Effective masses of electrons in n-type SrTiO3 determined from low-temperature specific heat capacities

We measured the low-temperature specific heat of Nb-doped and reduced (hydrogen treated) n-type SrTiO3 ranging from about 1.5 to 100 K. Displaying the data by the usual C-p/T vs. T-2 plot and applying a linear fit to the proper temperature range, the electronic density-of-states effective mass MD is derived resulting in m(D)/m(0) = 1.9 (+/- 0.1) for polycrystalline SrTiO3:Nb (5 at%) and m(D)/m(0) = 1.8 (+/- 0.2) for a reduced SrTiO3 single crystal. These results are in reasonable agreement with theoretical predictions, while in parts significantly deviating from experimentally obtained values, presented in literature. As far as specific heat measurements are concerned, we suggest that the higher values stated in most literature studies are due to an incorrect evaluation of specific heat data, as in n-type SrTiO3 one has to take care about a specific heat anomaly below about 5 K. Thermo-gravimetric oxidation of a reduced crystal combined with a Hall effect measurement leads to the conclusion that oxygen vacancies in this sample were doubly ionised even at liquid helium temperatures. (c) 2007 Elsevier B.V. All rights reserved.