Lab News & Scientific Highlights

In March 2018, the nine-week MOOC “Introduction to synchrotrons and x-ray free-electron lasers” (abbreviated to “SYNCHROTRONx”) came online via the edX provider of the École Polytechnique Fédérale de Lausanne (EPFL), created by Phil Willmott of the Swiss Light Source, Paul Scherrer Institute. “MOOC” is an acronym for “massive open online course”, a teaching platform started in the first decade of this century, which has become increasingly popular in the last five to six years. MOOCs have no limits to participation and are free. Some of the most popular MOOCs can attract many tens of thousands of participants. Even the most specialized subjects may have an initial enrollment of over a thousand, more than an order of magnitude larger than that typically found in traditional higher education. There were over 70 million MOOC enrollments covering nearly 10’000 subjects offered by the top five providers in 2017 alone!

A peculiar magnetic “dead layer” is detected at the surface of thin films of DyTiO3, a ferrimagnetic Mott insulator. Depth dependent X-ray absorption measurements performed at the X-Treme beamline in the Swiss Light Source indicate that this layer is associated with a deviation of the Ti valence from 3+ toward 4+ at the film surface, suppressing the magnetic coupling between Ti ions and unleashing a strong paramagnetic response from uncoupled Dy ions.

In this work, we have enhanced our originally proposed experiment-modeling synergy in Upadhyay et al. Acta Mat. 2016, to capture the stress evolution in the complex cruciform geometry during arbitrary multi-axial load path changes. We perform cruciform simulations using the implementation of the visco-plastic self-consistent (VPSC) model as a user material (UMAT) into the ABAQUS finite element (FE) solver. We also use the Elasto-viscoplastic fast Fourier transform (EVP-FFT) approach to compute yield surfaces. This experiment-modeling synergy is exploited to understand the mechanical response (including the elastic response, Bauschinger effect and hardening) of 316L stainless steel following biaxial load path changes.

The writing and deletion of magnetic Skyrmions is a fundamental step towards the fabrication of memory devices based on this promising spin configuration. Researchers at the Korea Institute of Technology have demonstrated the writing and deleting of isolated magnetic Skyrmions at room temperature in ferrimagnetic multilayer superlattice stacks using electrical currents.

X-ray Free Electron Lasers (XFELs) combine the properties of synchrotron radiation (short wavelengths) and laser radiation (high lateral coherence, ultrashort pulse durations). These outstanding machines allow to study ultra-fast phenomena at an atomic level with unprecedented temporal resolution for answering the most intriguing open questions in biology, chemistry and physics.

In the week of March 18-23 PSI welcomes 20 PhD students and postdocs taking part in the HERCULES 2018 school on Neutron and Synchrotron Radiation. They will attend lectures and perform two days of practical courses at several beam lines of the Swiss Light Source.

Using the FEMTO slicing source at SLS, we have studied the structural response during the photoinduced transition in a charge-ordered Pr_1-xCa_xMnO₃ thin films. By investigating the dynamics of both superlattice reflections and regular Bragg peaks, we disentangle the different structural contributions and analyze their relevant time-scales. Comparing these results with studies of the charge order and magnetic dynamics, a comprehensive picture of the phase transition linked to a single critical fluence f_c is proposed.

In-situ neutron diffraction studies performed on metastable 201 stainless steel combined with EBSD measurements confirm that ε-martensite is a precursor for α′-martensite during uniaxial and equibiaxial deformation at the same loading rate. In both loading states, the grains that contain martensite belong to orientations for which the leading partial dislocations have higher Schmid factor than the trailing partial dislocations. The martensitic transformation is suppressed during equibiaxial loading as a consequence of the different textures formed during deformation.

Researchers at PSI have established a link between the martensitic transformation, microstructural evolution and the mechanical behavior under multiaxial deformation in a NiTi alloy by using a unique combination of in situ high-resolution Digital Image Correlation (DIC), in situ X-ray diffraction and electron microscopy characterization.

Molecular dynamics simulations of transient stress drops have been carried out in different regimes on a nanocrystalline Aluminum sample with average grain size of 12 nm. Besides confirming the interpretation of experimental results obtained during in situ X-ray diffraction, the creep simulations performed at 2 or 3 orders of magnitude lower strain rates than usual reveal deformation mechanisms that have not been observed previously.

Current-induced spin-orbit torques hold a great potential for manipulation of magnetization at ultrafast timescales. Researchers at ETH Zürich have demonstrated, using time-resolved STXM imaging at the Swiss Light Source, the influence of spin-orbit torques on the switching behaviour of Pt/Co/AlO_x nanostructured elements.

Highly crystalline thin films of organic semiconductors offer great potential for high-performance, low-cost flexible electronics. Researchers at IMEC Belgium have developed a new double-step thin film fabrication process that offers higher performance devices. Soft X-ray spectro-microscopy at the Swiss Light Source was used to prove that the increased performance comes from larger areas of material sharing the same molecular orientation.

Multiaxial mechanical testing of sheet metals is far from trivial, which is mainly related to issues with sample design and fabrication. PSI scientists have developed a new methodology to produce cruciform shaped samples from thin sheet metals based on a novel bottom-up approach. A proof-of-principle experiment based on polymer lamination of an aluminum thin sheet demonstrates the effectiveness of this new approach.

The SPS 2017 Prize in Condensed Matter Physics, sponsored by IBM, has been awarded to Dr. Nan Xu for his excellent work on topological quantum states. Dr. Nan Xu is a joint postdoc of Paul Scherrer Institute (PSI) and the École Polytechnique Fédérale de Lausanne (EPFL).

PSI scientists have developed a new type of X-ray optics that allows for analyzing the emission in resonant inelastic x-ray scattering (RIXS) experiments. The new approach combines the energy dispersion with imaging capabilities. In a collaborative effort with research groups from Göttingen and Hamburg, two new classes of RIXS experiments, energy mapping and RIXS imaging, have been demonstrated.

The advent of x-ray free electron lasers has extended the unique capabilities of resonant x-ray spectroscopy techniques to ultrafast time scales. Here, in collaboration between researchers from PSI, Sorbonne Universités, HASYLAB/DESY, Synchrotron SOLEIL, CNRS, and Uppsala University, we report on a novel experimental method that allows retrieving with a single x-ray pulse the time evolution of an ultrafast process, not only at a few discrete time delays, but continuously over an extended time window.

Cruciform experiments are very useful to study non-proportional strain path change behavior of engineering metals and alloys. This work studies the stress response of 6 prominently used cruciform geometries deformed under tension. Results show that for most of the cruciform samples, the gauge stresses are non-linearly coupled to the applied forces in both arms. Cruciform geometries based on the ISO standard are able to decouple these stresses but negligible gauge plastic strains are reached prior to failure.

Controlled motions of atoms using ultrashort electric field pulses allow to manipulated the properties of a material on ultrafast timescales. Here we show how metallic structures can be used to enhance a THz electric field pulse and track the induced atomic motions with ultrashort x-ray pulses emitted by a X-ray free electron laser.

We employ time-resolved resonant x-ray diffraction to study the insulator-to-metal transition that is launched via resonant excitation of an infrared-active optical phonon mode in a half doped manganite. We find that the charge order reduces promptly with a highly nonlinear (quartic) dependence on excitation fluence.

Metal nanostructures can be fabricated by irradiation of suitable metal organic precursor molecules with a focused X-ray beam. This novel techniques offer the advantage of energy-selective deposition by switching of the incident photon energy due to the non-linear photon absorption cross-section of the precursor molecules for resonant excitation.

For decades, the mechanism of Mott phase in Ca₂RuO₄ has puzzled researchers. This material is a paradigmatic case of multi-band Mott physics including spin-orbit and Hund's coupling. Progress has been impeded by the lack of knowledge about the low-energy electronic structure. With our recent contribution, we provided-- using angle-resolved photoemission electron spectroscopy -- the band structure of the paramagnetic insulating phase of Ca₂RuO₄.

Turning the semimetal graphene into a technologically useful semiconductor is challenging. One way of opening a band gap is to cut graphene into nanometre-wide ribbons, but even atomic-level roughness at the ribbon edges can seriously degrade the mobility of charge carriers. Recent advances in on-surface chemistry have made it possible to obtain graphene nanoribbons with atomically precise edges through direct synthesis from molecular building blocks. Here, we report the synthesis, full structural and electronic characterization of 9-atom wide graphene nanoribbons with significantly improved electronic properties.

Researchers at PSI have developed a new unique miniaturized biaxial deformation rig, which allows to apply in-plane biaxial stress states with arbitrary stress ratios and to perform strain path changes on thin-sheet metals. The device is optimized for in situ usage inside a scanning electron microscope and at synchrotron beam lines.

Single holmium atoms adsorbed on few monolayers of magnesium oxide are extraordinarily stable magnets. They retain a significant fraction of their magnetization when the external magnetic field is switched off. This has been shown recently in a study combining x-ray magnetic circular dichroism performed at the Swiss Light Source (SLS) and at the European Synchrotron Radiation Facility (ESRF) as well as scanning tunneling microscopy. The results open perspectives of storing and processing information at ultrahigh density.

The first example of an insulating phase which is close to the superconducting phase in an iron-pnictide system has been recently observed in heavy Cu-doped NaFe_1-xCuxAs (x > 0.3). A combined study by angle-resolved photoemission spectroscopy (ARPES) and density functional theory (DFT) calculations revealed that on-site Coulomb repulsion and enhanced Hund’s rule coupling are responsible for the insulating behavior. The results show that the insulating phase in NaFe_0.5Cu_0.5As resembles the situation in the parent compounds of the high-T_c cuprate superconductors.

The use of spin-wave signals in future information processing devices can substantially reduce power consumption over present charge current based technologies. As part of an international research venture, scientists at PSI now introduced a concept to generate spin waves with nanoscale wavelengths exploiting the driven dynamics of magnetic vortex cores in magnetic heterostructures.