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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 Tc 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 TN comparable to their critical temperature Tc, and have the highest Néel temperatures TN 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.

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Jess H. Brewer