Positron Annihilation Studies on Stable and Undercooled Metal Melts at the Stuttgart Pelletron

摘要

If the phase transition liquid → solid is suppressed on cooling, metal melts may be transformed into the metastable state of the 'undercooled melt'. An MeV positron (e~+) beam, which can easily be implanted into all kind of samples facilitates considerably positron-lifetime and Doppler-broadening measurements on liquids. By taking advantage of the high data-acquisition rate of the beam-based β~+γΔE_γ-coincidence technique developed at the Stuttgart Pelletron measurements with good statistics in Ga, Bi, Sn, In, and Pb were performed not only in the solid states and in the 'stable' metal melts but also in the metastable undercooled melts at temperatures below the melting point T_m. As long as the metals stay in the liquid states, no sudden changes of electron density or electron momentum distribution at the positron site were observed at the melting point. Compared to the undisturbed solid, significantly longer positron lifetimes and much narrower Doppler broadening were observed in all metal melts investigated. These findings are explained by the formation of self-localized e~+ polaron states. Theory predicts two types of the 'polaronic' e~+ states which differ in their effective positron masses m~+. In Bi, Sn, In, and Pb 'low-m~(+') polaron states showing effective masses of 1 to 4 proton masses are found. In Ga 'high-m~(+') polaron states are observed with even much larger effective positron masses.