Phase formation and superconductivity of metal boronitrides

The discovery of superconductivity in rare-earth transition-metal borocarbides and boronitrides, RT2B2C and (LaN)3Ni2B2 eight years ago stimulated considerable research activity and revealed a number of outstanding features such as low-energy phonons, Fermi surface nesting and two band characteristics, nonlocality in the superconducting state, anisotropic upper critical field and vortex lattice transitions and phenomena related to the interplay of superconductivity and magnetism. A very special aspect of these compounds is their layered structure which indicates some analogies to MgB2 with Tc of even 40 K. Quaternary borocarbides have already been thoroughly investigated and have had considerable impact on literature. However, because of the difficult preparation of the boronitrides only a few studies on this system have been published although (LaN)3Ni2B2 appears to be a very interesting model type superconductor with a natural ABABA.. type heterostructure. Not even near at hand substitutions such as with magnetic rare earth elements have been seriously studied so far. The first task of this project is to advance the preparation techniques for the boronitrides and to search for chemical modifications of (LaN)3Ni2B2 such as chemical doping and substitution of La by magnetic rare earth elements. One primar aim is to tune the valence charge density distribution in a systematic way and to study the impact of these manipulations upon the superconducting state properties. The boronitrides are, thus, expected to provide a model-type reference system to test multiband theories (recently developed in the context with MgB2) which shall contribute a deeper insight to interrelations between electronic structure, superconductivity, pair-breaking effects and other physical properties of `conventional anisotropic (layered) superconductors. A further important aspect of this project is the progress expected in the physical and structural chemistry of higher order metal boronitrides. A challanging aim is the search for novel superconducting and magnetic higher order metal boronirtides and the investigation of the interplay of superconductivity and/or magnetism in layered compounds. It is planned to prepare alloys with compositions such as e.g. (LaN)(RN)(LaN)Ni2B2 with R= Ce, Pr and Nd. Such rare-earth transition metal boronitrides - if ordered - may form a natural multilayer with alternating superconducting and magnetic sheets and are thus candidates for exciting new phenomena resulting from the antagonistic nature of these two electronic ground states.

Studies on the phase formation, superconducting and magnetic properties of rare earth transition metal boronitride - an exotic superconductor with a layered crystal structure - and of related and novel materials have been performed. The key achievement with respect to the material chemistry has been the development of new preparation methods to obtain single phase La3Ni2B2N3 with well defined and reproducible stoichiometry. The methods allow adjustment of the nitrogen stoichiometry within +- 0.05 formula units and provide the basis for a systematic investigation of the high temperature phase diagram and thermal history effects. This allowed the width of formation with respect to the nitrogen stoichiometry in La3Ni2B2N(x) and the corresponding variation of the superconducting properties, for initial studies on crystal growth via zone melting the material to be explored. The methods have been used to investigate the effect of hydrogen absorption and enabled studies of phase formation of rare earth transition metal boronitrides with magnetic rare earth elements (Ce Pr, Nd). The latter revealed a limited solid solubility of Ce Pr, and Nd on the rare earth sublattice in (La,R)3Ni2B2N3, i.e. single phase material has been obtained on the La-rich side whereas the preparation of new quaternary phases R3Ni2B2N3 with magnetic rare earth elements failed. The investigation of the solid solution (La,R)3Ni2B2N3 with R= Ce and Nd provided initial information about magnetic pair-breaking effects in the boronitrides. Experimental studies on La3Ni2B2N3 by means of specific heat and magnetic susceptibility measurements have been complemented by theoretical studies by means of the Eliashberg formalism. The latter studies have finally focused on the analysis of thermodynamic and upper critical field data of the recently discovered beta-pyrochlore superconductor RbOs2O6 which is of particular interest because of its strongly anharmonic Rb-phonon modes. In this context our study provided information on the anisotropy of the electron-phonon coupling and the anisotropy of the Fermi velocity. Studies on the magnetic properties of the closely related compound GdNi2B2C contributed to the investigation of the magnetic structure, evaluation of Gd-Gd exchange interactions and provided important results in context with the investigation of magnetoelastic effects in Gd compounds. In the case of the layered borocarbide superconductor YB2C2 we have studied the intrinsic superconducting and thermodynamic properties complementing ab initio electronic structure calculations. Despite of the large interest on magnetic RB2C2, intrinsic superconductivity of YB2C2 was still unexplored. Motivated by the huge interest on our discovery of unconventional superconductivity in CePt3Si - a heavy fermion system without inversion symmetry - studies on the "hole analogue" YbPt3Si have been made, revealing a competition of Kondo and RKKY interactions on similar energy scales as observed in "electron analogue" CePt3Si.