Interaction of superconductivity and magnetism in borocarbide superconductors

Abstract

The interaction of rare-earth magnetism and superconductivity has been a topic of interest for many years. In classical magnetic superconductors (Chevrel phases, ternary rhodium borides, etc) as well as in the high-T c cuprates the superconducting state usually coexists with antiferromagnetic order on the rare-earth sublattice. In these compounds the magnetic ordering temperature T N is much below the superconducting transition temperature T c . The discovery of superconducting borocarbides RT 2 B 2 C with R = Sc, Y, La, Th, Dy, Ho, Er, Tm or Lu and T = Ni, Ru, Pd or Pt (where not all of these combinations of R and T result in superconductivity) has reanimated the research on the coexistence of superconductivity and magnetic order. Most of these borocarbides crystallize in the tetragonal LuNi 2 B 2 C type structure which is an interstitial modification of the ThCr 2 Si 2 type. Contrary to the behaviour of Cu in the cuprates Ni does not carry a magnetic moment in the borocarbides. Various types of antiferromagnetic structures on the rare-earth sublattice have been found to coexist with superconductivity in RNi 2 B 2 C for R = Tm, Er, Ho and Dy. Particularly of interest is the case of HoNi 2 B 2 C for which three different types of antiferromagnetic structures have been observed: (i) a commensurate one with Ho moments aligned ferromagnetically within layers perpendicular to the tetragonal c axis where consecutive layers are aligned in opposite directions, (ii) an incommensurate spiral along the c axis and (iii) an incommensurate a-axismodulated structure with a modulation vector τ ≈ (0.55, 0, 0). This wave vector emerges in various RNi 2 B 2 C compounds with magnetic as well as nonmagnetic R elements and is connected with Fermi surface nesting. Both incommensurate magnetization structures have been shown to be related to the near-reentrant behaviour observed in HoNi 2 B 2 C whereas the commensurate structure coexists well with the superconducting state in this compound. The variation of T N and T c with the de Gennes factor can roughly be drawn on straight lines from Lu to Gd and from Lu to Tb, respectively, with the exception of Yb. Consequently, T c > T N holds for Tm, Er, Ho and T c < T N for Dy. However, the study of various pseudoquaternary (R, R )Ni 2 B 2 C compounds has shown that this so-called de Gennes scaling is not universal for the borocarbides and it breaks down in some cases, which is attributed to effects of details

Superconductivity in the ternary ZrVGe compound

“…This compound crystallizes in CeScSi or UGeTe prototype structure belonging to the tetragonal I4/mmm space group with lattice parameters a = 3.754 Å and c = 14.488 Å. This prototype structure it is similar to the quaternary LuNi 2 B 2 C which is the prototype structure of many boron-carbide superconducting materials which offer interesting opportunities for the study of multiband behavior and the coexistence between superconductivity and magnetism [8][9][10][11][12][13][14]. Indeed, superconductivity in layer compounds often produces unusual results and at the same time offers a modern view for the occurrence of superconductivity [9][10][11][12][13][14].…”

Observation of an anisotropic effect of antiferromagnetic ordering on the superconducting gap in ErNi2B2C

“…Quaternary intermetallic nickel borocarbides (hereafter borocarbides) of the RNi 2 B 2 C type (R is a rareearth element) attract special interest (see surveys [1,2,3] and further references) as they include compounds with rather high superconducting transition temperatures (up to T c ≈17 K, R=Lu) and compounds with different types of magnetic ordering that include states with commensurate and incommensurate spin-density waves.…”

“…Borocarbides have a body centered tetragonal crystalline structure with the ratio c/a∼3 [1,2]. They have a rather complex Fermi surface (FS) consisting of several sheets [4,5].…”

“…They have a rather complex Fermi surface (FS) consisting of several sheets [4,5]. FS is anisotropic [1,2] and has two characteristic groups of electrons possessing different Fermi velocities ν F (this was mentioned in [6] from the de Haasvan Alphen experiments [7,8]. The T C of borocarbides is determined not by the total density of states N (E F ), but by the contribution to the density of states which is made by the slow electrons of the nodal regions [2].…”

“…The T C of borocarbides is determined not by the total density of states N (E F ), but by the contribution to the density of states which is made by the slow electrons of the nodal regions [2]. In the normal state the transport properties of borocarbides, in particular their resistivity ρ, are practically isotropic [1,2] because they are related to the groups of electrons that have relatively high velocities ν F with lower anisotropy and are unrelated to the nodal points at the Fermi surface [2].…”