Andreev reflection and order parameter symmetry in heavy-fermion superconductors: the case of CeCoIn$_5$

TL;DR

This review discusses Andreev reflection spectroscopy on CeCoIn$_5$, revealing a $d_{x^2-y^2}$-wave order parameter and proposing a Fano interference model to explain conductance features in heavy-fermion superconductors.

Contribution

It provides the first detailed analysis of Andreev reflection in CeCoIn$_5$ and introduces a phenomenological Fano interference model to explain conductance asymmetry and reduced signals.

Findings

Andreev reflection observed with ~13% of theoretical maximum

Order parameter symmetry identified as $d_{x^2-y^2}$-wave

Fano interference model explains conductance asymmetry

Abstract

We review the current status of Andreev reflection spectroscopy on the heavy fermions, mostly focusing on the case of CeCoIn$_5$, a heavy-fermion superconductor with a critical temperature of 2.3 K. This is a well-established technique to investigate superconducting order parameters via measurements of the differential conductance from nanoscale metallic junctions. Andreev reflection is clearly observed in CeCoIn$_5$ as in other heavy-fermion superconductors. The measured Andreev signal is highly reduced to the order of maximum $\sim$ 13% compared to the theoretically predicted value (100%). Analysis of the conductance spectra using the extended BTK model provides a qualitative measure for the superconducting order parameter symmetry, which is determined to be $d_{x^2-y^2}$-wave in CeCoIn$_5$. A phenomenological model is proposed employing a Fano interference effect between two conductance channels in order to explain both the conductance asymmetry and the reduced Andreev signal. This model appears plausible not only because it provides good fits to the data but also because it is highly likely that the electrical conduction occurs via two channels, one into the heavy electron liquid and the other into the conduction electron continuum. Further experimental and theoretical investigations will shed new light on the mechanism of how the coherent heavy-electron liquid emerges out of the Kondo lattice, a prototypical strongly correlated electron system. Unresolved issues and future directions are also discussed.