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Technology of High-Temperature Superconducting Films and Devicessketch of the preparation of YBC0 grain-boundary Josephson junctions at substrate steps.
Nb3Sn Films on Sapphire Substrates
Structure and Electronic PropertiesThe intermetallic system Nb3Sn belongs to the group of cubic A15 Structures, which contains more than 50 superconducting compounds [3]. Among these are the "classical high-temperature superconductors" with Tc > 10 K, which are binary compounds of the type A3B. Usually, "A" stands for a transition element like vanadium (V) or niobium (Nb), and "B" represents aluminum (A1), silicon (Si), gallium (Ga), germanium (Ge), or tin (Sn). A record value of Tc = 23.2 K was found for Nb3Ge [4]. However, in terms of mechanical and chemical stability, the discovery of Nb3Sn (Tc = 18 K) [5] turned out to be of major technological relevance. Reflecting the enormous impact on the science and technology of high-To type-II superconductors (e.g., for highfield magnets), the structural and electronic properties of the A15 compounds have been discussed in numerous reviews [3,[6][7][8].A unique feature of the binary A3B compounds is the formation of a bodycentered cubic lattice by the B atoms as shown in Fig. 5.1. The A atoms are arranged at the faces of the cubes, forming three orthogonal non-intersecting chains along the [001], [010] and [100] directions of the crystal. The distance between adjacent atoms along one chain (a/2 ~ 0.265 nm in NbaSn) is half of the lattice dimension (a = 0.529 nm). It is smaller than the separation of atoms between neighboring chains (_> 0.324 nm in Nb3Sn). As a result, the electronic wave functions extend mainly along the chains, although interactions between chains were found to contribute significantly to the electronic band structure [6].Most of the high-T~ compounds of the A15 superconductors display a softening of the phonon frequencies with decreasing temperature. Especially in Nb3Sn, a first-order phase transition from the cubic symmetry to a tetragonal distortion occurs around 43 K as illustrated in Fig. 5.2. At low temperatures, the lattice constant of the Sn cubus increases, as does the separation of the Nb atoms along the correspondingly oriented chain [7]. It is speculated that phonon softening and lattice instability are related to the high transition temperature. A theoretical approach to this electronic and structural interrelation was derived in terms of a one-dimensional model by Labb~ and Friedel [8, 9]. According to their model, which neglects electronic interactions between the chains, the electronic density of states exhibits singularities arising from the overlap between the d orbitals of adjacent chain atoms. A close proximity between one of these singularities and the Fermi energy provides the basis for both a high transition temperature (Chap. 1) and a structural phase transition. While this interpretation provides a reasonable qualitative
