Evan Constable, Institut für Festkörperphysik, Wien, AUSTRIA

January 21, 02:00 PM, Paris Time (GMT +1)

What can lattice dynamics teach us about frustrated magnets ?

The delicate balance between competing spin interactions in magnetically frustrated systems fosters a rich environment to explore novel states of matter with interesting collective behaviours. Small perturbations in this balance can result in a diversity of exotic phases such as highly degenerate spin liquids or spin ices, to long-range non-collinear magnetic ordering, stabilising multiferroic phases and even topological objects. While considerable effort has been made to understand the signatures of these different states of matter in their static lattice environment, little is known about their transient behaviours on a dynamic lattice. Indeed, a growing body of evidence suggests that perturbations from the lattice dynamics are fundamental to many of the more novel frustrated phases. An intriguing approach to probing these exotic states therefore involves direct investigation of spectroscopic signatures involving coupled spin and lattice degrees of freedom.

In this talk, I will present the results of spectroscopic investigations into two unique frustrated systems. One, a three-dimensional system based on the pyrochlore lattice — terbium titanate (Tb2Ti2O7). The other, a quasi-two-dimensional system based on the kagome lattice — francisite (Cu3BiSeO5Cl).

Terbium titinate is a close relative of the classical spin-ices Ho2Ti2O7, Dy2Ti2O7. However, unlike these systems, it exhibits no conventional spin-ice nor long-range magnetic order even down to very-low temperatures, making it a strong candidate for a spin-liquid. As I will discuss, this possibility is supported by a growing body of spectroscopic work including infrared/THz, magneto-optical and neutron scattering measurements. A key finding being that the peculiar magnetic behaviour of Tb2Ti2O7 is the result of entanglement between the spin degrees of freedom and the dynamics of the frustrated pyrochlore lattice.

Francisite on the other hand has its spin frustration lifted by a number anisotropic effects related to the lattice. Its resulting antiferromagnetic magnetic behavior is further complicated by the fact that this material additionally undergoes an antiferrfoelectic structural phase transition at 115 K. In particular this phase transition has been shown to exhibit a text-book example of a soft-mode driven anti-polar order. Thus francisite can bee seen as a novel form of multiferroic simultaneously exhibiting a frustrated antiferromagnetic phase and an antiferroelectric phase. The implications of coupling between these phases will be discussed in relation to the spin and lattice dynamics.

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