PHONON SPECTROSCOPY OF SUPERCONDUCTING NB USING POINT-CONTACT TUNNELING

Tunneling spectroscopy measurements have been made on single crystals of Nb using the point-contact technique with a Au tip. The current-voltage characteristics, I(V) and dI/dV, reproduce the results obtained from conventional, thin-film planar junctions. In the range of 5- to 50-meV bias, the tunneling conductance displays the characteristic structures associated with the phonon density of states for Nb, including a low-energy mode near 10 meV, previously observed only in junctions on the (100) crystal face. These results demonstrate that the point-contact tunneling method can be used to measure the electron-phonon interaction in superconductors with the same degree of sensitivity as found in thin-film tunneling spectroscopy. The high-bias conductance was always an increasing function of voltage, as expected for tunnel-barrier transmission, and never displayed the anomalous decreasing behavior found in low-resistance junctions on high-temperature superconductors (HTSs). We draw some inferences about the effects of heating and discuss them in relation to point-contact tunneling into the HTS materials. © 1990 The American Physical Society.