# Calendar of Events

## Events Calendar

### Trilinear couplings in ferroelectrics and charge density wave materials

Date and Time: Tuesday, November 16, 2021, 01:30pm -
Location: Zoom

Speaker: Tural Birol (Univ of Minnesota)

First principles calculations are historically very successful in both reproducing and predicting structural phase transitions in a wide range of crystalline materials. Density Functional Theory and related approaches enable calculating coefficients of crystal free energy expansions in terms of unstable structural modes, and the form of the free energies can be predicted using group and representation theories. In this talk, I am going to present our recent work on two different classes of materials that both have a trilinear term in their free energy, and discuss what this term entails. In the charge-density-wave metal CsV3Sb5, the trilinear coupling between different directions of the charge density wave order parameter emerges naturally from the hexagonal shape of the Brillouin zone, leads to distinct structural phases' with the same symmetry, and couples the loop current order parameters with Peierls-like distortions. In the hybrid-improper ferroelectric Sr3Sn2O7, the trilinear term between octahedral rotations and the polar mode is responsible of the ferroelectricity, and it also enables stabilization of a ferroelectric-metallic' phase, where the free carriers enhance the structural polarization present in the material.

References:
"Theory of the charge-density wave in AV3Sb5 kagome metals", Morten H. Christensen, Turan Birol, Brian M. Andersen, Rafael M. Fernandes, under review.
"Free carrier induced ferroelectricity in layered perovskites", Shutong Li, Turan Birol, Phys. Rev. Lett. 127, 087601 (2021).

Host:  David Vanderbilt

Extra Info: