Speaker: Ioannis Rousochatzakis, Max-Planck Insitute for the Physics of Complex Systems, Dresden
Abstract: We present an analytical and numerical study of the spin S=1/2 antiferromagnetic Heisenberg model on the Cairo pentagonal lattice. This is the dual of the Shastry-Sutherland lattice and has been discussed as a possible new candidate for having a spin liquid ground state . More recently a S=5/2 version of this model has been realized in the Bi2Fe4O9 system . Here we use a model with two different types of exchange couplings and investigate the nature of the ground state as a function of their ratio. This strategy allows us to understand the nature of a number of phases and derive effective models for their description with and without a magnetic field. Of particular interest is a surprising interplay between a collinear and a four-sublattice orthogonal phase due to an underlying order-by-disorder mechanism. Furthermore we address the issue of possible nonmagnetic ground states such as singlet and spin nematic phases.
 K. S. Raman, R. Moessner, and S. L. Sondhi, PRB 72, 064413 (2005)
 E. Ressouche, V. Simonet, B. Canals, M. Gospodinov, and V. Skumryev, PRL 103, 267204 (2009)