Correlated Electromigration of H in the Switchable MirrorYH3−δ

Electromigration of hydrogen in ${\mathrm{YH}}_{3-\delta }$ is studied by exploiting the H concentration dependence of the optical transmission of ${\mathrm{YH}}_{3-\delta }$. We find the effective valence $Z^{*}$ of H in ${\mathrm{YH}}_{3-\delta }$ to be negative. Its value is dominated by a huge wind-force-like term, i.e., $Z^{*}\approx K/\rho$, with $K\sim -60\mathrm{}\mathrm{m}\Omega \mathrm{cm}$. This value is 3 orders of magnitude larger than typical for H in metals. In an Arrhenius plot, the ratio of hydrogen and electron fluxes extrapolates to unity at infinite temperature, suggesting a one-to-one correlation of hydrogen and electron hopping. We discuss our results in the light of strong electron correlation theories which predict each proton to bind two electrons in a sort of Zhang-Rice singlet.