C-AXIS ORIENTED, IN PLANE TEXTURED BOROCARBIDES THIN FILMS DEPOSITED BY PULSED LASER DEPOSITION: STRUCTURE, SURFACE MORPHOLOGY AND PHYSICAL PROPERTIES

摘要

The borocarbide compounds discovered in 1994 have been intensively studied in following years for many reasons: first, their interesting properties, in particular when the rare earth is magnetic, second for the possibility to use standard liquid helium technology, due to the wide range of temperatures in which magnetic and superconducting transitions take place, and third for the availability of very large, high quality samples. In fact, a method for single crystal growth appeared in literature within a few month of the discovery of these materials; with this method it is possible to obtain single crystals of large dimensions. These bulk samples can be studied in a simple way, and they have given the principal answers about the behaviour of these systems. On the other hand, many groups spent a lot of efforts in the deposition of borocarbides thin films, but, independently from the used technique (magnetron spattering, in gas-laser ablation, in vacuum-laser ablation), the quality of the samples is, up to now, lower than in bulk samples. The main reason of this resides in the difficulties encountered in the deposition process, that happen at very high temperatures, and also in the choice of the right substrate. Indeed, the lattice parameters of this family don't match with any of the commonly used substrates (perovskites, sapphire, magnesium oxide) and this doesn't enable a simple epitaxial growth as in the case of cuprates. Here we would name the results of the group of Dresden, Germany, and Tsukuba, Japan, and the contribution given by the Italian groups in Naples and Genoa. In general, high quality thin films are needed to explore many physical properties not easily measured in bulk samples; in the case of superconducting materials, they can provide information about the superconducting gap by means of tunnelling measurements on junction devices and about penetration depth using surface impedance microwave measurements. This paper is focused on the recent results about the structural, morphological and superconducting properties of the first in-plain textured LuNi_2B_2C thin film. In this case, many properties are similar to the best single crystals reported in literature, as evidenced by the present measurements.