P. R. Willmott

Graphene on Ru(0001): a corrugated and chiral structure

We present a structural analysis of the graphene/Ru(0001) system obtained by surface x-ray diffraction. The data were fitted using Fourier-series-expanded displacement fields from an ideal bulk structure plus the application of symmetry constraints. The shape of the observed superstructure rods proves a reconstruction of the substrate, induced by strong bonding of graphene to ruthenium. Both the graphene layer and the underlying substrate are corrugated, with peak-to-peak heights of (0.82±0.15) A and (0.19±0.02) A for graphene and the topmost Ru-atomic layer, respectively. The Ru corrugation decays slowly over several monolayers into the bulk. The system also exhibits chirality, whereby in-plane rotations of up to 2.0° in those regions of the superstructure where the graphene is weakly bound are driven by elastic energy minimization.

Cluster method for analysing surface X-ray diffraction data sets using area detectors

An automated cluster algorithm is described, applicable to any image where a signal is to be analysed. The algorithm is employed in the context of surface X-ray diffraction data and extended to automate the data reduction process, which at present limits both the lead time to and the reliability of the retrieved structural information. A detailed evaluation of the constraints used to automate surface X-ray diffraction data analysis is provided. To overcome limitations of the algorithm and the experiment itself in certain geometries, the full field of view of area detectors is exploited to obtain orders of magnitude improvements in data collection. The method extends the surface X-ray diffraction technique to new systems and highlights the often archaic approach to the analysis of data collected with a two-dimensional detector.

La-doped BaTiO3 heterostructures: Compensating the polarization discontinuity

We demonstrate a route to manipulate the polarization and internal electric field of a complex oxide heterostructure using a layering sequence based on the LaAlO3-SrTiO3 interface. By combining sensitive atomic-level mapping of the structure using direct x-ray phase-retrieval methods with theoretical modeling of the electrostatic charge and polarization, we have devised a novel single-domain polar heterostructure. We find that ionic rearrangement results in strain and free energy minimization, and eliminates the polarization discontinuity leading to a two-fold increase of the spontaneous polarization towards the surface of an ultra-thin single-domain BaTiO3 film.

Technical Reports: Pulsed Laser Deposition and in situ Surface X-ray Diffraction at the Materials Science Beamline at the Swiss Light Source

One of the primary challenges of condensed matter physicists and materials scientists is the discovery and/or design of novel materials and their detailed characterization [1]. One can argue that this scientific odyssey began with the discovery of superconductivity in ceramic cuprates in 1984 [2], and more recently, colossal magnetoresistance in the manganates [3]. One of the consequences of this has been a concerted effort to produce high-quality thin films of systems such as YBa2Cu3O7 (YBCO), La1-xSrxMnO3 (LSMO), the ruthenates, vanadates, and other complex metal oxides, driven both by technological applications, and a desire to better understand the underlying physics of these fascinating systems.

Demonstration of femtosecond X-ray pump X-ray probe diffraction on protein crystals

The development of X-ray free-electron lasers (XFELs) has opened the possibility to investigate the ultrafast dynamics of biomacromolecules using X-ray diffraction. Whereas an increasing number of structures solved by means of serial femtosecond crystallography at XFELs is available, the effect of radiation damage on protein crystals during ultrafast exposures has remained an open question. We used a split-and-delay line based on diffractive X-ray optics at the Linac Coherent Light Source XFEL to investigate the time dependence of X-ray radiation damage to lysozyme crystals. For these tests, crystals were delivered to the X-ray beam using a fixed-target approach. The presented experiments provide probe signals at eight different delay times between 19 and 213 femtoseconds after a single pump event, thereby covering the time-scales relevant for femtosecond serial crystallography. Even though significant impact on the crystals was observed at long time scales after exposure with a single X-ray pulse, the collected diffraction data did not show significant signal reduction that could be assigned to beam damage on the crystals in the sampled time window and resolution range. This observation is in agreement with estimations of the applied radiation dose, which in our experiment was clearly below the values expected to cause damage on the femtosecond time scale. The experiments presented here demonstrate the feasibility of time-resolved pump-multiprobe X-ray diffraction experiments on protein crystals.

Structure of ultrathin heteroepitaxial superconducting YBa{sub 2}Cu{sub 3}O{sub 7-x} films

The atomic structures of ultrathin YBa{sub 2}Cu{sub 3}O{sub 7-x} (YBCO) films on SrTiO{sub 3}(001) (STO) and (La{sub x}Sr{sub 1-x})(Al{sub y}Ta{sub 1-y})O{sub 3}(001) (LSAT) were investigated with sub-Angstrom resolution using surface x-ray diffraction and the phase-retrieval direct-method difference map using the constraints of atomicity and film shift (DCAF). The model-independent electron densities which emerge from random initializations in DCAF are exceedingly stable. The films grow with a well-defined stacking sequence even when grown on substrates with mixed terrace termination. Only very minor out-of-plane deviations from bulk YBCO are observed in the film structures, although they are perfectly strained to the substrate and are therefore tetragonal. The films are superconducting, with critical temperatures for growth on STO and LSAT of 43 K and 70 K, respectively. These results have important implications for reliable structure determination of technologically relevant complex-metal oxide surfaces and interfaces.