M. Arif

Conformations in dioctyl substituted polyfluorene: A combined theoretical and experimental Raman scattering study

The structural properties of polyfluorenes (PF) are extremely sensitive to the choice of functionalizing side chains. Dioctyl substituted PF (PF8) adopts metastable structures that depend upon the thermal history and choice of solvents used in film forming conditions. We present a detailed study of the changes in the backbone and side chain morphology in PF8, induced by the various crystallographic phases, using Raman scattering techniques. The vibrational frequencies and intensities of fluorene oligomers are calculated using hybrid density-functional theory with a 3-21G(*) basis set. The alkyl side chains are modeled as limiting conformations: all anti, anti-gauche-gauche, and end gauche representations. The calculated vibrational spectra of single chain oligomers in conjunction with our experimental results demonstrate the beta phase, which is known to originate in regions of enhanced chain planarity as a direct consequence of the alkyl side chain conformation.

Neutron interference induced by gravity: New results and interpretations☆

Abstract We report the results of neutron interferometric experiments, extending the range and precision of the COW gravitationally-induced quantum interference experiment of Staudenmann and co-workers. These experiments provide a test of the principle of equivalence in the quantum limit. High precision data (1 part in 1000) is presented. The frequency of the quantum interference oscillations, and the loss of contrast observed as a function of increasing gravitational potential energy difference are compared with the recent interferometer dynamical diffraction calculations of Bonse and Wroblewski and of Horne. Theory and experiment are found to differ by 0.8%.

Fizeau effect for neutrons passing through matter at a nuclear resonance

Abstract We have measured the phase shift of a neutron de Broglie wave induced by the motion of Sm-149 through which the neutron is propagating. This experiment differs from a number of ‘neutron Fizeau’ experiments carried out in recent years. The observed phase shift is caused by the motion of the matter itself, and not by the motion of its boundaries, as was the case with the earlier experiments. In this regard our experiment is a true neutron analog of the famous Fizeau experiment of 1859, where phase shift of the light wave interference pattern was induced by the motion of matter.

Full rotation gravitationally induced quantum interference experiment

We present here a preliminary description of second generation neutron interferometry experiments in which quantum interference is induced by the gravitational field of the Earth. The interferometer is mounted in such a way that it can be rotated a full 360 degrees about the incident beam. In situ X-ray experiments are also carried out to monitor the effects of bending of the perfect Si crystal LLL device. We are studying in detail the contrast variations as a function of rotation.

Neutron interferometry investigation of the Aharonov-Casher effect☆

Abstract This paper describes our progress on a neutron interferometry search for the Aharonov-Casher (A-C) effect. Unpolarized neutrons are passed through a 40 kV/mm vacuum electrode system. The spin-dependent phase is set to maximum sensitivity with a magnetic field, and the spin-independent phase is adjusted to zero (modulo 2π) using the Earths gravitational field. Upon reversal of the electric field, the predicted A-C phase shift is 0.2°. Currently, our results are statistically limited.

The study of the electrical charge distribution inside the neutron by neutron scattering methods

The neutron scattering cross section contains significant information about the internal electrical structure of the neutron. Recent achievements in neutron interferometry allow the precision measurements of the neutron scattering length (Δb/b∼10−5) at the crucial point E∼0, where the cross-section measurements are strongly influenced by small-angle scattering at the sample and are not very reliable.

Neutron interferometric observation of the topological Aharonov-Casher effect

Abstract The Aharonov-Casher effect has been observed experimentally using a single crystal neutron interferometer.

Mathematical methods in the solution of the Hamilton-Darwin and the Takagi-Taupin equations☆

Abstract The diffraction of neutrons by a single crystal is intrinsically a multiple scattering problem. For an ideally imperfect mosaic crystal the Hamilton-Darwin transfer equations describe the coupling of the incident and diffracted beams; whereas, for a perfect crystal one must use the dynamical theory of diffraction, which can be recast in the form of two coupled partial differential equations commonly referred to as the Takagi-Taupin equations. From a mathematical point of view these two problems are equivalent, although the physical manifestations of the solutions are quite different. For the occasion of Professor Shulls seventieth birthday celebration, we bring together in this paper some of the mathematical techniques which we have found useful in elucidating the subtleties of the Bragg diffraction of neutron by crystals.

Does quantum entanglement invalidate contrast variation practices in neutron scattering

Abstract A surprising observation of the anomalous deep inelastic neutron scattering from liquid H2O–D2O mixtures was reported recently in PRL. It has also been suggested that such anomalous scattering can manifest itself as a deviation of the neutron scattering length density Nb from the conventional theory, based upon the assumption that the molecular volumes of the constituents are additive in the mixture. For H2O–D2O mixtures deviations as much as 5–10% are predicted from the conventionally expected value Nb. This fact would result in serious consequences for the widely used technique of contrast variation in neutron scattering. We have used an advanced neutron interferometry technique in order to verify these predictions and confirmed the experimental practice of isotopic contrast variation with high precision.

Neutron interferometric measurement of the topological Aharonov‐Casher phase shift

We have measured the phase shift predicted by Aharonov and Casher (AC) for a magnetic dipole diffracting around a charged electrode for the case of thermal neutrons, using a neutron interferometer containing a 30‐kV/mm vacuum electrode system. Tilting the interferometer about the incident beam direction introduces a spin‐independent gravitationally‐induced phase shift which enables unpolarized neutrons to be used. A supplementary magnetic bias field of the correct magnitude allows first‐order sensitivity to the AC phase shift to be achieved. Nevertheless, the theoretically predicted phase shift is only 1.52 mrad for the geometry and conditions of the experiment. We observe a phase shift of 2.11±0.34 mrad. A detailed description of the experiment and its interpretation is given in this paper.