Two-channel point-contact tunneling theory of superconductors

Abstract

We introduce a two-channel tunneling model to generalize the widely used BTK theory of pointcontact 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 CeCoIn5 and CeRhIn5 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 temperatureindependent 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 CeCoIn5 consistent with standard d x 2 −y 2 -wave pairing, assuming an equal mixture of tunneling into [100] and [110] crystallographic interfaces. Whereas in CeRhIn5 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.

Superconductivity and magnetism in noncentrosymmetric RhGe

“…We used the novel soft PCS technique in which the contact to the surface of a superconductor is made by gluing it with a small M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT droplet of silver epoxy to the end of fine 25-µm-diameter Pt wire. This technique give stable contact and was used earlier for studies of the superconducting order parameter of various superconductors at ambient pressure [40] and even at high pressure [41], where the opening of an energy gap near 2 GPa was found in the heavy fermion superconductor CeRhIn 5 .…”

High-pressure studies on heavy fermion systems

“…[23,26] In recent years, PCS has also been extended to study the hybridization between localized f electrons and conduction electrons in heavy fermion systems, where asymmetric Fano-line shape or hybridization gap is observed due to the quantum interference between the local f and itinerant conduction electrons. [27][28][29][30] PCS has become an effective method to study the systematical evolution of the hybridization in heavy fermion systems tuned by pressure as another dimension.…”

Spin-polarized currents and noise in normal-metal/superconductor junctions with Yu-Shiba-Rusinov impurities

“…It means that the contact area is much smaller than the superconducting coherence length, A c ξ 2 0 . This circumstance allows us to assume the order parameter to be approximately spatially independent 41,42 .…”