Forschung zu Covid-19

Discovering fundamentally new ways to manipulate magnetic spins is crucial for research into advanced technologies. Magnetic Skyrmions, which are topologically stable whirls of magnetic spins, are promising candidates for new device components since those found in metallic host materials can be manipulated using electric currents.

The interaction with light weakens the superconducting ground state in classical superconductors. The situation in cuprate superconductors is more complicated: illumination increases the charge carrier density, a photo-induced effect that persists below room temperature. Furthermore, systematic investigations in underdoped YBa₂Cu₃O_6+x (YBCO) have shown an enhanced critical temperature T_c. Until now, studies of photo-persistent conductivity (PPC) have been limited to investigations of structural and transport properties, as well as the onset of superconductivity.

The need for reducing the operating temperature of solid-oxide fuel cells (SOFCs) imposed by cost reduction has pushed significant progress in fundamental understanding of the individual components, as well as materials innovation and device engineering. Proton-conducting oxides have emerged as potential alternative electrolyte materials to oxygen-ion conducting oxides for operation at low and intermediate temperatures.

Plasma chemistry and scattering strongly affect the congruent, elemental transfer during pulsed laser deposition of target metal species in an oxygen atmosphere. Studying the plasma properties of La_0.6Sr_0.4MnO₃, we demonstrate for as grown La_0.6Sr_0.4MnO_3-δ films that a congruent transfer of metallic species is achieved in two pressure windows: ∼10⁻³ mbar and ∼2 × 10⁻¹ mbar.

Using angle-resolved photoemission spectroscopy, we show that the recently discovered surface state on SrTiO₃ consists of nondegenerate t_2g states with different dimensional characters.

Strontium titanate, SrTiO3, is an important material for the realization of next-generation electronic devices. A famous example is the interface of LaAlO3 grown on SrTiO3, which is metallic and magnetic at its interface, even though the individual compounds are insulating and nonmagnetic in bulk form. The physics behind how novel interface states form on SrTiO3 - and how they become endowed with such surprising properties - is not well understood.

EuTiO₃, which is a G-type antiferromagnet below T_N = 5.5 K, has some fascinating properties at high temperatures, suggesting that macroscopically hidden dynamically fluctuating weak magnetism exists at high temperatures. This conjecture is substantiated by magnetic field dependent magnetization measurements, which exhibit pronounced anomalies below 200 K becoming more distinctive with increasing magnetic field strength. Additional results from muon spin rotation experiments provide evidence for weak fluctuating bulk magnetism induced by spin-lattice coupling which is strongly supported in increasing magnetic field.

Partially amorphous La_0.6Sr_0.4CoO_3-δ (LSC) thin-film cathodes are fabricated using pulsed laser deposition and are integrated in free-standing micro-solid oxide fuel cells (micro-SOFC) with a 3YSZ electrolyte and a Pt anode. A low degree of crystallinity of the LSC layers is achieved by taking advantage of the miniaturization of the cells, which permits low-temperature operation (300–450 °C).

The temperature dependence of the gapped triplet excitations (triplons) in the 2D Shastry-Sutherland quantum magnet SrCu₂(BO₃)₂ is studied by means of inelastic neutron scattering. The excitation amplitude rapidly decreases as a function of temperature, while the integrated spectral weight can be explained by an isolated dimer model up to 10 K.

The smaller pixel size, high frame rate, and high dynamic range of next-generation photon counting pixel detectors expedites measurements based on coherent diffractive imaging (CDI). The latter comprises methods that exploit the coherence of X-ray synchrotron sources to replace imaging optics by reconstruction algorithms. Researchers from the Paul Scherrer Institut have recently demonstrated fast CDI image acquisition above 25,000 resolution elements per second using an in-house developed Eiger detector. This rate is state of the art for diffractive imaging and even on a par with the fastest scanning X-ray transmission instruments. High image throughput is of crucial importance for both materials and biological sciences for studies with representative population sampling.

The exploration of the interaction of structural and electronic degrees of freedom in strongly correlated electron systems on the femtosecond time scale is an emerging area of research. One goal of these studies is to advance our understanding of the underlying correlations, another to find ways to control the exciting properties of these materials on an ultrafast time scale.

The exploration of the interaction of structural and electronic degrees of freedom in strongly correlated electron systems on the femtosecond time scale is an emerging area of research. One goal of these studies is to advance our understanding of the underlying correlations, another to find ways to control the exciting properties of these materials on an ultrafast time scale. So far a general model is lacking that provides a quantitiative description of the correlations between the structural and electronic degrees of freedom.

Large negative oxygen-isotope (¹⁶O and ¹⁸O) effects (OIEs) on the static spin-stripe-ordering temperature T_so and the magnetic volume fraction V_m were observed in La_2−xBa_xCuO₄(x=1/8) by means of muon-spin-rotation experiments. The corresponding OIE exponents were found to be α_Tso=-0.57(6) and α_Vm=-0.71(9), which are sign reversed to α_TC=0.46(6) measured for the superconducting transition temperature T_c. This indicates that the electron-lattice interaction is involved in the stripe formation and plays an important role in the competition between bulk superconductivity and static stripe order in the cuprates.

Topological Kondo insulators have been proposed as a new class of topological insulators in which non-trivial surface states reside in the bulk Kondo band gap at low temperature due to strong spin–orbit coupling. In contrast to other three-dimensional topological insulators, a topological Kondo insulator is truly bulk insulating. Furthermore, strong electron correlations are present in the system, which may interact with the novel topological phase. By applying spin- and angle-resolved photoemission spectroscopy, here we show that the surface states of SmB6 are spin polarized. The spin is locked to the crystal momentum, fulfilling time reversal and crystal symmetries.

Using the small angle neutron scattering (SANS) technique we investigated the vortex lattice (VL) in the mixed state of the stannide superconductor Yb₃Rh₄Sn₁₃. We find a single domain VL of slightly distorted hexagonal geometry for field strengths between 350 and 18 500 G and temperatures between T=0.05 and 6.5 K. We observe a clear in-plane rotation of the VL for different magnetic field directions relative to the crystallographic axes.

Neutron inelastic scattering has been used to probe the spin dynamics of the quantum (S=1/2) ferromagnet on the pyrochlore lattice Lu₂V₂O₇. Well-defined spin waves are observed at all energies and wave vectors, allowing us to determine the parameters of the Hamiltonian of the system.

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.

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.

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.