Possibility of unconventional superconductivity of SrTiO3-δ

SrTiO3-delta can show metallic behavior and superconductivity at T < 500 mK. Due to its low carrier concentration (similar or equal to10(20) cm(-1)) and huge dielectric polarisability (epsilon similar or equal to 300) this compound is considered as a candidate for a polaronic superconducting pairing mechanism. It is demonstrated that by variation of the annealing temperature in vacuum the transport properties of SrTiO3-delta can be tuned continuously from semiconducting to metallic. We present measurements of the upper critical field B-c2 (T) which show near T. a positive curvature. This unusual temperature dependence is consistent with a model of weakly interacting charged bosons which condense in the superconducting state (local pairing). However, measurements of current-voltage curves reveal only small critical currents of our samples. This observation is discussed in the framework of doping inhomogeneities. (C) 2003 Elsevier Science B.V. All rights reserved.