Scientific Highlights
Coherent structural dynamics of a prototypical Charge-Density-Wave-to-Metal transition
In so called charge-density-wave compounds, the peculiar shape of the Fermi surface as well as electron-phonon coupling lead to a low-temperature broken symmetry ground state. This state is characterized by a modulation of the charge density (hence the name) and, via electron-phonon coupling, a distortion of the equilibrium lattice positions.
Origin of Anomalous Slip in Tungsten
Low-temperature deformation of body-centered cubic metals shows a significant amount of plastic slip on planes with low shear stresses, a phenomenon called anomalous slip. Despite progress in atomistic modeling of the consequences of complex stress states on dislocation mobility, the phenomenon of anomalous slip remained elusive. Using in situ Laue microdiffraction and discrete dislocation dynamics in micrometer sized tungsten single crystals, we demonstrate the occurrence of significant anomalous slip. It occurs as a consequence of cross kinks, topological configurations generated by prior dislocation interactions.
Identification of transitions between liquid water and ice with dual spectrum neutron imaging
The ability to start up at sub-zero Celsius temperatures is a prerequisite for the use of fuel cells in automotive applications, but specific measures need to be taken to prevent the product water to freeze and block the gas supply pathways. In this context, a method for imaging the distribution of liquid water and ice from neutron imaging experiments was developed.
First beam from the SwissFEL electron gun
The new 3 GHz photocathode gun will provide the electron bunches for SwissFEL and has recently been installed in the SwissFEL injector test facility. There, it replaced the CTF2-gun 5, borrowed from CERN. The new gun is capable now of operation with 100Hz repetition frequency and a higher field on cathode and improved field symmetry. After RF conditioning of about 4 days, the gun reached the nominal acceleration gradient of 100 MV/m at an input power of about 17 MW and pulse-width of 1 microsecond.
Muon Cooling: Longitudinal Compression
A 10 MeV/c positive muon beam was stopped in helium gas of a few mbar in a magnetic field of 5T. The muon 'swarm' has been efficiently compressed from a length of 16cm down to a few mm along the magnetic field axis (longitudinal compression) using electrostatic fields. The simulation reproduces the low energy interactions of slow muons in helium gas. Phase space compression occurs on the order of microseconds, compatible with the muon lifetime of 2μs. This paves the way for the preparation of a high- quality low-energy muon beam, with an increase in phase space density relative to a standard surface muon beam of 107. The achievable phase space compression by using only the longitudinal stage presented here is of the order of 104.
Understanding the μSR spectra of MnSi without magnetic polarons
Transverse-field muon-spin rotation (μSR) experiments were performed on a single crystal sample of the noncentrosymmetric system MnSi. The observed angular dependence of the muon precession frequencies matches perfectly the one of the Mn-dipolar fields acting on the muons stopping at a 4a position of the crystallographic structure. The data provide a precise determination of the magnetic dipolar tensor. In addition, we have calculated the shape of the field distribution expected below the magnetic transition temperature TC at the 4a muon site when no external magnetic field is applied.
Controlling Magnetic Order and Quantum Disorder in Molecule-Based Magnets
We investigate the structural and magnetic properties of two molecule-based magnets synthesized from the same starting components. Their different structural motifs promote contrasting exchange pathways and consequently lead to markedly different magnetic ground states. Through examination of their structural and magnetic properties we show that [Cu(pyz)(H2O)(gly)2](ClO4)2 may be considered a quasi-one- dimensional quantum Heisenberg antiferromagnet whereas the related compound [Cu(pyz)(gly)](ClO4), which is formed from dimers of antiferromagnetically interacting Cu2+ spins, remains disordered down to at least 0.03 K in zero field but shows a field-temperature phase diagram reminiscent of that seen in materials showing a Bose-Einstein condensation of magnons.
On the protonation of water
Imaging photoelectron photoion coincidence (iPEPICO) spectroscopy on isolated water molecules and water dimers establishes a new route to determining the water proton affinity (PA) with unprecedented accuracy. A floating thermochemical cycle constructed from the OH+ and H3O+ appearance energies and three other spectroscopic values establishes the water PA as 683.22 ± 0.25 kJ mol−1 at 0 K, which converts to 688.81 ± 0.25 kJ mol−1 at room temperature.
Lattice-Site-Specific Spin Dynamics in Double Perovskite Sr2CoOsO6
Magnetic properties and spin dynamics have been studied for the structurally ordered double perovskite Sr2CoOsO6. Neutron diffraction, muon-spin relaxation, and ac-susceptibility measurements reveal two antiferromagnetic (AFM) phases on cooling from room temperature down to 2 K. In the first AFM phase, with transition temperature TN1=108K, cobalt (3d7, S=3/2) and osmium (5d2, S=1) moments fluctuate dynamically, while their average effective moments undergo long-range order. In the second AFM phase below TN2=67K, cobalt moments first become frozen and induce a noncollinear spin-canted AFM state, while dynamically fluctuating osmium moments are later frozen into a randomly canted state at T≈5K.
Chloroplast remodeling during state transitions in green algae
In oxygenic photosynthesis photochemical reactions occur in two different photosystems (PSs) and the light-energy conversion is regulated by balancing their activity. Such a power balance requires a sophisticated regulatory mechanism called state transitions, which involve reversible phosphorylation of the light-harvesting complex proteins (LHCIIs) to redistribute absorbed excitation energy between the two photosystems.