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arXiv · 0910.4230

Point-contact spectroscopic studies on normal and superconducting AFe_2As_2-type iron-pnictide single crystals

Xin Lu·W. K. Park·H. Q. Yuan·G. F. Chen·G. L. Luo·N. L. Wang·A. S. Sefat·M. A. McGuire·R. Jin·B. C. Sales·D. Mandrus·J. Gillett·Suchitra E. Sebastian·L. H. Greene
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Abstract

Point-contact Andreev reflection spectroscopy (PCARS) is applied to investigate the gap structure in iron pnictide single crystal superconductors of the AFe_2As_2 (A=Ba, Sr) family ("Fe-122"). The observed point-contact junction conductance curves, G(V), can be divided into two categories: one where Andreev reflection is present for both (Ba_{0.6}K_{0.4})Fe_2As_2 and Ba(Fe_{0.9}Co_{0.1})_2As_2, and the other with a V^{2/3} background conductance universally observed extending even up to 100 meV for Sr_{0.6}Na_{0.4}Fe_2As_2 and Sr(Fe_{0.9}Co_{0.1})_2As_2. The latter is also observed in point-contact junctions on the nonsuperconducting parent compound BaFe_2As_2. Mesoscopic phase-separated coexistence of magnetic and superconducting orders is considered to explain distinct behaviors in the superconducting samples. For Ba_{0.6}K_{0.4}Fe_2As_2, double peaks due to Andreev reflection with strongly-sloping background are frequently observed for point-contacts on freshly-cleaved c-axis surfaces. If normalized by a background baseline and analyzed by the Blonder-Tinkham-Klapwijk model, the data show a gap size ~3.0-4.0 meV with 2Δ_0/k_BT_c ~ 2.0-2.6, consistent with the smaller gap size reported in the LnFeAsO family ("Fe-1111"). For the Ba(Fe_{0.9}Co_{0.1})_2As_2, G(V) curves typically display a zero-bias conductance peak.

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Xin Lu, W. K. Park, H. Q. Yuan, G. F. Chen, G. L. Luo, N. L. Wang, A. S. Sefat, M. A. McGuire, R. Jin, B. C. Sales, D. Mandrus, J. Gillett, Suchitra E. Sebastian, L. H. Greene. 2009-11-30. Point-contact spectroscopic studies on normal and superconducting AFe_2As_2-type iron-pnictide single crystals. https://doi.org/10.1088/0953-2048%2F23%2F5%2F054009

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