Electronic structure of Ni-based superconducting quaternary compounds: YNi2B2X (X=B, C, N, and O)

Magnetic structures of quaternary intermetallic borocarbides RCo(2)B(2)C (R = Dy, Ho, Er)

The magnetic structures of the title compounds have been studied by neutron diffraction. In contrast to the isomorphous RNi(2)B(2)C compounds, wherein a variety of exotic incommensurate modulated structures has been observed, the magnetic structure of ErCo(2)B(2)C is found to be a collinear antiferromagnet with [Formula: see text] while those of HoCo(2)B(2)C and DyCo(2)B(2)C are observed to be simple ferromagnets. For all studied compounds, the moments are found to be confined within the basal plane and their magnitudes are comparable to the values obtained from the low-temperature isothermal magnetization measurements. The absence of modulated magnetic structures in the RCo(2)B(2)C series (for ErCo(2)B(2)C, verified down to 50 mK) is attributed to the quenching of the Fermi surface nesting features.

Unconventional superconductivity in YNi(2)B(2)C

We use the semiclassical (Doppler shift) approximation to calculate magnetic field angle-dependent density of states and thermal conductivity κ(zz) for a superconductor with a quasi-two-dimensional Fermi surface and line nodes along k(x) = 0 and k(y) = 0. The results are shown to be in good quantitative agreement with experimental results obtained for YNi(2)B(2)C (Izawa K et al 2002 Phys. Rev. Lett. 89 137006).

On the ferromagnetic structure of the intermetallic borocarbide TbCo(2)B(2)C

Based on magnetization, specific heat, magnetostriction and neutron-diffraction studies on single-crystal TbCo(2)B(2)C, it is found out that the paramagnetic properties, down to liquid nitrogen temperatures, are well described by a Curie-Weiss behavior of the Tb(3+) moments. Furthermore, below T(c) = 6.3 K, the Tb sublattice undergoes a ferromagnetic (FM) phase transition with the easy axis being along the (100) direction and, concomitantly, the unit cell undergoes a tetragonal-to-orthorhombic distortion. The manifestation of an FM state in TbCo(2)B(2)C is unique among all other isomorphous borocarbides, in particular TbNi(2)B(2)C (T(N) = 15 K, incommensurate modulated magnetic state) even though the Tb ions in both isomorphs have almost the same crystalline electric field properties. The difference among the magnetic modes of these Tb-based isomorphs is attributed to a difference in their exchange couplings which are in turn caused by a variation in their lattice parameters and in the position of their Fermi levels.

Anisotropy of the superconducting gap of the borocarbide superconductor YNi2B2C with ultrasonic attenuation

The ultrasonic attenuation alpha of the highly anisotropic s-wave superconductor YNi2B2C has been measured for all the symmetrically independent elastic modes to explore the location of the zero superconducting gap region on the Fermi surface. The attenuation of the longitudinal mode shows a pronounced anisotropy in the superconducting state: While alpha shows a thermally activated behavior along [110] and [001] directions, it shows T-linear dependence along [100]. These results together with those for the transverse modes demonstrate the presence of point nodes or zero-gap regions along [100] and [010] directions. This is a clear demonstration of ultrasonic attenuation as a powerful probe for the structure of the anisotropic superconducting gap.

Ultrahigh-resolution photoemission spectroscopy of Ni borocarbides: direct observation of the superconducting gap and a change in gap anisotropy by impurity

We have performed ultrahigh-resolution photoemission spectroscopy of Y(Ni1-xPtx)2B2C ( x = 0.0 and 0.2) in order to study the changes in the density of states across the superconducting transition. Because of a drastic increase in energy resolution, we clearly observe the opening of superconducting gaps across T(c) in both compounds. Furthermore, we find a small but significant difference in the superconducting-state spectral shape. This can be explained in terms of reduction in gap anisotropy by introducing impurities and provides spectroscopic evidence for an anisotropic s-wave gap in YNi2B2C.