Correlated electromigration of H in the switchable mirror YH3-delta

Electromigration of hydrogen in YH3-delta is studied by exploiting the H co ncentration dependence of the optical transmission of YH3-delta. We find th e effective valence Z* of H in YN3-delta to be negative. Its value is domin ated by a huge wind-force-like term, i.e., Z* approximate to K/rho, with K similar to -60 m Omega cm. This value is 3 orders of magnitude larger than typical for H in metals. In an Arrhenius plot, the ratio of hydrogen and el ectron fluxes extrapolates to unity at infinite temperature, suggesting a o ne-to-one correlation of hydrogen and electron hopping. We discuss our resu lts in the light of strong electron correlation theories which predict each proton to bind two electrons in a sort of Zhang-Rice singlet.