Temperature dependence of plasmons in Nb-doped SrTiO3

The temperature dependence of infrared reflection spectra of single crystals of n-type strontium titanate doped with 0.3 to 1.5 at. % niobium are reported from 10 to 5 000 cm-1 in the temperature range 80-900 K. Results are analyzed by means of various factorized forms of the dielectric function. A decoupling scheme allows the temperature dependence of the plasmon to be deduced. The plasma frequency of the carriers is found to increase upon cooling, a very unusual phenomenon which, together with Hall-effect measurements, may imply that the effective mass of the carriers decreases upon cooling. The dependence of the soft ferroelectric mode on temperature and Nb doping is also reported and discussed. © 1993 The American Physical Society.