In doped SrTiO3 superconductivity persists down to an exceptionally low concentration of mobile electrons. This restricts the relevant energy window and possible pairing scenarios. We present a study of quantum oscillations and superconducting transition temperature, Tc as the carrier density is tuned from 10 17 to 10 20 cm −3 and identify two critical doping levels corresponding to the filling thresholds of the upper bands. At the first critical doping, which separates the single-band and the two-band superconducting regimes in oxygen-deficient samples, the steady increase of Tc with carrier concentration suddenly stops. Near this doping level, the energy dispersion in the lowest band displays a downward deviation from parabolic behavior. The results impose new constraints for microscopic pairing scenarios.Superconductiviy is induced in insulating SrTiO 3 by introducing n-type charge carriers through chemical doping[1] and survives over three orders of magnitude of carrier concentration. The transition temperature, T c , peaks to 0.45 K around a carrier density of n H ∼ 10 20 cm −3 [2]. A superconducting dome has also been detected in the metallic interfaces of SrTiO 3 [3] when the carrier density is modulated by a gate voltage bias [4]. In unconventional superconductors, such as high-T c cuprates, superconducting domes are often attributed to the proximity of a magnetic order or a Mott insulator. The recent discovery of superconducting dome in gate-tuned MoS 2 [5] in absence of a competing order, however, highlights the limits of our current understanding of the interplay between carrier concentration and superconductivity and motivates a fresh reexamination of superconducting domes. In the specific case of SrTiO 3 , superconductivity occurs in the vicinity of an aborted ferroelectric order[6] and survives deep inside the dilute metallic regime when the Fermi temperature becomes more than one order of magnitude lower than the Debye temperature [7]. This is a second puzzle in addition to the one raised by the drop in T c on the overdoped side. These two questions, raised at the opposite limits of the superconducting dome, remain unsettled.According to band calculations[9-11], doping SrTiO 3 with n-type carriers can fill three bands one after the other. Once the critical threshold for the occupation of a band is attained, a new Fermi surface concentric with the previous one emerges. Previous studies of quantum oscillations in bulk doped SrTiO 3 [7, 12-14] have detected both multiple-frequency [7,13,14] and singlefrequency [7,14] oscillations at different doping levels, but did not determine these critical doping levels. Moreover, according to tunneling experiments, doped SrTiO 3 beyond a carrier density of 10 19 cm −3 is a multi-gap superconductor [8]. The interplay between multi-band occupation in the normal state and multi-gap superconductivity has been a subject of recent theoretical attention [20].We present a systematic study of quantum oscillations and superconducting transition as a function of carrier con...

The thermal conductivity of the heavy fermion superconductor Pr(Os(4)Sb(12) was measured down to T(c)/40 throughout the vortex state. At lowest temperatures and for magnetic fields H approximately 0.07H(c2), already 40% of the normal state thermal conductivity is restored. This behavior (similar to that observed in MgB2) is a clear signature of multiband superconductivity in this compound.