Recherche sur le Covid-19

We employ grazing-incidence femtosecond x-ray diffraction to characterize the coherent, femtosecond laser-induced lattice motion of a bismuth crystal as a function of depth from the surface with a temporal resolution of ~200 fs. The data show direct consequences on the lattice motion from carrier diffusion and electron-hole interaction, allowing us to estimate the effective diffusion rate for the highly excited carriers and the electron-hole interaction time.

We introduce a coherent diffractive imaging technique that utilizes multiple exposures with modifications to the phase profile of the transmitted wave front to compensate for the missing phase information.

A type of X-ray imaging that shows detail otherwise lost, and which is compatible with conventional radiography instrumentation is now feasible, reports a study published online in Nature Materials. This technique offers unprecedented resolution for several applications, including medical imaging, security screening and industrial non-destructive testing.

The typical time scale of atomic motion during fundamental physical processes such as phase transitions in solids or molecular dynamics in chemical reactions ranges from ten to hundreds of femtoseconds. The direct observation of these processes on an atomic length scale therefore requires utrashort light pulses at wavelengths capable of resolving the underlying atomic structures.

The typical time scale of atomic motion during fundamental physical processes such as phase transitions in solids or molecular dynamics in chemical reactions ranges from ten to hundreds of femtoseconds. The direct observation of these processes on an atomic length scale therefore requires utrashort light pulses at wavelengths capable of resolving the underlying atomic structures.

In 2004, it was discovered that when a layer of LaAlO3 (LAO) is in contact with a layer of SrTiO3 (STO), an ultrathin layer of highly conducting material is formed where they contact one another, despite the fact that both LAO and STO are insulators. The underlying physics responsible for this phenomenon is still much disputed, despite a worldwide concerted research drive since then to explain it.

In this story of TiSe2, experiment and theory meet to provide an explanation for a long-standing enigma. In this system, the electrons rearrange themselves spontaneously at low temperature, resulting in a new periodicity from that of the original lattice. This phase change is driven by a decrease in the total energy of the system. However, the nature of this transition has been a matter of controversy for a long time.

In 2004 researcher discovered that a single crystalline metal is stronger when the sample volume is reduced to the micron or even submicron range. In an ongoing debate on the origin of this phenomenon classical deformation theories are questioned. The suspicion that structural defects, i.e. deviations from perfect crystalline structure would play an important role in the smaller is stronger effect, could not be verified because of the lack of an appropriate measuring technique.

Using x-rays scientists have learned to make the invisible visible. Since almost 100 years doctors use the difference in x-ray absorption between bones and tissue to diagonose their patients.

Nature has found remarkable ways to protect sensitive objects. One example is the seed of a nut which is protected by its shell. Another are viruses from the cypovirus family. They are hidden inside tiny natural crystals where they can survive harsh conditions until they meet their target, the gut of the silkworm. Here the virus is released from the crystal causing a virus infection of the worm. Researchers have now unraveled the structure of these natural protein crystals.

It is known since a long time that x-rays, which are nothing but light of very short wave length, can be used for microscopy. This is particularly attractive because due to the small wave length x-rays allow studying objects which are invisibly small in an optical microscope.

Although only recently discovered, the fossil record of embryonic development has already begun to challenge cherished hypotheses on the origin of major animal groups. Synchrotron-based X-ray Tomographic Microscopy has provided unparalleled insight into the anatomy and preservation of these fossil remains and this has allowed us to test competing hypotheses on their nature.

Neutrons are usually considered as small massive particles with a size of about 10^-15 meters. Due to the wave-particle duality of quantum mechanics, however, they can equivalently be considered as matter wave packets whose spatial extent may be large enough to show interference effects similar to what can be observed with visible laser light.

When growing thin films of novel materials, smooth surfaces are a must. How else could one stack them layer by layer, as needed in optical coatings, sensors or conductors? One method known to produce atomically smooth films is Pulsed Laser Deposition (PLD).

X-ray radiographic absorption imaging is an invaluable tool in medical diagnostics and materials science. For biological tissue samples, polymers, or fiber composites, however, the use of conventional X-ray radiography is limited due to their weak absorption. This is resolved at highly brilliant X-ray synchrotron or micro-focus sources by using phase-sensitive imaging methods to improve contrast.

More than 20 years ago researchers in Switzerland discovered that certain materials transport electrical current without any loss. For this they need to be cooled, but because they do so at relatively high temperatures (up to -150°C) they are called high temperature superconductors. How exactly the electrons transport current in such materials is still a mystery. But the electrons can be studied using the photoelectric effect where light of high energy knocks an electron out of the material.