Quantum order-by-disorder in geometrically frustrated magnets

by Emil J. Bergholtz (Max Planck Institute for the Physics of Complex Systems)

Tuesday 23 Aug 2011, 11:00 → 12:00 Europe/Stockholm

122:026

I will give an introduction to the physics of frustrated magnets and explain why this class of systems can give rise to exotic (classical as well as quantum) phases realized in real materials. In particular, I will discuss the possibility of formation of complex ordering patterns of strong singlet pairs: so-called valence bond crystals (VBC's). In their simplest form, such states are formed due to inequivalent bonds. A more intriguing possibility is that VBC order may be spontaneously induced by quantum fluctuations, thus realizing a quantum version of the order-by-disorder phenomena encountered in classical statistical mechanics. In this context, I will describe our recent and ongoing series expansion work on antiferromagnetic spin-1/2 Heisenberg models on a number of highly frustrated lattices. In the case of the quasi-two-dimensional SCGO (kagome bilayer) and the three-dimensional hyperkagome we find VBC's with huge unit (84 and 72 sites respectively) cells. On the three-dimensional pyrochlore lattice we have preliminary evidence that the ground state is a VBC with a novel self-similar structure, in which each (larger) length scale of the crystal is associated with a certain (smaller) energy scale. In all these cases, the VBC scenario has interesting observable implications such as spatially anisotropic neutron scattering spectra and multiple finite temperature signatures in the magnetic specific heat due to a multi-step breaking of discrete symmetries, including a clearly resolved Ising transition at relatively high temperature. The hyperkagome part of this work is published in: E.J. Bergholtz, A.M. Läuchli and R. Moessner, Phys. Rev. Lett. 105, 237202 (2010).