The Nickel Borocarbide Family

The nickel borocarbides form a recently discovered class of compounds exhibiting unusual superconducting and magnetic characteristics. Their constituents combine in the ratio RNi₂B₂C, where R stands for a rare earth element or yttrium (Y), giving rise to the lattice shown in Figure 3.1.

  

Figure: RNi₂B₂C body centred tetragonal crystal structure (space group I4/mmm), from reference [26].

Each nickel atom tetrahedrally co-ordinates with the four closest borons to build Ni₂B₂ layers which alternate with RC sheets. Despite this planar composition, band structure calculations predict isotropic electronic properties [27] and resistivity measurements [28] confirm this. These materials provide an excellent opportunity to study the connection between superconductivity and magnetism.

Superconducting members of the nickel borocarbide family manifest relatively high critical temperatures T_c and mysterious field driven vortex lattice symmetry transitions. The substances are clean type II superconductors [29] with high Ginzburg-Landau parameters $\kappa \sim 5 - 15$ [30] and intermediately strong electron phonon interaction [27]. As the external magnetic field varies the vortex lattice geometry evolves from hexagonal to square, a phenomenon attributed to the squarish flux line cross section [31]. Under this transformation TmNi₂B₂C ( $T_c = 11.0\,\mathrm{K}$) exhibits simultaneous transitions in magnetic order [30].

Magnetic nickel borocarbide superconductors become antiferromagnetically ordered below a Néel temperature T_N comparable to their critical temperature T_c, and have the highest Néel temperatures T_N of any superconductor [32]. ErNi₂B₂C ( $T_c = 10.5\,\mathrm{K}$, $T_N = 6.0\,\mathrm{K}$) displays weak ferromagnetism as well when cooled below $T_{WFM} = 2.5\,\mathrm{K}$ [30]. The wave vector for maximum generalised electronic susceptibility in LuNi₂B₂C also characterises the incommensurate magnetic structures in superconducting ErNi₂B₂C and HoNi₂B₂C, and in the nonsuperconductors TbNi₂B₂C and GdNi₂B₂C [33] [34]. A phonon mode near this wave vector softens greatly on cooling, and is comparable to the superconducting gap in energy. These observations have triggered speculation that phonon softening and magnetic ordering stem from a common Fermi surface nesting and compete to decrease the system energy. Investigation of the superconductivity of this material family, without the complications introduced by magnetism, is possible with the nonmagnetic members YNi₂B₂C and LuNi₂B₂C.