Two-channel point-contact tunneling theory of superconductors

Mikael Fogelström, Matthias J. Graf, V. A. Sidorov, Xin Lu, E. D. Bauer, J. D. Thompson
2014 Physical Review B  
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_5
more » ... and CeRhIn_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_5 consistent with standard d_x^2-y^2-wave pairing, assuming an equal mixture of tunneling into [100] and [110] crystallographic interfaces. Whereas in CeRhIn_5 at 1.8 and 2.0 GPa the signal is described by a d_x^2-y^2-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.
doi:10.1103/physrevb.90.104512 fatcat:zie7qnoekbgfzd2m2acuf2j5g4