Andreev Reflection in Heavy-Fermion Superconductors and Order Parameter Symmetry in CeCoIn_5

TL;DR

This study provides spectroscopic evidence for d_{x^2-y^2} symmetry in CeCoIn_5 heavy-fermion superconductor through Andreev reflection measurements, revealing characteristic conductance features and proposing a comprehensive phenomenological model.

Contribution

First spectroscopic confirmation of d_{x^2-y^2} symmetry in CeCoIn_5 using Andreev reflection and a new model fitting the conductance data.

Findings

Observation of conductance asymmetry and Andreev reflection with reduced signal

Consistent spectroscopic features across multiple junctions

Successful modeling of conductance spectra with a phenomenological approach

Abstract

Differential conductance spectra are obtained from nanoscale junctions on the heavy-fermion superconductor CeCoIn$_5$ along three major crystallographic orientations. Consistency and reproducibility of characteristic features among the junctions ensure their spectroscopic nature. All junctions show a similar conductance asymmetry and Andreev reflection-like conductance with reduced signal (~ 10%-13%), both commonly observed in heavy-fermion superconductor junctions. Analysis using the extended Blonder-Tinkham-Klapwijk model indicates that our data provide the first spectroscopic evidence for $d_{x^2-y^2}$ symmetry. To quantify our conductance spectra, we propose a model by considering the general phenomenology in heavy fermions, the two-fluid behavior, and an energy-dependent density of states. Our model fits to the experimental data remarkably well and should invigorate further investigations.