| Transport property measurement is a powerful method to detect electronic structure, low-energy excitation and various intrinsic scattering mechanisms of materials. Therefore it has been widely used in the research of superconductivity and quantum phase transitions. Especially, ther-mopower, a kind of thermoelectric effect, is believed to be very sensitive to the change of electronic structure as it mainly depends on the energy deviation of Fermi surface. In this thesis, we carry out the thermopower and other transport measurements on three low-dimensional systems:Ⅰ) The local noncentrosymmetricity and spin-momentum locking in Sr3Ru207; Ⅱ) The Lifshitz transition between the two superconducting domes in LaFeAs1-xPxO; Ⅲ) The effect of carrier density and other competing orders on the superconductivity in Nb2(Pd1-xRx)0.76S5(R=Ir, Ag).Ⅰ) Strong spin-orbital coupling (SOC) was found previously to lead to dramatic effects in quantum materials, such as those found in topological insulators. Interestingly, the SOC of itinerant electrons can come from origins other than the atomic orbitals. It was shown theoretically that local noncentrosymmetricity resulting from the rotation of RuO6 octahedral in Sr3Ru2O7 will also give rise to an effective stagger SOC. In the presence of a magnetic field applied along a specific in-plane direction, the Fermi surface was predicted to undergo a reconstruction. The results of our in-plane magnetoresistivity and magnetothermopower measurements on single crystals of Sr3Ru2O7 with an electrical or a thermal current applied along specific crystalline directions and a magnetic field rotating in the ab plane shows a minimal value for field along Γ-X directions due to the gap opening on γ2 bands predicted by the local noncentrosymmetricity theory. Furthermore, the thermopower, and therefore, the electron entropy, were found to be suppressed as the field was applied perpendicular to the thermal current, which suggests that the spin and the momentum in Sr3Ru2O7 are locked over substantial parts of the Fermi surface, likely originating from local noncentrosymmetricity as well.Ⅱ) Two distinct superconducting (SC) domes were discovered in the phase diagram of P-doped iron pnictide superconductors LaFeAs1-xPxO with x ranging from 0 to 1. The first SC dome is centered around x= 0.3 and then disappears as P content increases to x= 0.5. For x> 0.5, a second SC dome appears and Tc reaches a maximum at x= 0.7. Although the maximum of Tc in the two domes is close to each other, the estimated Hc2 value is significantly larger in the first SC dome, implying a difference in the nature between the two domes. Further band calculations and thermoelectric measurements suggest that there could be a Lifshitz transition in the electronic structure between the two SC domes.Ⅲ) Pd site doping effect on superconductivity was investigated in Nb2(Pd1-x;Rx)0.76S5(R=Ir, Ag) by measuring resistivity, Hall effect and magnetic susceptibility. It was found that super-conducting transition temperature (Tc) is firstly slightly enhanced by partial substitution of Pd with Ir and then it is suppressed gradually as Ir content increases. Meanwhile Ag substitution quickly suppresses the sample to a non-superconducting metallic ground state. Hall effect mea-surements indicate the variations of charge carrier density caused by Ir or Ag doping. The estab-lished phase diagram implies that the charge carrier density (or the band filling) could be one of the crucial controlling factors to determine Tc in this system. In addition, the phase diagram in Nb2(Pd1-xRx)0.76S5(R=Ir, Ag) is dome-like, which mimics the general phase diagrams of high-Tc superconducting cuprates and some other unconventional superconductors. Further theoretical and experimental studies are expected to explore if there exists a competing order or a magnetic quantum critical point (QCP) accompanied by the emergence of superconductivity in this system. |
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