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22 results on '"Als-Nielsen, J"'

1. Critical behavior of the order-disorder phase transition in ß-brass investigated by x-ray scattering.

Author

Madsen, A., Als-Nielsen, J., Hallmann, J., Roth, T., and Lu, W.

Subjects
*PHASE transitions, *X-ray scattering, *SCALING hypothesis (Statistical physics)
Abstract

ß-brass exhibits an archetypical example of an order-disorder transition with a critical behavior that was previously investigated by neutron scattering. The data were well described by the three-dimensional (3d) Ising model but the relatively crude experimental resolution prevented an in-depth examination of the single-length scaling hypothesis, a cornerstone in the theory of critical phenomena. With the development of synchrotron x-ray experiments, high-resolution data could be recorded and surprisingly it was found that the single-length scaling did not hold in most critical systems, possibly due to strain originating from surface defects and/or impurities. In this paper we demonstrate single-length critical behavior using high-resolution x-ray scattering in ß-brass. The investigations confirm that ß-brass behaves like a 3d Ising system over a wide range of length scales comprising correlated clusters of millions of atoms. To vary the surface sensitivity, experiments have been performed both in Bragg reflection and Laue transmission geometries but without any substantial differences observed in the scaling and critical behavior. [ABSTRACT FROM AUTHOR]

Published
2016
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22. Observation of algebraic decay of positional order in a smectic liquid crystal

Author

Cyrus R. Safinya, A. Lindegaard-Andersen, Robert J. Birgeneau, Jens Aage Als-Nielsen, J. D. Litster, S. Mathiesen, and M. Kaplan

Subjects
Physics, Crystal, Singularity, Condensed matter physics, Liquid crystal, Resolution (electron density), Order (ring theory), Gravitational singularity, Algebraic number, Density wave theory
Abstract

A smectic-$A$ liquid crystal in three dimensions has been predicted to exhibit algebraic decay of the layer correlations rather than true long-range order. As a consequence, the smectic Bragg peaks are expected to be power-law singularities of the form ${q}_{\ensuremath{\parallel}}^{\ensuremath{-}2+\ensuremath{\eta}}$ and ${q}_{\ensuremath{\perp}}^{\ensuremath{-}4+2\ensuremath{\eta}}$, where $\ensuremath{\parallel}(\ensuremath{\perp})$ is along (perpendicular to) the smectic density wave vector direction, rather than $\ensuremath{\delta}$ function peaks. Observation of these phenomena requires very high instrumental resolution together with a resolution function with wings which drop off much more rapidly than ${q}_{\ensuremath{\parallel}}^{\ensuremath{-}2}({q}_{\ensuremath{\perp}}^{\ensuremath{-}4})$. We show that these requirements may be met by using a three crystal x-ray spectrometer with multiple-reflection channel cut crystals as monochromator and analyzer. We find that the smectic-$A$ Bragg peaks observed in the liquid-crystal octyloxy-cyanobiphenyl are indeed consistent with the predicted power-law singularity form. Furthermore, the explicit values of $\ensuremath{\eta}$ required to describe the measured profiles are in accordance with calculations of $\ensuremath{\eta}$ using the harmonic approximation with empirically determined splay and layer compressibility elastic constants.

Published
1980

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