•Gernot Goll¹, Hilbert v. Löhneysen^1,2, Vivien S. Zapf³, Eric D. Bauer³ und M. Brian Maple^3
1Physikalisches Institut, Universität Karlsruhe, 76128 Karlsruhe
²Forschungszentrum Karlsruhe, Institut für Festkörperphysik, 76021 Karlsruhe
³Institute for Pure and Applied Physical Sciences, University of California, San Diego, La Jolla, CA 92093, USA

The ternary rare-earth compound CeCoIn₅ becomes superconducting for temperatures T £ 2.3 K, the highest transition temperature among the heavy-fermion superconductors. Power-law behavior of the specific heat and the thermal conductivity in the superconducting state give evidence that the superconductivity in this material is unconventional [1]. We report on investigations of CeCoIn₅ by point-contact spectrocopy with Pt as the normal-metal counterelectrode. Andreev reflection of quasiparticles at a normal metal/superconductor interface leads to characteristic features in the differential conductance dI/dV as a function of applied bias V. We measured spectra which show either an enhanced conductance for bias |V| < D/e or a single maximum of dI/dV for V = 0, i. e. a zero-bias anomaly, depending on the transparency of the interface barrier. The observation of a zero-bias conductance anomaly is expected only if the order parameter exhibits a sign change as a function of [k\vec] which leads to a bound state at the surface. Therefore the data support possible unconventional superconductivity in CeCoIn₅. Different order-parameter scenarios will be discussed.
 

[1] R. Movshovich et al., Phys. Rev. Lett. 86, 5152 (2001).