orbital ordering of the Mn3+ions within an almost cubic crystal field in addition to valence fluctuations of a Mn3+and Mn2+type. - 10:20Talk 8 Dr. U. Staub (SLS, Switzerland)
- From Charge to Orbitals: Ordering Phenomena
Studied by Resonant Soft and Hard X-Ray Scattering
ABSTRACT: It is shown how charge (monopoles), magnetic (dipoles) and orbitals (quadrupoles and higher multipoles) can be accessed by resonant X-ray scattering. Examples are given by the perovskite RNiO 3, where hard and soft X-ray resonant X-ray scattering is used to study the charge and magnetic ordering of the Ni ions. The consistent interpretation of these signals and the absence of a detectable orbital contribution indicates that the unusual magnetic structure is not based on a particular scheme of orbital ordering. It is shown how these ordering phenomena relate to the metal-to-insulator transition.
The second example concerns the layered manganite La 0.5Sr 1.5MnO 4, where a detailed soft X-ray scattering study deduces the magnetic and orbital ordering. It is shown that polarization analysis and azimuthal scans are required to understand the observed signals. Moreover, the different temperature dependence of different features in the spectra points either to a strong interference of a magnetic signal originated by short range correlations with the orbital signal, or to a different order parameter of the Jahn-Teller distortion and the orbital ordering itself.
In a last example, if time allows, higher-multipole ordering is discussed in the 4f electron material DyB 2C 2. The energy dependence of the resonant soft X-ray scattering exhibits clear interference effects, which can be explained by an intra-atomic quadrupole interaction giving rise to a splitting of the core states. This makes the signal sensitive to quadrupoles (rank 2), hexadecapoles (rank 4) and hexacontatetrapoles (rank 6). - 10:55Concluding remarks
- Prof. G. van der Laan (Daresbury Laboratory)
- 11:00Close
