X-Ray Diffraction Studies on Alicyclic Saturated Discotic Liquid Crystals

The discotic liquid crystalline phase of a new class of disc-like molecules (hexa-O-alkanoylscyllo- inositoles) is studied by small angle X-ray scattering, DSC and optical observations. Comparison of these three methods allows the determination of the structural arrangement. X-ray film exposures of oriented samples clearly demonstrate the hexagonal columnar ordered type of mesophases. A simple model calculation shows that the alkyl chains of neighbouring columns interpenetrate each other to a small extent

Alpha-band rhythms in visual task performance: phase-locking by rhythmic sensory stimulation

Oscillations are an important aspect of neuronal activity. Interestingly, oscillatory patterns are also observed in behaviour, such as in visual performance measures after the presentation of a brief sensory event in the visual or another modality. These oscillations in visual performance cycle at the typical frequencies of brain rhythms, suggesting that perception may be closely linked to brain oscillations. We here investigated this link for a prominent rhythm of the visual system (the alpha-rhythm, 8-12 Hz) by applying rhythmic visual stimulation at alpha-frequency (10.6 Hz), known to lead to a resonance response in visual areas, and testing its effects on subsequent visual target discrimination. Our data show that rhythmic visual stimulation at 10.6 Hz: 1) has specific behavioral consequences, relative to stimulation at control frequencies (3.9 Hz, 7.1 Hz, 14.2 Hz), and 2) leads to alpha-band oscillations in visual performance measures, that 3) correlate in precise frequency across individuals with resting alpha-rhythms recorded over parieto-occipital areas. The most parsimonious explanation for these three findings is entrainment (phase-locking) of ongoing perceptually relevant alpha-band brain oscillations by rhythmic sensory events. These findings are in line with occipital alpha-oscillations underlying periodicity in visual performance, and suggest that rhythmic stimulation at frequencies of intrinsic brain-rhythms can be used to reveal influences of these rhythms on task performance to study their functional roles

Roadmap on photonic metasurfaces

Funding: C.R. and U.L. acknowledge support through the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) under Germany’s Excellence Strategy via the Excellence Cluster 3D Matter Made to Order (EXC-2082/1, Grant No. 390761711). A.B.E. acknowledges support through the Cluster of Excellence PhoenixD (EXC 2122, Project ID No. 390833453). I.F.-C. and C.R. acknowledge support through the CRC Waves: Analysis and Numerics (SFB 1173, Grant No. 258734477. K.A. acknowledges funding from the Swiss National Science Foundation (Project No. PZ00P2_193221).Here we present a roadmap on Photonic metasurfaces. This document consists of a number of perspective articles on different applications, challenge areas or technologies underlying photonic metasurfaces. Each perspective will introduce the topic, present a state of the art as well as give an insight into the future direction of the subfield.Peer reviewe

Rietveld analysis for energy dispersive x-ray diffraction under high pressure with synchrotron radiation

A new program has been developed for the conversion of energy-dispersive x-ray diffraction spectra obtained from powder samples at high pressure in a diamond anvil cell (DAC) into conventional pseudo angle-dispersive data. The program is compatible with a conventional Rietveld program. This allows the determination of the structural parameters of the samples investigated. Results of a synchrotron radiation study of polycrystalline SrFCl in the tetragonal phase at high pressure are presented

Texture-sensitive energy dispersive X-ray diffraction on powders at high pressure with synchrotron radiation

An instrument is described which allows for texture-sensitive energy dispersive X-ray diffraction on powders with the use of diamond anvil cells for the generation of high pressures. The instrument is primarily adapted to the use of synchrotron radiation and includes as special features a eucentric cradle carrying a conical slit collimator for the scattered radiation with variable slit width and a high-resolution four-channel solid state detector which allows polarization-independent intensity collection in very short times with very small samples or with high spatial resolution

Pressure and temperature dependence of the Eu valence in EuNi2Ge2EuNi_2Ge_2 and related systems studied by Mössbauer effect, X-ray absorption and X-ray diffraction

The effect of pressure and temperature on the valence state of Eu in EuNi2Ge2 systems and the related systems EuNi2Si2, EuNi2Si0.5Ge1.5 and EuPl2Ge2 was studied by 151Eu-Mössbauer spectroscopy, EuLIII X-ray absorption and X-ray diffraction. Pressure-induced valence transitions from a divalent towards a trivalent state were observed in EuNi2Ge2 around 5 GPa and in the quasi-ternary EuNi2Si0.5Ge1.5 system around 0.5 GPa. In EuNi2Ge2 we found from Mössbauer spectra measured at various pressures and temperatures a new type of valence transition, where an initially homogeneous valence state near to divalency segregates into two different valent states. From these data we derive for EuNi2Ge2 the pressure and temperature dependence of the Eu valence and magnetism