Dresden 2017 – scientific programme

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BP: Fachverband Biologische Physik

BP 40: Colloids and Complex Fluids II (Joint Session CPP/BP/DY)

BP 40.7: Talk

Wednesday, March 22, 2017, 12:15–12:30, ZEU 260

Determing helicity modulus in systems with orientational order from microscopic properties through Zwanzig-Mori formalism — •Johannes Häring and Matthias Fuchs — FB Physik, Universität Konstanz, 78457 Konstanz, Germany

Up to now, we have studied crystals with point disorder and applied the theory to crystals of soft core particles, so-called cluster crystals [1]. Now, systems with orientational order like nematic liquid crystals are considered. With the Zwanzig-Mori formalism it is possible to calculate the helicity modulus for all temperatures in the ordered phase, even near the critical point.

The Zwanzig-Mori formalism is a way to treat many-body systems systematically. Projection Operators are used to focus on the dominant variables of the system and their correlation functions. Simulations of the three dimensional XY model are done to test the accuracy of the approach. The XY model consists of particles with one orientational degree of freedom which are fixed on a lattice.

It is known from the Mermin-Wagner theorem that two dimensional systems show no conventional long range order, i.e have vanishing order parameters. That leads to problems in calculating the helicity modulus. We discuss how it is still possible to obtain a solution.

[1] J. M. Häring, C. Walz, G. Szamel, and M. Fuchs, Phys. Rev. B 92, 184103 (2015)