Superconductivity and magnetism and their interplay in quaternary borocarbides RNi₂B₂C

Abstract

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 RNi₂B₂C, 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 RNi₂B₂C (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 RNi₂B₂C 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 k ~ 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. RNi₂B₂C 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 mechanism. Magnetic superconductors RNi₂B₂C (R = Dy, Ho, Er, Tm) reveal several phenomena, not observed earlier, associated with the interplay of superconductivity and magnetism. Microscopic evidence (via square FLL interacting with magnetism) of the coexistence of magnetism and superconductivity; intrinsic FLL-pinning by magnetic ions; weak ferromagnetism (local moment) coexisting with superconductivity (down to the lowest temperature) and the spontaneous vortex phase (ErNi₂B₂C); superconductivity setting in an already magnetically ordered lattice (DyNi₂B₂C) and pair-breaking by nonmagnetic ions in such materials; rich and complex magnetic structures and double (nearly) re-entrant superconductivity (HoNi₂B₂C) and changes in the FLL-symmetry in the vicinity of magnetic transition (TmNi₂B₂C) and 4f-quadrupole ordering (TmNi₂B₂C) are several exciting phenomena that magnetic superconductors RNi₂B₂C exhibit. At the end of this review are indicated some possible further studies in quaternary borocarbide superconductors. These studies may turn out to be important not only with respect to borocarbides themselves but also from the standpoint of superconductivity in general.