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Please use this identifier to cite or link to this item: http://hdl.handle.net/1807/32024

Title: Probing Exotic Superconducting Order Parameters Using Point-contact Andreev Reflection Spectroscopy
Authors: Turel, Cyrus Sam
Advisor: Wei, John
Department: Physics
Issue Date: 17-Jan-2012
Abstract: Point-contact Andreev reflection spectroscopy measurements were performed on various unconventional superconducting systems, as a function of temperature and magnetic field, to probe their superconducting order parameter symmetries. Low-temperature differential conductance measurements on the heavy-fermion system PrOs4Sb12 revealed multiple spectral features, including distinct evidence for gap nodes in the superconducting order parameter. A field-driven change in order parameter symmetry was also observed confirming the existence of multiple superconducting phases with different symmetries in PrOs4Sb12. The effect of Ru doping in Pr(Os1−xRux)4Sb12 was studied and the spectral evolution is interpreted in terms of multigap pairing. Our data which indicates that the nodal order parameter is rapidly suppressed by the introduction of Ru is consistent with other experimental results. Point-contact tips were fabricated from the high-Tc superconductor YBa2Cu3O7− (YBCO) and used to form high impedance junctions with CrO2 to determine how the formation of Andreev bound states is effected by spin polarization. Strong suppression of the d-wave Andreev reflection characteristics were observed in the YBCO/CrO2 junctions indicating that spin polarization prevents Andreev surface states from forming. The point contacts were estimated to be on the order of several nanometers in size, attesting to their microscopic nature and demonstrate the feasibility of using superconducting high-Tc tips as spin-sensitive nanoprobes of itinerant ferromagnets. Point-contact Andreev spectroscopy measurements were performed on single crystal samples of the heavy-fermion superconductor CeCoIn5. The temperature dependence of the differential conductance spectrum, measured down to 150 mK, showed multiple features whose characteristics depended on the junction impedance. Spectral analysis revealed two co-existing order parameters with nodal symmetry indicating a highly unconventional pairing symmetry in CeCoIn5. The magnetic field dependence of the differential conductance spectrum, where the applied field was parallel to the c-axis, was measured and the upper critical field observed was consistent with that determined by other experimental studies. Low-energy peak structures were observed in low-temperature differential conductance measurements performed on the itinerant ferromagnet ZrZn2. The temperature and field dependence of the spectra were measured and discussed within the context of surface superconductivity known to exist in ZrZn2.
URI: http://hdl.handle.net/1807/32024
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