Identifier

etd-04032017-150350

Degree

Doctor of Philosophy (PhD)

Department

Physics and Astronomy

Document Type

Dissertation

Abstract

The discovery of Fe-based superconductors marked the beginning of a new era in unconventional superconductors. To date, the underlying superconducting mechanism has not yet been fully understood. Among Fe-based superconductors, the relatively new members are Ca10PtnAs8(Fe2As2)5 (n = 3, 4) and CaFeAs2. They are unique because they contain special adjacent layers, i.e., metallic Pt4As8 in Ca10Pt4As8(Fe2As2)5, semiconducting Pt3As8 in Ca10Pt3As8(Fe2As2)5, and As chains in CaFeAs2. The focus of this research is to understand the electrical and magnetic properties of Ca10PtnAs8(Fe2As2)5 (n = 3, 4) and CaFeAs2 in both the normal and superconducting state. Single crystals Ca10Pt4As8(Fe2As2)5 and Ca10Pt3As8(Fe2As2)5 with different dopings have been grown by the self-flux method. Using the same growth method, polycrystals CaFeAs2 and Ca0.85La0.15FeAs2 have been obtained. The experimental investigation of electrical resistivity, Hall effect, magnetoresistivity, magnetization, vortex imaging of Ca10PtnAs8(Fe2As2)5 (n = 3, 4) single crystals has been proceeded. The electrical resistivity, magnetoresistivity and magnetization of CaFeAs2 have also been measured. We observed anisotropy in the superconducting and normal state of Ca10PtnAs8(Fe2As2)5 (n = 3, 4) which is larger than many Fe-based superconductors. The results we obtained for Ca10Pt3As8(Fe2As2)5 with different doping not only reveal the doping effect on the in-plane and out-of-plane transport properties, but also indicate superconducting transition temperature Tc increases with increasing interlayer distance d. Among different doping, La is the most efficient in enhancing Tc and its normal state shows a total metallic behavior. Instead of having quadratic dependence to applied magnetic field like normal conductors, magnetoresistivity of Ca10PtnAs8(Fe2As2)5 (n = 3, 4) is linear to H at fixed temperatures. It is rare in materials without strong disorder or Dirac-cone like electronic structure around the Fermi surface. Although its cause remains uncertain, we discussed the possibility that it may be related to the quantum interference effect. Through growth of CaFeAs2, we found La doping on the Ca site helps form stable crystal phase. Surprisingly, although CaFeAs2 has Dirac-cone like electronic structure around the Fermi surface which should lead to the linear H dependence of MR according to the quantum model developed by Abrikosov, magnetoresistivity of Ca0.85La0.15FeAs2 shows a quadratic dependence on H.

Date

2016

Document Availability at the Time of Submission

Release the entire work immediately for access worldwide.

Committee Chair

Jin, Rongying

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