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Novel Physical Phenomena of Iron-Based Superconductors Revealed Through Transport and Thermodynamic Measurements

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, PHD, Kent State University, College of Arts and Sciences / Department of Physics.

The first line of research presented here is experimental studies of the abnormal free-flux-flow resistivity in the mixed state of Ba(Fe1-xCox)2As2 single crystals. The flux vortices present in the mixed state of these superconductors are induced by applying magnetic field. We performed I-V measurements on these single crystals covering a wide range of Co doping, from under-doped (x = 0.044 and 0.056), near optimally-doped (x = 0.060 and 0.072), to the over-doped (x = 0.100) regimes.

We revealed an unusual flux-flow resistivity (increase in the flux-flow resistivity with decreasing temperature or applied magnetic field) over the whole doping range of Ba(Fe1-xCox)2As2 crystals. Significantly enhanced flux-flow resistivity was observed at x = 0.060, possibly related to the existence of the boundary between the purely superconducting phase and the phase where superconductivity co-exists with the SDW order. This abnormal behavior is suppressed on both the under-doped and over-doped sides of the phase diagram. In addition, the I-V measurements also reveal universal scaling behavior of flux-flow resistivity measured at different temperatures and magnetic fields. This behavior observed in all doping levels implies that the upturn is governed by the dissipation in the bulk excitations. In addition, our results imply that the viscosity coefficient has strong magnetic field and temperature dependence governed by the systems' proximity to a magnetic instability.

Moreover, a relationship between critical de-pinning current and corresponding flux-flow resistivity was extracted from these I-V data in the Ba(Fe1-xCox)2As2 system. This relationship holds over the whole doping range of Ba(Fe1-xCox)2As2, as well as canonical single band conventional superconductor Nb and multiband unconventional superconductor CeCoIn5. The sign difference of the merging point in the V vs I plots between the magnetic and non-magnetic systems reveals the effect of spin fluctuations on the scattering of quasi-particles. In addition, this relationship in optimally doped Ba(Fe1-xCox)2As2 shows a temperature-dependent behavior, while it is temperature independent for the under-doped and over-doped samples. This latter result reflects the presence of a temperature-dependent secondary phase (in addition to the superconducting phase) most likely the spin density wave phase or the micro-emulsion phase.

During a second line of research, heat capacity measurements were performed to study the gap structure in the hole-doped system Ba0.05K0.95Fe2As2. Our results show evidence for the existence of a three-component superconducting order parameters. The gap values are comparable with the reported values of the KFe2As2 system. The magnetic field dependence of Sommerfeld coefficient can be fitted with a square root term, a linear term, and a constant. The square root term indicates that the nodes of one of the gap are most likely accidental. It also shows that the orbital effect is important in the low magnetic field. The linear term reveals that the Zeeman effect is the dominant pair breaker at high field. After suppressing the small gap with magnetic field, this system turns to a gapless superconducting state when magnetic field is greater than 4 T.

Carmen Almasan (Advisor)
Maxim Dzero (Committee Member)
Elizabeth Mann (Committee Member)
Anatoly Khitrin (Committee Member)
Scott Bunge (Committee Member)

Recommended Citations

Citations

  • Huang, X. (n.d.). Novel Physical Phenomena of Iron-Based Superconductors Revealed Through Transport and Thermodynamic Measurements [Doctoral dissertation, Kent State University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=kent1491943156716437

    APA Style (7th edition)

  • Huang, Xinyi. Novel Physical Phenomena of Iron-Based Superconductors Revealed Through Transport and Thermodynamic Measurements. Kent State University, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=kent1491943156716437.

    MLA Style (8th edition)

  • Huang, Xinyi. "Novel Physical Phenomena of Iron-Based Superconductors Revealed Through Transport and Thermodynamic Measurements." Doctoral dissertation, Kent State University. Accessed MARCH 29, 2024. http://rave.ohiolink.edu/etdc/view?acc_num=kent1491943156716437

    Chicago Manual of Style (17th edition)