Tunnelling spectroscopy of high‐temperature superconductors

We measured samples of La_1·85Sr_0·15CuO_4‐y and YBa₂Cu₃O_7‐y with a UHV low‐temperature scanning tunnelling microscope using tips made out of tungsten, aluminium and indium. The samples are either sintered pellets, single crystals, or thin films. A wide variety of I‐V curves were obtained ranging from nearly ideal S‐I‐N and S‐I‐S characteristics, to strongly asymmetric and multiple‐peaked characteristics. The best data are obtained with thin film samples and indium tips. Large values of 2δ/kT_c (∼ 11) are observed in most cases. We discuss possible explanations for these large values. New data are presented obtained with thin films of Y‐Ba‐Cu‐O made by e‐beam evaporation in our laboratory. A system of loadlocks is used to ensure that these films are never exposed to the atmosphere. 1988 Blackwell Science Ltd