Borocarbide superconductors

Borocarbide superconductors with LuNi2B2C-type structure and their superconducting transition temperature Tc... 

The symmetry changes of the vortex lattice in borocarbide superconductors affect their pinning properties as was shown for YNi2B2C (Silhanek et al. 2001). For the field orientation // c, the reorientation transition of the vortex lattice mentioned above was found to be associated with a significant kink in the volume pinning force Fp, whereas in the basal plane (for H c) the signature of nonlocal effects is a fourfold periodicity of Fp. 

In addition to the vortex lattice occupying the main part of the H-T-phase diagram of borocarbide superconductors, several other vortex phases have been identified in the nonmagnetic borocarbides. Mun et al. (1996) found, by transport measurements on YNi2B2C,... 

The development in the research on borocarbide superconductors is remarkable for just a several years past from their discovery till the end of 2001. In the following we will select some ideas and conclusions from Lynn et al. (2001). 

The discovery of rare-earth nickel borocarbide superconductors... 

Soon after the discovery of the quaternary borocarbide superconductors in 1994 a remarkable progress in the investigation of their physical properties could be asserted (Muller and Narozhnyi, 2001b). One reason for this rapid progress is the favorable synthesis properties of this class of materials, which resulted in high-quality polycrystalline samples over a wide range of compositions, as well as thin... 

After a short report on the discovery of the quaternary borocarbide superconductors in Section 1.1, for comparison a limited survey of other superconductors based on boron and/or carbon will be presented in Section 1.2 whereas the interplay of superconductivity and magnetism in other materials is discussed in Section 1.3. At the end of this section some special features of the RNi2B2C compounds and review articles in this field can be found in Section 1.4 as well as the further outline of this chapter. 

A typical dc susceptibility-versus-temperature transition curve for polycrystalline LuNi2E>2C and YNi2E>2C with Tc 16.5 K and 15 K, respectively, is shown in Figure 1. The growth of very high-quality single crystals of nickel borocarbide superconductors (see, e.g., Xu et al., 1994) almost immediately after their discovery has had a profound impact on the quality of the work performed. Thus many of the pitfalls of the early research on other complex materials, such as high-Tc superconductors, carried out on poly crystalline samples of variable quality, have essentially been avoided (Cava, 2001). 

The non-local effects can result in an anisotropy of Hc2 microscopically due to the anisotropy of the pairing state (Shiraishi et al., 1999) or directly to the anisotropy in the shape of the Fermi surface (Metlushko et al., 1997). The anisotropy of the Fermi surface sheets (see Section 3.2) has been assumed to cause the mentioned basal anisotropy of Hc2 because the borocarbide superconductors are usually clean-limit type-II superconductors. In the clean limit for an anisotropic Fermi surface the non-local corrections to Hc2 are given by... 

With the discovery of superconductivity (Tc = 15.5 K) in the Y-Ni-B-C system [6, 80], a new class of quaternary borocarbide superconductors has emerged. Superconductivity has been observed in several rare earth (Lu, Tm, Er and Ho) nickel borocarbides[80], and with transition metals such as Pd and Pt. The superconducting phase having the composition of YNi2B2C, crystallizes [81] in a tetragonal structure with alternating Y-C and Ni2B2 layers. Band structure calculations [82] indicate that these materials, unlike cuprate superconductors, are three-dimensional metals. 

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