The Energetics of Li Off-Centering in K1−x_{1-x}Lix_xTaO3_3; First Principles Calculations

K$_{1-x}$Li$_{x}$TaO$_3$ (KLT) solid solutions exhibit a variety of interesting physical phenomena related to large displacements of Li-ions from ideal perovskite A-site positions. First-principles calculations for KLT supercells were used to investigate these phenomena. Lattice dynamics calculations for KLT exhibit a Li off-centering instability. The energetics of Li-displacements for isolated Li-ions and for Li-Li pairs up to 4th neighbors were calculated. Interactions between nearest neighbor Li-ions, in a Li-Li pair, strongly favor ferroelectric alignment along the pair axis. Such Li-Li pairs can be considered "seeds" for polar nanoclusters in KLT. Electrostriction, local oxygen relaxation, coupling to the KT soft-mode, and interactions with neighboring Li-ions all enhance the polarization from Li off-centering. Calculated hopping barriers for isolated Li-ions and for nearest neighbor Li-Li pairs are in good agreement with Arrhenius fits to experimental dielectric data.Comment: 14 pages including 10 figures. To Physical Review B. Replaced after corrections due to referees' remark

Modeling of 110 GHz electron cyclotron wave propagation and absorption on DIII-D

In modeling electron cyclotron (EC) waves for heating (ECH) and current drive (ECCD) applications, the standard approach describes wave propagation using geometric optic ray tracing with cold plasma dispersion and wave absorption using a relativistic warm plasma expression. However, recent vertical O-mode transmission measurements on Tore Supra indicate that wave trajectories near the fundamental resonance frequency can significantly deviate from predictions of cold plasma analysis. The experimental results were attributed to warm plasma refraction effects caused by the anomalous dispersion associated with wave-particle resonance. Here, warm plasma refraction effects on wave propagation and absorption are examined in the context of a slab model in the parameter regime of interest to the upcoming 110 GHz electron cyclotron heating and current drive experiments on DIII-D