Re-Ni2B2C superconducting borocarbides: ''In situ'' growth of thin films, transport and point contact spectroscopy

High quality c-axis oriented films of the intriguing intermetallic superconducting compound YNi2B2C have been obtained ''in situ'' by magnetron sputtering on MgO substrates held at about 800 degrees C. The films showed maximum T-c = 15.3 K, Delta T-c approximate to 0.1 K, room temperature resistivity rho approximate to 50 mu Omega.cm, critical current J(c) approximate to 10(5) A/cm(2) and B-c2 approximate to 6 T. Superconducting films were also obtained on Al2O3 and LaAlO3 single-crystal substrates. From the rho(T) dependence a value of the Debye temperature Theta(D) = 330 +/- 20 K can be deduced. At low temperatures the resistivity follows a quadratic power law possibly indicative of a high value of the electron-phonon interaction parameter lambda. In order to clarify the role of lambda in these compounds, point contact spectroscopy measurements have been performed on YNi2B2C and HoNi2B2C bulk samples prepared by inductive melting using a Low Temperature Scanning Tunneling Microscope (LTSTM). In the point contact regime clear evidence of a superconducting gap have been found in both compounds, corresponding to a moderate strong coupling behaviour (2 Delta/KTc approximate to 3.8).