Fig. 4: TKE in K⁺ β″-alumina.

a, Normalized time-domain transient birefringence in thin polycrystalline K⁺ β″-alumina at 300 K (blue) and 620 K (pink). The shaded regions are ±1 s.e.m. The inset shows that in β″-aluminas, mobile ions (purple) hop (arrows) between equivalent four-coordinate sites. b, Normalized simulated (sim.) anisotropy of hopping directions in polycrystalline K⁺ β″-alumina under applied electric field centred at 0.7 THz mimicking the experimental pump, 300 K (blue) and 600 K (pink). The black dashed line indicates the square of the simulated pump electric field. The inset zooms in on the post-pump times and simulated hopping anisotropy at 300 K is non-zero for more than or equal to 5 ps. c, Simulated anisotropy of hopping in polycrystalline K⁺ β″-alumina at 300 K under applied electric fields centred at 0.7 THz (teal) and 2.5 THz (orange). The inset shows simulated in-plane optical K⁺ conductivity of K⁺ β″-alumina in the terahertz region (blue). Peaks at 1, 3, and 5 THz agree with the literature⁴⁹. The spectra of simulated pumps with centre frequencies 0.7 THz (teal) mimicking the experimental pump (a,b), and 2.5 THz (orange) are shaded.