Superconductivity and magnetism and their interplay in quaternary borocarbides RNi2B2C

Since 1986, most of the interest in superconductivity became focused on high-T c cuprates. The discovery of the superconducting quaternary borocarbide system Y-Ni-B-C with T c as high as ∼12 K inspired research into intermetallic superconductors (IMS) once again. Several reasons can be attributed to this revival of interest in IMS: (i) In the tetragonal quaternary magnetic superconductors RNi 2 B 2 C, superconductivity and magnetism occur with T c and T N ∼ 10 K, thereby allowing studies of exotic phenomena associated with, and arising from, the interplay of superconductivity and magnetism. (ii) High T N 's and a variety of commensurate and incommensurate magnetic structures in RNi 2 B 2 C (Fermi surface nesting playing a central role) strongly suggest that R-spins are coupled via the RKKY-exchange interaction. Hence, unlike in most other magnetic superconductors known so far, conduction electrons take part in superconductivity and magnetism. (iii) Quaternary borocarbides open up new pathways to try and synthesize multicomponent intermetallic superconductors. Their remarkable intrinsic superconducting and magnetic properties and the availability of high quality samples (bulk polycrystalline, large single crystals and thin films) make RNi 2 B 2 C particularly special to investigate. Several unusual phenomena have been reported, such as, to name a few, dramatic phonon mode softening at T c , H c2 (T) exhibiting a positive curvature near T c and a four-fold anisotropy in the basal plane; a variety of exceptional and fascinating flux line lattice (FLL) related effects - FLL-symmetry transformations and alignments with the underlying crystal lattice as a function of applied field (manifestation of nonlocal electrodynamics despite high κ ∼ 10, and thermal fluctuation effects even though T c , ∼ 16 K, is not too high) and a four-fold symmetric star-shaped (in real space) vortex core. RNi 2 B 2 C are strong coupling s-wave BCS superconductors and, remarkably, have a superconducting gap with extreme anisotropy. Strong experimental evidence shows that the four-fold symmetric superconducting gap has point nodes along the 100 - and 10 -directions, a feature that has been shown consistent with (s + g)-Cooper pairing. An energy gap with such strong anisotropy is unusual for an s-wave superconductor and, hence, calls for a pairing mechanism different from conventional electron-phonon coupling. Antiferromagnetic fluctuations possibly play an important role in the mechani