Transport Measurement Of Pr1-ySryFe1-xCoxAsO And SrTiO3Superconductors

Iron-pnictide materials are newly discovered superconductors, which attracts worldwide attention and brings enormous scientific interests in recent years. The superconductors can be divided into several main groups, such as1111phase,122phase,111phase,11phase and so on. In this thesis, we first focus on the investigation of1111phase superconductors with a parent compound PrFeAsO. Element substitution can induce superconductivity (SC) in this system, due to carrier doping. Partial Sr substituted into Pr site introduces hole-type carriers, leading to superconductivity with a maximum superconducting critical temper-ature Tc=14K, while Co doped into Fe site introduces electron-type carriers, which also leads to superconductivity with a maximum Tc=16K. A electron-hole compensation effect occurs in the Sr and Co doped PrFeAsO system. Co doped into Pr0.8Sr0.2FeAsO first suppresses Tc to low temperature in low Co con-tent region, then Tc goes up with further Co doping. Hole-type carriers dominate in lower Co content region and electron-type carries dominate in higher content region. Tc evolves systematically with Co content. The phase-diagram in the electron-type side seems similar to that of PrFe1-xCoxAsO. So the superconduc-tivity in Pr0.8Sr0.2FeAsO system is not sensitive to impurity doping, but only depends on the carrier density.Tc of Pr0.8Sr0.2FeAsO is rarely suppressed by non-magnetic impurity (Zn) doping, implying an isotropic paring symmetry without nodes in this system. The high temperature broad hump structure of the resistivity is obvious in hole-type1111phase superconductors. The origin of the structure is still an open question. This structure in Pr0.8Sr0.2FeAsO is hardly suppressed by Zn dop-ing, contrary to that of structure/AFM transition anomaly in LaFe1-xZnxAsO systems. Combining with magneto-resistance, Hall, specific heat, magnetic sus-ceptibility measurements, we conclude that this hump would not come from the residual Fe AFM order, but relate to the multi-band electronic structure of1111 phase systems, e.g., the mobility of electrons and holes evolves with temperature in different ways.We investigate the transport properties of an oxide superconductor SrTiO3in the second part. SrTiO3is a well investigated band insulator with a band gap△=3.2eV. Tiny electron doping can induce insulator-metal transition and super-conductivity in SrTiO3. The superconducting SrTiO3is the first discovered oxide superconductor with a maximum Tc below1K. We re-investigate the bulk SrTi03with Nb substitution or oxygen deficiency and find the most dilute superconduc-tor with a Tc=87mK, whose carrier concentration is only5.5×1017cm-3, one order of magnitude smaller than previous reports. Prom our Nernst oscillation measurements, this superconductor has a single, tiny and nearly isotropic Fermi surface, with a Fermi energy of1.1meV on top of such a giant band gap. The oc-currence of superconducting instability in an extremely small, single-component and nearly isotropic Fermi surface implies strong constraints for the identification of the pairing mechanism. The Fermi surface evolution with carrier concentration in SrTiO3is also shown here according to Nernst quantum oscillations.