Two-channel point-contact tunneling theory of superconductors

• Published in: Physical review. B, Condensed matter and materials physics Condensed matter and materials physics, 2014-09, Vol.90 (10), Article 104512
• Main Authors: Fogelström, Mikael, Graf, Matthias J., Sidorov, V. A., Lu, Xin, Bauer, E. D., Thompson, J. D.
• Format: Article
• Language: English
• Subjects: Channels; Condensed matter; Conductance; Contact; Mathematical models; Silver; Slopes; Superconductors; Tunneling
• ISSN: 1098-0121; 1550-235X
• DOI: 10.1103/PhysRevB.90.104512

We introduce a two-channel tunneling model to generalize the widely used BTK theory of point-contact conductance between a normal metal contact and superconductor. Tunneling of electrons can occur via localized surface states or directly, resulting in a Fano resonance in the differential conductance G = dI/dV. We present an analysis of G within the two-channel model when applied to soft point contacts between normal metallic silver particles and prototypical heavy-fermion superconductors CeCoIn sub(5) and CeRhIn sub(5) at high pressures. In the normal state the Fano line shape of the measured G is well described by a model with two tunneling channels and a large temperature-independent background conductance. In the superconducting state a strongly suppressed Andreev reflection signal is explained by the presence of the background conductance. We report Andreev signal in CeCoIn sub(5) consistent with standard d sub(x2-y2)-wave pairing, assuming an equal mixture of tunneling into [100] and [110] crystallographic interfaces, whereas in CeRhIn sub(5) at 1.8 and 2.0 GPa the signal is described by a d sub(x2-y2)-wave gap with reduced nodal region, i.e., increased slope of the gap opening on the Fermi surface. A possibility is that the shape of the high-pressure Andreev signal is affected by the proximity of a line of quantum critical points that extends from 1.75 to 2.3 GPa, which is not accounted for in our description of the heavy-fermion superconductor.