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 Title: Point-contact Andreev reflection studies on superconductors Author(s): Lu, Xin Director of Research: Greene, Laura H. Doctoral Committee Chair(s): Eckstein, James N. Doctoral Committee Member(s): Greene, Laura H.; Peng, Jen-Chieh; Leggett, Anthony J. Department / Program: Physics Discipline: Physics Degree Granting Institution: University of Illinois at Urbana-Champaign Degree: Ph.D. Genre: Dissertation Subject(s): Point-contact Andreev reflection spectroscopy (PCARS) superconductors superconducting gap order parameter symmetry Sharvin ballistic limit planar tunneling density of states (DOS) Blonder-Thinkham-Klapwijk Model NbSe2 borocarbide iron-based superconductor Abstract: Point-contact Andreev reflection spectroscopy (PCARS) and planar tunneling spectroscopy are used to investigate the density of states and superconducting gap structure of several novel superconducting systems. Theoretical and experimental background is given with an emphasis on PCARS as applied to various unconventional superconductors, such as NbSe$_2$, the borocarbide family RNi$_2$B$_2$C (R=Lu, Y) and the recently-discovered, iron-based superconductors AFe$_2$As$_2$ (A=Ba, Sr) with both hole and electron doping. The superconducting gap distribution observed from point-contacts on NbSe$_2$, with energies between 0.9 and 1.5 meV at 2 K, is consistent with the multigap scenario claimed by other experiments. The superconducting gap follows the BCS temperature dependence. PCARS probes a small gap anisotropy in both LuNi$_2$B$_2$C and YNi$_2$B$_2$C for the three major crystallographic orientations, while point-nodes are reported to exist in \textit{a} and \textit{b} axes and predicted to originate from the antiferromagnetic fluctuations due to nesting structure on the 17th band Fermi surface. This can be explained by the other Fermi surface sheets, with $\Delta\geq$2.1 meV in [100] direction, masking the point-node feature and resulting in a small gap anisotropy, as observed in our PCARS. A large tunneling cone in PCARS measurements, which would smear the differences in three directions, may be another origin of the observed small gap anisotropy. For the Fe-122 family of the iron-based superconductors, Andreev reflection is present for PCARS on (Ba$_{0.6}$K$_{0.4}$)Fe$_2$As$_2$ and Ba(Fe$_{0.9}$Co$_{0.1}$)$_2$As$_2$ crystals but measured conductances are distinct. A V-shape conductance valley (VCV) feature is observed for point-contacts on Sr(Fe$_{0.9}$Co$_{0.1}$)$_2$As$_2$ and (Sr$_{0.6}$Na$_{0.4}$)Fe$_2$As$_2$ crystals. This VCV feature is also observed for the non-superconducting parent compound BaFe$_2$As$_2$ and the superconducting (Ba$_{0.6}$K$_{0.4}$)Fe$_2$As$_2$ crystals. The coexistence of phase-separated magnetic and superconducting orders on the mesoscopic scale is considered to explain the range of behaviors in the superconducting samples. Issue Date: 2010-05-14 URI: http://hdl.handle.net/2142/15554 Rights Information: Copyright 2010 Xin Lu Date Available in IDEALS: 2010-05-142012-05-15 Date Deposited: May 2010
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