The concept for hard X-ray beamline optics at SLS 2.0.

In the scope of the latest upgrade of the Swiss Light Source, five hard X-ray beamlines will be constructed or rebuilt. To use synergies between these beamline projects, a concept is developed here for hard X-ray beamlines that is tailored to the new storage ring. Herein, this concept is described from the source, via the front end, to the beamline optics.

Microstructure and crystal order during freezing of supercooled water drops.

Supercooled water droplets are widely used to study supercooled water, ice nucleation and droplet freezing. Their freezing in the atmosphere affects the dynamics and climate feedback of clouds and can accelerate cloud freezing through secondary ice production. Droplet freezing occurs at several timescales and length scales and is sufficiently stochastic to make it unlikely that two frozen drops are identical.

Thickness-Dependent Perovskite Octahedral Distortions at Heterointerfaces.

In this study, we analyze how the octahedral tilts and rotations of thin films of LaNiO and LaAlO grown on different substrates, determined using synchrotron X-ray diffraction-measured half-integer Bragg peaks, depend upon the total film thickness. We find a striking difference between films grown on SrTiO and LaAlO substrates which appears to stem not only from the difference in epitaxial strain state but also from the level of continuity at the heterointerface. In particular, the chemically and structurally discontinuous LaNiO/SrTiO and LaAlO/SrTiO interfaces cause a large variation in the octahedral network as a function of film thickness whereas the rather continuous LaNiO/LaAlO interface seems to allow from just a few unit cells the formation of a stable octahedral pattern corresponding to that expected only given the applied biaxial strain.

Femtosecond phase-transition in hard x-ray excited bismuth.

The evolution of bismuth crystal structure upon excitation of its A phonon has been intensely studied with short pulse optical lasers. Here we present the first-time observation of a hard x-ray induced ultrafast phase transition in a bismuth single crystal at high intensities (~10 W/cm). The lattice evolution was followed using a recently demonstrated x-ray single-shot probing setup.

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.

SLS-2 - the upgrade of the Swiss Light Source.

An upgrade of the Swiss Light Source (SLS) is planned for 2021-2024 and includes the exchange of the existing storage ring by a new one providing about 40-50 times lower emittance in user operation mode. This will extend the performance of SLS in particular in the fields of coherent imaging, full-field tomography, soft X-ray angle-resolved photoelectron spectroscopy and resonant inelastic X-ray scattering. A science case and a conceptual design for the machine have been established.

Negative Pressures and Spallation in Water Drops Subjected to Nanosecond Shock Waves.

Most experimental studies of cavitation in liquid water at negative pressures reported cavitation at tensions significantly smaller than those expected for homogeneous nucleation, suggesting that achievable tensions are limited by heterogeneous cavitation. We generated tension pulses with nanosecond rise times in water by reflecting cylindrical shock waves, produced by X-ray laser pulses, at the internal surface of drops of water. Depending on the X-ray pulse energy, a range of cavitation phenomena occurred, including the rupture and detachment, or spallation, of thin liquid layers at the surface of the drop.

Chemistry and structure of homoepitaxial SrTiO3 films and their influence on oxide-heterostructure interfaces.

The properties of single-crystal SrTiO3 substrates and homoepitaxial SrTiO3 films grown by pulsed laser deposition have been compared, in order to understand the loss of interfacial conductivity when more than a critical thickness of nominally homoepitaxial SrTiO3 is inserted between a LaAlO3 film and a SrTiO3 substrate. In particular, the chemical composition and the structure of homoepitaxial SrTiO3 investigated by low-energy ion-scattering and surface X-ray diffraction show that for insulating heterointerfaces, a Sr-excess is present between the LaAlO3 and homoepitaxial SrTiO3. Furthermore, an increase in the out-of-plane lattice constant is observed in LaAlO3, indicating that the conductivity both with and without insertion of the SrTiO3 thin film originates from a Zener breakdown associated with the polar catastrophe.

The Materials Science beamline upgrade at the Swiss Light Source.

The Materials Science beamline at the Swiss Light Source has been operational since 2001. In late 2010, the original wiggler source was replaced with a novel insertion device, which allows unprecedented access to high photon energies from an undulator installed in a medium-energy storage ring. In order to best exploit the increased brilliance of this new source, the entire front-end and optics had to be redesigned.

Interface Fermi states of LaAlO3/SrTiO3 and related heterostructures.

The interfaces of LaAlO3/SrTiO3 and (LaAlO3)(x)(SrTiO3)(1-x)/SrTiO3 heterostructures have been investigated by soft x-ray photoelectron spectroscopy for different layer thicknesses across the insulator-to-metal interface transition. The valence band and Fermi edge were probed using resonant photoemission across the Ti L(2,3) absorption edge. The presence of a Fermi-edge signal originating from the partially filled Ti 3d orbitals is only found in the conducting samples.

Tunable conductivity threshold at polar oxide interfaces.

The physical mechanisms responsible for the formation of a two-dimensional electron gas at the interface between insulating SrTiO(3) and LaAlO(3) have remained a contentious subject since its discovery in 2004. Opinion is divided between an intrinsic mechanism involving the build-up of an internal electric potential due to the polar discontinuity at the interface between SrTiO(3) and LaAlO(3), and extrinsic mechanisms attributed to structural imperfections. Here we show that interface conductivity is also exhibited when the LaAlO(3) layer is diluted with SrTiO(3), and that the threshold thickness required to show conductivity scales inversely with the fraction of LaAlO(3) in this solid solution, and thereby also with the layer's formal polarization.

Atomic imaging and direct phase retrieval using anomalous surface x-ray diffraction.

The application of multi-wavelength anomalous diffraction to thin films, interfaces and surface structures is presented. The method directly determines the amplitudes and phases of the complex surface structure factors from surface x-ray diffraction data, measured at three different energies around the absorption edge of one of the elements present in the film. Thereby, one is able to directly Fourier transform the data, which immediately provides meaningful and unambiguous electron-density distributions.

Electrostriction at the LaAlO3/SrTiO3 interface.

We present a direct comparison between experimental data and ab initio calculations for the electrostrictive effect in the polar LaAlO(3) layer grown on SrTiO(3) substrates. From the structural data, a complete screening of the LaAlO(3) dipole field is observed for film thicknesses between 6 and 20 uc. For thinner films, an expansion of the c axis of 2% matching the theoretical predictions for an electrostrictive effect is observed experimentally.

Evolution of the interfacial structure of LaAlO3 on SrTiO3.

The evolution of the atomic structure of LaAlO_{3} grown on SrTiO_{3} was investigated using surface x-ray diffraction in conjunction with model-independent, phase-retrieval algorithms between two and five monolayers film thickness. A depolarizing buckling is observed between cation and oxygen positions in response to the electric field of polar LaAlO_{3}, which decreases with increasing film thickness. We explain this in terms of competition between elastic strain energy, electrostatic energy, and electronic reconstructions.

Stages in molecular beam epitaxy growth of GaAs nanowires studied by x-ray diffraction.

GaAs nanowires were grown by molecular beam epitaxy and studied by glancing-angle x-ray diffraction during five different stages of the growth process. An entire forest of randomly positioned epitaxial nanowires was sampled simultaneously and a large variation in the Au-Ga catalyst was found. Au, AuGa, AuGa(2) and the hexagonal beta phase were all identified in several orientations and in similar amounts.

Synchrotron radiation hardness studies of PILATUS II.

A synchrotron beam has been used to investigate the radiation tolerance of a PILATUS II module. It has been demonstrated that radiation-induced threshold shifts become significant above 30 Mrad. Individual adjustment of pixel thresholds after irradiation enabled retention of standard behaviour in excess of 40 Mrad.

Profiling the interface electron gas of LaAlO3/SrTiO3 heterostructures with hard x-ray photoelectron spectroscopy.

The conducting interface of LaAlO3/SrTiO3 heterostructures has been studied by hard x-ray photoelectron spectroscopy. From the Ti 2p signal and its angle dependence we derive that the thickness of the electron gas is much smaller than the probing depth of 4 nm and that the carrier densities vary with increasing number of LaAlO3 overlayers. Our results point to an electronic reconstruction in the LaAlO3 overlayer as the driving mechanism for the conducting interface and corroborate the recent interpretation of the superconducting ground state as being of the Berezinskii-Kosterlitz-Thouless type.

Performance of single-photon-counting PILATUS detector modules.

PILATUS is a silicon hybrid pixel detector system, operating in single-photon-counting mode, that has been developed at the Paul Scherrer Institut for the needs of macromolecular crystallography at the Swiss Light Source (SLS). A calibrated PILATUS module has been characterized with monochromatic synchrotron radiation. The influence of charge sharing on the count rate and the overall energy resolution of the detector were investigated.

Graphene on Ru(0001): a 25 x 25 supercell.

The structure of a single layer of graphene on Ru(0001) has been studied using surface x-ray diffraction. A surprising superstructure containing 1250 carbon atoms has been determined, whereby 25 x 25 graphene unit cells lie on 23 x 23 unit cells of Ru. Each supercell contains 2 x 2 crystallographically inequivalent subcells caused by corrugation.

Conducting interfaces between polar and non-polar insulating perovskites.

Two-dimensional electron gases in semiconductors have found use in applications such as optoelectronics, high-power radio-frequency and magnetoelectronic devices. The ability to grow heterostructures of oxides exhibiting similar effects is a significant step towards the fabrication of all-oxide devices. Here, we give an overview of recent studies of two-dimensional electron gases formed at the interface between polar and non-polar perovskites.

Oxide surfaces.

Although the history of metal oxides and their surfaces goes back several decades to landmark studies, such as Mott and Peierls' explanation of electrical insulation in materials that are predicted in band theory to be conducting, or the observation by Morin of the superfast metal-to-insulator transition in vanadium dioxide, it is only in the last two decades that the world of condensed matter physics has become increasingly dominated by research into complex metal oxides. This has been driven most notably by an attempt to better understand and describe the fundamental physical processes behind their seemingly endless spectrum of properties, which in turn has also led to the discovery of novel phenomena, most prominently demonstrated by the discovery of high-temperature superconductivity in 1986, colossal magnetoresistance in 1994, and most recently, the formation of a two-dimensional conducting layer at the interface between two band insulators in 2004. One important reason why metal oxides, particularly in the form of thin films, have become such a popular subject for basic condensed matter research is that they offer a uniquely versatile materials base for the development of novel technologies.

Production and characterization of Ti:sapphire thin films grown by reactive laser ablation with elemental precursors.

Crystalline Ti:sapphire (Ti:Al(2)O(3)) thin films were grown at low temperatures upon Al(2)O(3) (0001) substrates by reactive crossed-beam laser ablation at 248 nm by use of a liquid Ti-Al alloy target and O(2) . The films were investigated ex situ by x-ray diffraction, x-ray photoelectron spectroscopy, and Rutherford backscattering spectrometry. Low-temperature luminescence was identical to that for Ti(3+) ions in bulk samples of Al(2)O(3) .

Structural basis for the conducting interface between LaAlO3 and SrTiO3.

The complete atomic structure of a five-monolayer film of LaAlO3 on SrTiO3 has been determined for the first time by surface x-ray diffraction in conjunction with the coherent Bragg rod analysis phase-retrieval method and further structural refinement. Cationic mixing at the interface results in dilatory distortions and the formation of metallic La(1-x)SrxTiO3. By invoking electrostatic potential minimization, the ratio of Ti{4+}/Ti{3+} across the interface was determined, from which the lattice dilation could be quantitatively explained using ionic radii considerations.

Surface of strontium titanate.

We report the first complete determination, using surface x-ray diffraction, of the surface structure of TiO2-terminated SrTiO3(001), both at room temperature in vacuum, and also hot, under typical conditions used for thin film growth. The cold structure consists of a mixture of a (1x1) relaxation and (2x1) and (2x2) reconstructions. The latter disappear over several minutes upon heating.

Confinement-induced liquid ordering investigated by x-ray phase retrieval.

Using synchrotron x-ray diffraction, we have determined the ensemble-averaged density profile of colloidal fluids within confining channels of different widths. We observe an oscillatory ordering-disordering behavior of the colloidal particles as a function of the channel width, while the colloidal solution remains in the liquid state. This phenomenon has been suggested by surface force studies of hard-sphere fluids and also theoretically predicted, but here we see it by direct measurements of the structure for comparable systems.