Visualizing nodal heavy fermion superconductivity in CeCoIn5

By means of low-temperature scanning tunnelling spectroscopy, a heavy fermion material in its superconducting and mixed states can be imaged. Besides probing the superconducting gap symmetry, the measurements also reveal a pseudogap. Understanding the origin of superconductivity in strongly correlated electron systems continues to be at the forefront of the unsolved problems of physics 1 . Among the heavy f -electron systems, CeCoIn 5 is one of the most fascinating, as it shares many of the characteristics of correlated d -electron high- T c cuprate and pnictide superconductors 2 , 3 , 4 , including competition between antiferromagnetism and superconductivity 5 . Although there has been evidence for unconventional pairing in this compound 6 , 7 , 8 , 9 , 10 , 11 , high-resolution spectroscopic measurements of the superconducting state have been lacking. Previously, we have used high-resolution scanning tunnelling microscopy (STM) techniques to visualize the emergence of heavy fermion excitations in CeCoIn 5 and demonstrate the composite nature of these excitations well above T c (ref. 12 ). Here we extend these techniques to much lower temperatures to investigate how superconductivity develops within a strongly correlated band of composite excitations. We find the spectrum of heavy excitations to be strongly modified just before the onset of superconductivity by a suppression of the spectral weight near the Fermi energy ( E F ), reminiscent of the pseudogap state 13 , 14 in the cuprates. By measuring the response of superconductivity to various perturbations, through both quasiparticle interference (QPI) and local pair-breaking experiments, we demonstrate the nodal d -wave character of superconducting pairing in CeCoIn 5 .