The effects of lateral halogen substituents on the low-temperature cybotactic nematic phase in oxadiazole based bent-core liquid crystals

We have previously demonstrated that the incorporation of lateral methyl groups on oxadiazole-based liquid crystals leads to relatively low-temperature cybotactic nematic phases which, in some cases, supercool to room temperature. We report here the synthesis and phase behaviour of related compounds that possess lateral halogen groups and in some cases, lateral methyl groups as well. Derivatives with three lateral substituents (one halogen and two methyl groups) in a specific pattern supercool in the nematic phase to room temperature. As was the case with the previously reported trimethylated derivatives, the low-temperature nematic phase is glassy in nature. Two of the new trisubstituted derivatives (with bromo and chloro groups) remain in the nematic phase upon subsequent heating until transitioning to the isotropic phase indicating that the low-temperature nematic phase may be more stable than that shown by the trimethylated analogue. Preliminary X-ray diffraction analysis confirms the presence of a tilted cybotactic nematic phase. In addition, the splitting observed in the wide-angle scattering feature is indicative of enhanced local biaxial packing.     

Insights into Biaxial Ordering of Bent-Core Nematics: X-Ray Diffraction Evidence

Recently we have reported the first X-ray diffraction evidence of biaxial order in the fluid cybotactic nematic phase of a pair of trimethylated bent-core mesogens. This evidence is based on the splitting of the wide-angle diffuse scattering in the plane normal to the nematic director. The additional experimental data presented here provide further insights into this unprecedented behavior observed over the entire nematic phase. Although we could not determine the spatial extent of biaxial order—intracluster or intercluster—our observations reveal an enhanced biaxial orientational correlation in the transverse molecular packing, possibly reflecting stronger anisotropic interactions between nearest-neighbor mesogens.     

Sustained oscillations in GaInP/AlGalnP visible diode lasers

We report the observation of sustained oscillations and pulsation behaviour in various GaInP/AlGaInP 670 nm diode lasers. We present experimental dependences of the oscillation strength and frequency as functions of diode structure, temperature and drive current. We explore the resonant excitation of this oscillation using both an r.f. current signal and a tunable-repetition-rate picosecond laser source. When a diode laser with a weak internal oscillation is coupled to an external cavity, strong pulsing behaviour is observed, and we attribute this behaviour to self-mode-locking.     

Evidence for narrow bound states related to the pp system

The γ-ray spectrum accompanying the p$\text{p}$ annihilation at rest has been measured with a large NaI(Tl) spectrometer. Three γ-lines have been observed with energies of 183 ± 7 MeV, 216 ± 9 MeV, and 420 ± 17 MeV at confidence levels of 99.0%, 97.5%, and 98.2%, respectively. The width of the lines corresponds to the detector resolution.     

π0 and η spectra fromp\bar p annihilations at rest

The low-energy part of theπ ⁰ spectrum associated with \(p\bar p\) annihilation at rest was measured in order to search for bound baryonium-like states. The upper limit for reaching such states via the emission of monochromaticπ ⁰'s was found to be 8% per annihilation in the mass region of 1650 MeV. The low-energy part of the η spectrum from \(p\bar p\) annihilations at rest was also observed.     

Precision measurement of the muonic 5-4 transitions in Pb and 4-3 transitions in ba as a test for the validity of QED

The real parts of the optical model potentials for 104 MeV alpha-particle and 156 MeV⁶Li ion scattering from^{40, 48}Ca are calculated in terms of folding model approaches. The validity of different procedures is tested by comparing the differential cross section predictions with experimental data measured with high angular accuracy. It is found that a refined folding potential accounting for density dependence of an effective nucleon-nucleon interaction is appropriate for alpha particle scattering without any parameter adjustment. However, for⁶Li ion scattering renormalization of the depth of the real potential is necessary.     

Measurement of the 4f strong interaction level width in light antiprotonic atoms

The yields of the atomic 4→3 transitions in antiprotonic¹⁴N,^16,17,18O,¹⁹F, and²³Na were measured at the CERN antiproton facility, LEAR. From these, the widths Γ_up of the 4f levels were determined to be 136±19meV (¹⁴N); 603±22 meV (¹⁶O); 731±35 meV (¹⁷O); 795±23 meV (¹⁸O); 2.79±0.16 eV (¹⁹F); and 23.8±7.4 eV (²³Na).     

Remeasurement of the magnetic moment of the antiproton

A high-precision measurement of the finestructure splitting in the circular 11→10 X-ray transition of \(\bar p^{208} Pb\) was performed. The experimental value of 1199(5) eV is in agreement with QED calculations. From that value the magnetic moment of the antiproton was deduced to be ?2.8005(90)μ _nucl. With this result the uncertainty of the previous world average value was reduced by a factor of ≈2. A comparison with the corresponding quantity of the proton now yields: \({{\left( {\mu _p - \left| {\left\langle {\mu _{\bar p} } \right\rangle } \right|} \right)} \mathord{\left/ {\vphantom {{\left( {\mu _p - \left| {\left\langle {\mu _{\bar p} } \right\rangle } \right|} \right)} {\mu _p }}} \right. \kern-0em} {\mu _p }} = \left( { - 2.4 \pm 2.9} \right) \times 10^{ - 3} \) .     

Intensities and strong interaction attenuation of kaonic X-rays

Relative intensities of numerous kaonic X-ray transitions have been measured for the elements C,P,S, and C1, from which level widths due to the strong K-nucleus absorption have been determined. From these and earlier published data, optical potential parameters have been derived and possible consequences on the nuclear matter distribution are discussed.