Andreev reflection spectroscopy of the heavy-fermion superconductor CeCoIn$_5$ along three different crystallographic orientations

TL;DR

This study uses Andreev reflection spectroscopy on CeCoIn$_5$ single crystals along three orientations, revealing features consistent with d-wave symmetry and providing the first spectroscopic evidence for the order parameter symmetry in this heavy-fermion superconductor.

Contribution

It presents the first spectroscopic evidence for d-wave symmetry of the order parameter in CeCoIn$_5$, addressing previous controversies and analyzing conductance spectra across multiple orientations.

Findings

Conductance spectra show asymmetric background conductance due to heavy-fermion liquid.

Energy gap of approximately 460 μeV was estimated using the d-wave BTK model.

Spectral weight shifts and suppressed Andreev reflection indicate limitations of extended BTK models.

Abstract

Andreev reflection spectroscopy has been performed on the heavy-fermion superconductor (HFS) CeCoIn$_5$ single crystals along three different crystallographic orientations, (001), (110), and (100), using Au tips as counter-electrodes. Dynamic conductance spectra are reproducible over wide temperature ranges and consistent with each other, ensuring the spectroscopic nature. Features common to all directions are: i) asymmetric behaviors of the background conductance, which we attribute to the emerging coherent heavy-fermion liquid; ii) energy scales (~1 meV) for conductance enhancement due to Andreev reflection; iii) magnitudes of enhanced zero-bias conductance (10 - 13 %). These values are an order of magnitude smaller than the predicted value by the Blonder-Tinkham-Klapwijk (BTK) theory, but comparable to those for other HFSs. Using the d-wave BTK model, we obtain an energy gap of ~ 460 ueV. However, it is found that extended BTK models considering the mismatch in Fermi surface parameters do not account for our data completely, which we attribute to the shift of spectral weight to low energy as well as to the suppressed Andreev reflection. A qualitative comparison of the conductance spectra with calculated curves shows a consistency with d$_{x^2-y^2}$-symmetry, providing the first spectroscopic evidence for the order parameter symmetry and resolving the controversy over the location of the line nodes.