The relationship between the chemical structure of nematic liquid crystals and their pretilt angles

About 50 different additives in one or more of three different nematic mixtures have been investigated to clarify the relationship between the chemical structure of the liquid crystal and the pretilt angle on a polyimide surface. The pretilts found for cells have been explained within our recently proposed population distribution model. For compounds with cyano-groups at one end, we find that the in-plane order is governed both by the surface-mesogen interaction and by the relative strength of the intermolecular interactions in the nematic phase. This strength is nearly linear in alkyl chain length for the compounds investigated. Changes in the strength due to variations in the core of the molecules can be calculated easily by using group contributions from the known Parachors. The in-plane order can be treated as a simple product of the contributions from the liquid crystal and from the polyimide. Different polar end groups will give different angles between the surface and the optical axis of the individual mesogens in the first monolayer. The cyano-group gives the highest angle and alkyl groups the lowest. For nitro-compounds the dimers formed are so strongly bound that they do not break up at the surface. Nitro-compounds will thus act as dialkyl compounds. For dialkyl compounds the pretilt angles are dominated by the difference between the chain lengths at the two ends of the molecule.     

Optical pumping and a nondestructive readout of a single magnetic impurity spin in an InAs/GaAs quantum dot

We report on the resonant optical pumping of the | ± 1? spin states of a single Mn dopant in an InAs/GaAs quantum dot which is embedded in a charge tunable device. The experiment relies on a W scheme of transitions reached when a suitable longitudinal magnetic field is applied. The optical pumping is achieved via the resonant excitation of the central Λ system at the neutral exciton X(0) energy. For a specific gate voltage, the redshifted photoluminescence of the charged exciton X- is observed, which allows a nondestructive readout of the spin polarization. An arbitrary spin preparation in the | + 1? or |-1? state characterized by a polarization near or above 50% is evidenced.     

A standardised test environment to compare aircraft noise calculation programs

A new standardised test environment for aircraft noise calculation programs is presented. By means of this test environment different programs for aircraft noise calculation can be compared systematically. For this purpose a complete data set was developed, containing all relevant information needed to perform unambiguous aircraft noise calculations in the vicinity of an artificial airport. The input data is provided in a generic, non-program-specific form in order to be applicable to programs based on different calculation methods and data structures. In addition to the input data, guidelines on the file format of the results of calculations performed with different programs are specified to facilitate direct comparisons. The test environment and its data structure are outlined and examples of calculations according to this scheme are presented.     

Neutral strange particle exclusive production in charged current high-energy antineutrino interactions

The cross section of the quasi-elastic reactions \(\bar v_\mu p \to \mu ^ + \Lambda (\Sigma ^0 )\) in the energy range 5–100 GeV is determined from Fermilab 15′ bubble chamber antineutrino data. TheQ ² analysis of quasi-elastic Λ events yieldsM _A=1.0±0.3 GeV/c² for the axial mass value. With zero µΛ K ⁰ events observed, the 90% confidence level upper limit \(\sigma (\bar v_\mu p \to \mu ^ + \Lambda {\rm K}^0 )< 2.0 \cdot 10^{ - 40} cm^2 \) is obtained. At the same time, we found that the cross section of reaction \(\bar v_\mu p \to \mu ^ + \Lambda {\rm K}^0 + m\pi ^0 \) is equal to \(\left( {3.9\begin{array}{*{20}c} { + 1.6} \\ { - 1.3} \\ \end{array} } \right) \cdot 10^{ - 40} cm^2 \) .     

Optically probing the fine structure of a single Mn atom in an InAs quantum dot

We report on the optical spectroscopy of a single InAs/GaAs quantum dot doped with a single Mn atom in a longitudinal magnetic field of a few Tesla. Our findings show that the Mn impurity is a neutral acceptor state A0 whose effective spin J=1 is significantly perturbed by the quantum dot potential and its associated strain field. The spin interaction with photocarriers injected in the quantum dot is shown to be ferromagnetic for holes, with an effective coupling constant of a few hundreds of mueV, but vanishingly small for electrons.