You can find explicit answer to your question in Sec. 2.7 of the book Principles of Condensed Matter Physics, by P.M. Chaikin and T.C. Lubensky (Cambridge University Press, 2000). The ring pattern you are referring to is to be seen in Fig. 2.7.4. (a) (p. 63) of this book.
In the high temperature, unaligned regime, the system has rotational space invariance. In 2D this reflects in a structure factor with an isotropic maximum on a ring (Kx^2+Ky^2=(2Pi/lambda)^2, at a wave vector modulus (2Pi/lambda) of the order of the inverse size of your molecules (lambda). On the aligned regime, isotropy is broken, the nematic phase is an space anisotropic phase, in which one direction is prefered by the molecules to align. While this alignment is not perfect for all the molecules, it reflects in the emergence of diffuse arcs, with an amplitude corresponding to the angle fluctuations of the principal axis of the molecules.
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