NMR investigation of rapid structural changes and non-metal-to-metal transition in the molten eutectic Ge-Te alloy

Resonance shift, K125, spin-lattice relaxation time, T1 and spin-spin relaxation time, T2 for 125Te nuclei in molten Ge15Te85 and Te in deeply under-cooled state have been measured as a function of temperature. The temperature dependence of K125 is compared with that of the extent of structural change, C, and it is shown that the rapid increase of K125 with increasing temperature can be attributed to an increase of the density of states at the Fermi level proportional to the extent of structural change. At high temperatures, the dependences on the temperature of K125 and T1 conform to the Korringa relation with an enhancement factor or the relaxation mechanism expected metallic nature. At low temperatures, in contrast, the Redfield theory assuming localized spins of which the density is proportional to C can explain the overall temperature dependences of T1 and T2. These results provide the first clear evidence for a crossover transition from extended to localized states in the electron system accompanying the rapid structural changes.