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Temperature and junction-type dependency of Andreev reflection in MgB2

R. S. Gonnelli·A. Calzolari·D. Daghero·G. A. Ummarino·V. A. Stepanov·P. Fino·G. Giunchi·S. Ceresara·G. Ripamonti
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Abstract

We studied the voltage and temperature dependency of the dynamic conductance of normal metal-MgB2 junctions obtained either with the point-contact technique (with Au and Pt tips) or by making Ag-paint spots on the surface of high-quality single-crystal-like MgB2 samples. The fit of the conductance curves with the generalized BTK model gives evidence of pure s-wave gap symmetry. The temperature dependency of the gap, measured in Ag-paint junctions (dirty limit), follows the standard BCS curve with 2Delta/kTc = 3.3. In out-of-plane, high-pressure point contacts we obtained almost ideal Andreev reflection characteristics showing a single small s-wave gap Delta = 2.6 +/- 0.2 (clean limit). These results support the two-gap model of superconductivity, the presence of a modified layer at the surface of the crystals and an important and non-conventional role of the impurities in MgB2.

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R. S. Gonnelli, A. Calzolari, D. Daghero, G. A. Ummarino, V. A. Stepanov, P. Fino, G. Giunchi, S. Ceresara, G. Ripamonti. 2001-07-11. Temperature and junction-type dependency of Andreev reflection in MgB2. https://doi.org/10.1016/s0022-3697(02)00229-9

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