Electrotoroidic systems can exhibit optical activity if the electrical toroidal moment G of their electrical vortices couples to a spontaneous electrical polarization P [1]. Here we use large-scale atomistic simulations to explore electric field and temperature control of optical activity and related properties in chiral phases of an electrotoroidic nanocomposite. Specifically, we consider a BaTiO3 nanowire in a SrTiO3 matrix exhibiting electrical vortices with both axial G and P. Molecular dynamics for a second-principles effective Hamiltonian (Heff) finds the gyrotropic coefficient measuring optical activity is maximized at room temperature for some critical bias field. A temperature-electric field phase diagram of chiral and non-chiral phases is obtained from Monte Carlo simulations. We then follow the recipe of [2] in a low-temperature regime: we apply an electric field anti-parallel to the polarization to switch from the chiral vortex to a chiral, Bloch-like electrical skyrmion. However, this skyrmion exhibits insignificant optical activity, so we probe other optoelectronic properties. To this end, we extend work of [3] that used the Heff method to obtain equilibrium relaxed atomic configurations as input for the linear-scaling 3D fragment (LS3DF) method, a first-principles method for calculating electronic structure in >10,000 atom systems. Band gap and alignment change more significantly under temperature control of the vortex than under electric field control of the skyrmion, suggesting the former regime is superior for novel optoelectronic applications.
References
[1] Prosandeev et al., PRB 87, 195111 (2013)
[2] Nahas et al., Nat. Comm. 6, 8542 (2015).
[3] Gui et al., NanoLett 15, 3224 (2015).