Point-contact spectroscopy of heavy-fermion superconductors and metals

Speaker: Wan Kyu Park, University of Illinois, Urbana-Champaign

When: March 23, 2007 (Fri), 12:00PM to 02:00PM (add to my calendar)
Location: SCI 352

This event is part of the Biophysics/Condensed Matter Seminar Series.

Identifying the order parameters in a superconductor is a crucial step toward an elucidation of the underlying pairing mechanism. The recently discovered heavy-fermion compounds CeTIn5 (T = Co, Rh, Ir) and their doped variants have attracted a great deal of interest due to their novel and rich physical properties, including coexistence of superconductivity and magnetism, quantum criticality, and intrinsically inhomogeneous superconductivity. We have employed a point-contact Andreev reflection spectroscopy technique [1] in order to investigate the detailed order parameter structures in these heavy fermions. This is a proven technique probing the Andreev reflection process, which is a retro-reflection of an injected electron as a hole at a normal-metal/superconductor interface, transferring a Cooper pair and doubling the electrical conductance in an ideal setting. In this talk, I will present our recent point-contact spectroscopy results on heavy-fermion superconductors, pure and Cd-doped CeCoIn5, and heavy-fermion metals such as CeCoIn5 above its Tc (= 2.3 K), CeRhIn5, and YbAl3. Our Andreev conductance data on CeCoIn5 junctions along three major crystallographic orientations provide the first spectroscopic evidence for dx2-y2 symmetry of the superconducting order parameter in this compound. Andreev signal in CeCoIn5 appears heavily reduced, similar to those in other heavy-fermion superconductors but in contrast to that in Nb probed by using heavy-fermion metals. Asymmetry in the background conductance is observed in all junctions involving heavy-fermions although it differs in material-dependent detail. Physical explanations for these behaviors will be given in the context of a two-fluid model [3] and an energy-dependent density of states in heavy fermions. Conductance spectra on 10% Cd-doped CeCoIn5 exhibit intriguing behaviors as a function of temperature and magnetic field, undergoing antiferromagnetic and superconducting transitions. Their implications will be discussed in terms of competing or coexisting electronic phases.

[1] W. K. Park and L. H. Greene, Rev. Sci. Instrum. 77, 023905 (2006). [2] W. K. Park et al., Phys. Rev. B 72, 052509 (2005); cond-mat/0507353; cond-mat/0606535; W. K. Park and L. H. Greene, Phys. Rev. Lett. 96, 259702 (2006). [3] S. Nakatsuji, D. Pines and Z. Fisk, Phys. Rev. Lett. 92, 016401 (2004).

In collaboration with Heiko Stalzer, Laura Greene, John Sarrao, Joe Thompson, Long Pham, Zachary Fisk, Josh Frederick, Paul Canfield.

Pizza at 11:30 AM