Correlation of refractive index based and THz streaking arrival time tools for a hard X-ray free-electron laser.

To fully exploit ultra-short X-ray pulse durations routinely available at X-ray free-electron lasers to follow out-of-equilibrium dynamics, inherent arrival time fluctuations of the X-ray pulse with an external perturbing laser pulse need to be measured. In this work, two methods of arrival time measurement were compared to measure the arrival time jitter of hard X-ray pulses. The methods were photoelectron streaking by a THz field and a transient refractive index change of a semiconductor. The methods were validated by shot-to-shot correction of a pump-probe transient reflectivity measurement. An ultimate shot-to-shot full width at half-maximum error between the devices of 19.2 ± 0.1 fs was measured.

Non-equilibrium dynamics of spin-lattice coupling.

Quantifying the dynamics of normal modes and how they interact with other excitations is of central importance in condensed matter. Spin-lattice coupling is relevant to several sub-fields of condensed matter physics; examples include spintronics, high-Tc superconductivity, and topological materials. However, experimental approaches that can directly measure it are rare and incomplete. Here we use time-resolved X-ray diffraction to directly access the ultrafast motion of atoms and spins following the coherent excitation of an electromagnon in a multiferroic hexaferrite. One striking outcome is the different phase shifts relative to the driving field of the two different components. This phase shift provides insight into the excitation process of such a coupled mode. This direct observation of combined lattice and magnetization dynamics paves the way to access the mode-selective spin-lattice coupling strength, which remains a missing fundamental parameter for ultrafast control of magnetism and is relevant to a wide variety of materials.

A compact gas attenuator for the SwissFEL ATHOS beamline realized using additive manufacturing.

Gas attenuators are important devices providing accurate variation of photon intensity for soft X-ray beamlines. In the SwissFEL ATHOS beamline front-end the space is very limited and an innovative approach has been taken to provide attenuation of three orders of magnitude up to an energy of 1200 eV. Additive manufacturing of a differential pumping system vacuum manifold allowed a triple pumping stage to be realized in a space of less than half a meter. Measurements have shown that the response of the device is as expected from theoretical calculations.

Spectral monitoring at SwissFEL using a high-resolution on-line hard X-ray single-shot spectrometer.

The performance and parameters of the online photon single-shot spectrometer (PSSS) at the Aramis beamline of the SwissFEL free-electron laser are presented. The device operates between the photon energies 4 and 13 keV and uses diamond transmission gratings and bent Si crystals for spectral measurements on the first diffraction order of the beam. The device has an energy window of 0.7% of the median photon energy of the free-electron laser pulses and a spectral resolution (full width at half-maximum) ΔE/E on the order of 10-5. The device was characterized by comparing its performance with reference data from synchrotron sources, and a parametric study investigated other effects that could affect the reliability of the spectral information.

Proximity-Induced Novel Ferromagnetism Accompanied with Resolute Metallicity in NdNiO3 Heterostructure.

Employing X-ray magnetic circular dichroism (XMCD), angle-resolved photoemission spectroscopy (ARPES), and momentum-resolved density fluctuation (MRDF) theory, the magnetic and electronic properties of ultrathin NdNiO3 (NNO) film in proximity to ferromagnetic (FM) La0.67 Sr0.33 MnO3 (LSMO) layer are investigated. The experimental data shows the direct magnetic coupling between the nickelate film and the manganite layer which causes an unusual ferromagnetic (FM) phase in NNO. Moreover, it is shown the metal-insulator transition in the NNO layer, identified by an abrupt suppression of ARPES spectral weight near the Fermi level (EF ), is absent. This observation suggests that the insulating AFM ground state is quenched in proximity to the FM layer. Combining the experimental data (XMCD and AREPS) with the momentum-resolved density fluctuation calculation (MRDF) reveals a direct link between the MIT and the magnetic orders in NNO systems. This work demonstrates that the proximity layer order can be broadly used to modify physical properties and enrich the phase diagram of RENiO3 (RE = rare-earth element).

Advances in long-wavelength native phasing at X-ray free-electron lasers.

Long-wavelength pulses from the Swiss X-ray free-electron laser (XFEL) have been used for de novo protein structure determination by native single-wavelength anomalous diffraction (native-SAD) phasing of serial femtosecond crystallography (SFX) data. In this work, sensitive anomalous data-quality indicators and model proteins were used to quantify improvements in native-SAD at XFELs such as utilization of longer wavelengths, careful experimental geometry optimization, and better post-refinement and partiality correction. Compared with studies using shorter wavelengths at other XFELs and older software versions, up to one order of magnitude reduction in the required number of indexed images for native-SAD was achieved, hence lowering sample consumption and beam-time requirements significantly. Improved data quality and higher anomalous signal facilitate so-far underutilized de novo structure determination of challenging proteins at XFELs. Improvements presented in this work can be used in other types of SFX experiments that require accurate measurements of weak signals, for example time-resolved studies.

Transmission measurement at the Bernina branch of the Aramis Beamline of SwissFEL.

The transmission of the optical components of the Bernina branch of the Aramis beamline at SwissFEL has been measured with an X-ray gas monitor from DESY and compared with a PSI gas detector upstream of the optical components. The transmission efficiencies of the Mo, Si and SiC mirror coatings of the Aramis beamline and the various other in-beam components were evaluated and compared with theoretical calculations, showing an agreement of 6% or better in all cases. The experiment has also shown the efficacy of the high-harmonic rejection mirrors at the Bernina branch of the Aramis beamline at SwissFEL, and characterized the transmission efficiency of the on-line spectrometer in the Aramis beamline. The theoretical transmission of the mirror coatings match the experimental data to within 7%. The accuracy of these measurements was checked against a radiative bolometer from a Japanese collaboration and found to agree to a level of 4% or better. Further comparisons with a diamond detector from a US-based inter-institute collaboration demonstrated a good agreement for the attenuator settings of the beamline.

Wavefront sensing at X-ray free-electron lasers.

Here a direct comparison is made between various X-ray wavefront sensing methods with application to optics alignment and focus characterization at X-ray free-electron lasers (XFELs). Focus optimization at XFEL beamlines presents unique challenges due to high peak powers as well as beam pointing instability, meaning that techniques capable of single-shot measurement and that probe the wavefront at an out-of-focus location are desirable. The techniques chosen for the comparison include single-phase-grating Talbot interferometry (shearing interferometry), dual-grating Talbot interferometry (moiré deflectometry) and speckle tracking. All three methods were implemented during a single beam time at the Linac Coherent Light Source, at the X-ray Pump Probe beamline, in order to make a direct comparison. Each method was used to characterize the wavefront resulting from a stack of beryllium compound refractive lenses followed by a corrective phase plate. In addition, difference wavefront measurements with and without the phase plate agreed with its design to within λ/20, which enabled a direct quantitative comparison between methods. Finally, a path toward automated alignment at XFEL beamlines using a wavefront sensor to close the loop is presented.

Experimental station Bernina at SwissFEL: condensed matter physics on femtosecond time scales investigated by X-ray diffraction and spectroscopic methods.

The Bernina instrument at the SwissFEL Aramis hard X-ray free-electron laser is designed for studying ultrafast phenomena in condensed matter and material science. Ultrashort pulses from an optical laser system covering a large wavelength range can be used to generate specific non-equilibrium states, whose subsequent temporal evolution can be probed by selective X-ray scattering techniques in the range 2-12 keV. For that purpose, the X-ray beamline is equipped with optical elements which tailor the X-ray beam size and energy, as well as with pulse-to-pulse diagnostics that monitor the X-ray pulse intensity, position, as well as its spectral and temporal properties. The experiments can be performed using multiple interchangeable endstations differing in specialization, diffractometer and X-ray analyser configuration and load capacity for specialized sample environment. After testing the instrument in a series of pilot experiments in 2018, regular user operation begins in 2019.

SwissFEL Aramis beamline photon diagnostics. Erratum.

The list of authors in the paper by Juranic et al. (2018) [J. Synchrotron Rad. 25, 1238-1248] is corrected.

X-ray induced damage of B4C-coated bilayer materials under various irradiation conditions.

In this report, we analyse X-ray induced damage of B4C-coated bilayer materials under various irradiation geometries, following the conditions of our experiment performed at the free-electron-laser facility SACLA. We start with the discussion of structural damage in solids and damage threshold doses for the experimental system components: B4C, SiC, Mo and Si. Later, we analyze the irradiation of the experimentally tested coated bilayer systems under two different incidence conditions of a linearly polarized X-ray pulse: (i) grazing incidence, and (ii) normal incidence, in order to compare quantitatively the effect of the pulse incidence on the radiation tolerance of both systems. For that purpose, we propose a simple theoretical model utilizing properties of hard X-ray propagation and absorption in irradiated materials and of the following electron transport. With this model, we overcome the bottleneck problem of large spatial scales, inaccessible for any existing first-principle-based simulation tools due to their computational limitations for large systems. Predictions for damage thresholds obtained with the model agree well with the available experimental data. In particular, they confirm that two coatings tested: 15 nm B4C/20 nm Mo on silicon wafer and 15 nm B4C/50 nm SiC on silicon wafer can sustain X-ray irradiation at the fluences up to ~10 µJ/µm2, when exposed to linearly polarized 10 keV X-ray pulse at a grazing incidence angle of 3 mrad. Below we present the corresponding theoretical analysis. Potential applications of our approach for design and radiation tolerance tests of multilayer components within X-ray free-electron-laser optics are indicated.

SwissFEL Aramis beamline photon diagnostics.

The SwissFEL Aramis beamline, covering the photon energies between 1.77 keV and 12.7 keV, features a suite of online photon diagnostics tools to help both users and FEL operators in analysing data and optimizing experimental and beamline performance. Scientists will be able to obtain information about the flux, spectrum, position, pulse length, and arrival time jitter versus the experimental laser for every photon pulse, with further information about beam shape and size available through the use of destructive screens. This manuscript is an overview of the diagnostics tools available at SwissFEL and presents their design, working principles and capabilities. It also features new developments like the first implementation of a THz-streaking based temporal diagnostics for a hard X-ray FEL, capable of measuring pulse lengths to 5 fs r.m.s. or better.

A compact and versatile tender X-ray single-shot spectrometer for online XFEL diagnostics.

One of the remaining challenges for accurate photon diagnostics at X-ray free-electron lasers (FELs) is the shot-to-shot, non-destructive, high-resolution characterization of the FEL pulse spectrum at photon energies between 2 keV and 4 keV, the so-called tender X-ray range. Here, a spectrometer setup is reported, based on the von Hamos geometry and using elastic scattering as a fingerprint of the FEL-generated spectrum. It is capable of pulse-to-pulse measurement of the spectrum with an energy resolution (ΔE/E) of 10-4, within a bandwidth of 2%. The Tender X-ray Single-Shot Spectrometer (TXS) will grant to experimental scientists the freedom to measure the spectrum in a single-shot measurement, keeping the transmitted beam undisturbed. It will enable single-shot reconstructions for easier and faster data analysis.

Opportunities for Chemistry at the SwissFEL X-ray Free Electron Laser.

X-ray techniques have long been applied to chemical research, ranging from powder diffraction tools to analyse material structure to X-ray fluorescence measurements for sample composition. The development of high-brightness, accelerator-based X-ray sources has allowed chemists to use similar techniques but on more demanding samples and using more photon-hungry methods. X-ray Free Electron Lasers (XFELs) are the latest in the development of these large-scale user facilities, opening up new avenues of research and the possibility of more advanced applications for a range of research. The SwissFEL XFEL project at the Paul Scherrer Institute will begin user operation in the hard X-ray (2.1-12.4 keV) photon energy range in 2018 with soft X-ray (240-1930 eV) user operation to follow and here we will present the details of this project, it's operating capabilities, and some aspects of the experimental stations that will be particularly attractive for chemistry research. SwissFEL is a revolutionary new machine that will complement and extend the time-resolved chemistry efforts in the Swiss research community.

THz streak camera method for synchronous arrival time measurement of two-color hard X-ray FEL pulses.

The two-color operation of free electron laser (FEL) facilities allows the delivery of two FEL pulses with different energies, which opens new possibilities for user experiments. Measuring the arrival time of both FEL pulses relative to the external experimental laser and to each other improves the temporal resolution of the experiments using the two-color FEL beam and helps to monitor the performance of the machine itself. This work reports on the first simultaneous measurement of the arrival times of two hard X-ray FEL pulses with the THz streak camera. Measuring the arrival time of the two FEL pulses, the relative delay between them was calculated and compared to the set values. Furthermore, we present the first comparison of the THz streak camera method to the method of FEL induced transient transmission. The results indicate a good agreement between the two methods.

Perspective: Opportunities for ultrafast science at SwissFEL.

We present the main specifications of the newly constructed Swiss Free Electron Laser, SwissFEL, and explore its potential impact on ultrafast science. In light of recent achievements at current X-ray free electron lasers, we discuss the potential territory for new scientific breakthroughs offered by SwissFEL in Chemistry, Biology, and Materials Science, as well as nonlinear X-ray science.

Surface science at the PEARL beamline of the Swiss Light Source.

The Photo-Emission and Atomic Resolution Laboratory (PEARL) is a new soft X-ray beamline and surface science laboratory at the Swiss Light Source. PEARL is dedicated to the structural characterization of local bonding geometry at surfaces and interfaces of novel materials, in particular of molecular adsorbates, nanostructured surfaces, and surfaces of complex materials. The main experimental techniques are soft X-ray photoelectron spectroscopy, photoelectron diffraction, and scanning tunneling microscopy (STM). Photoelectron diffraction in angle-scanned mode measures bonding angles of atoms near the emitter atom, and thus allows the orientation of small molecules on a substrate to be determined. In energy scanned mode it measures the distance between the emitter and neighboring atoms; for example, between adsorbate and substrate. STM provides complementary, real-space information, and is particularly useful for comparing the sample quality with reference measurements. In this article, the key features and measured performance data of the beamline and the experimental station are presented. As scientific examples, the adsorbate-substrate distance in hexagonal boron nitride on Ni(111), surface quantum well states in a metal-organic network of dicyano-anthracene on Cu(111), and circular dichroism in the photoelectron diffraction of Cu(111) are discussed.

Science opportunities at the SwissFEL X-ray Laser.

Next-generation X-ray sources, based on the X-ray Free Electron Laser (XFEL) concept, will provide highly coherent, ultrashort pulses of soft and hard X-rays with peak intensity many orders of magnitude higher than that of a synchrotron. These pulses will allow studies of femtosecond dynamics at nanometer resolution and with chemical selectivity. They will produce diffraction images of organic and inorganic nanostructures without deleterious effects of radiation damage.

Science Opportunities at the SwissFEL X-ray Laser.

Next-generation X-ray sources, based on the X-ray Free Electron Laser (XFEL) concept, will provide highly coherent, ultrashort pulses of soft and hard X-rays with peak intensity many orders of magnitude higher than that of a synchrotron. These pulses will allow studies of femtosecond dynamics at nanometer resolution and with chemical selectivity. They will produce diffraction images of organic and inorganic nanostructures without deleterious effects of radiation damage.

Persistent high-energy spin excitations in iron-pnictide superconductors.

Motivated by the premise that superconductivity in iron-based superconductors is unconventional and mediated by spin fluctuations, an intense research effort has been focused on characterizing the spin-excitation spectrum in the magnetically ordered parent phases of the Fe pnictides and chalcogenides. For these undoped materials, it is well established that the spin-excitation spectrum consists of sharp, highly dispersive magnons. The fate of these high-energy magnetic modes upon sizable doping with holes is hitherto unresolved. Here we demonstrate, using resonant inelastic X-ray scattering, that optimally hole-doped superconducting Ba(0.6)K(0.4)Fe(2)As(2) retains well-defined, dispersive high-energy modes of magnetic origin. These paramagnon modes are softer than, though as intense as, the magnons of undoped antiferromagnetic BaFe(2)As(2). The persistence of spin excitations well into the superconducting phase suggests that the spin fluctuations in Fe-pnictide superconductors originate from a distinctly correlated spin state. This connects Fe pnictides to cuprates, for which, in spite of fundamental electronic structure differences, similar paramagnons are present.