K. Krug

UPPER CRITICAL FIELD PECULIARITIES OF SUPERCONDUCTING YNI2B2C AND LUNI2B2C

We present new upper critical field Hc2(T) data in a broad temperature region from 0.3K to Tc for LuNi2B2C and YNi2B2C single crystals with well characterized low impurity scattering rates. The absolute values for all T, in particular Hc2(0), and the sizeable positive curvature (PC) of Hc2(T) at high and intermediate T are explained quantitatively within an effective two-band model. The failure of the isotropic single band approach is discussed in detail. Supported by de Haas van Alphen data, the superconductivity reveals direct insight into details of the electronic structure. The observed maximal PC near Tc gives strong evidence for clean limit type II superconductors.

Superconducting rare earth transition metal borocarbides

Abstract We present an overview of selected properties of quaternary intermetallic rare earth transition metal borocarbides and related boronitride compounds, as well as of theoretical calculations with possible relevance to the mechanism of superconductivity. The interplay of superconductivity and magnetism for compounds with pure and mixed rare earth components is considered. We suggest that the incommensurate magnetic structure modulated along the a -axis is responsible for the pair breaking in Ho x R 1− x Ni 2 B 2 C; R=Y, Lu samples. The effect of doping (Cu, Co) at the transition metal site is considered experimentally and theoretically. The possible role of correlation effects due to the presence of the transition metal component in determining the electronic structure is discussed comparing the band structure calculation results with various electronic spectroscopies as well as de Haas–van Alphen data. Important thermodynamic properties of these systems are analyzed within multiband Eliashberg theory with special emphasis on the upper critical field H c2 ( T ) and the specific heat. In particular, the unusual positive curvature of H c2 ( T ) near T c observed for high-quality single crystals, polycrystalline samples of YNi 2 B 2 C, LuNi 2 B 2 C as well as to a somewhat reduced extent also for the mixed system Y 1− x Lu x Ni 2 B 2 C is explained microscopically. It is shown that in these well-defined samples the clean limit of type II superconductors is achieved. The values of H c2 ( T ) and of its positive curvature near T c (as determined both resistively and from magnetization as well as from specific heat measurements is an intrinsic quantity generic for such samples) decrease with growing impurity content. Both quantities thus provide a direct measure of the sample quality.

Upper-critical-field anisotropy and magnetic phase diagram of HoNi2B2C

Abstract To study the influence of the magnetic structure in the electronic properties, resistivity measurements were performed on high-quality HoNi 2 B 2 C single-crystal samples as a function of temperature or magnetic field. The temperature range investigated reaches from the superconducting transition temperature T c = 8.85 K down to 50 mK. Magnetic fields up to 5 T have been applied along the [001], [100] and [110] directions, respectively. In addition to the temperature dependence of the upper critical field, anomalies in the normal state resistivity could be assigned to metamagnetic phase transitions. Both the superconducting-normal and the magnetic phase diagram show large anisotropies.

Superconductivity and the magnetic phase diagram of DyNi2B2C at very low temperatures

Abstract Detailed measurements of the resistivity ρ ab as a function of temperature, magnetic field and orientation have been carried out on single crystals of tetragonal bodycentered DyNi 2 B 2 C. From the angular dependence of the metamagnetic transitions a magnetic phase diagram has been obtained. For B||[1 0 0] , an anomalously large hysteresis effect of the metamagnetic and also of the superconducting transitions were observed in the low temperature range ( T 2 B 2 C.

Superconductivity and magnetism in the pseudo-quaternary system (ErxTb1−x)Ni2B2C

Abstract Magnetic phase diagram and superconductivity in the (Er x Tb 1− x )Ni 2 B 2 C system were studied by measurements of resistivity and susceptibility as a function of temperature and magnetic field. In the range 0.45≤ x ≤0.75, antiferromagnetic transition temperature T N exhibits an anomalous minimum. The suppression of superconductivity in the (Er x Tb 1− x )Ni 2 B 2 C system can be explained using the AG pair breaking theory. The possible reason for the absence of superconductivity in TbNi 2 B 2 C is discussed.

Electronic Structure of Y and Lu Borocarbides Determined by de Haas—van Alphen Experiments

The Fermi surfaces of the nonmagnetic borocarbide superconductors YNi2B2C and LuNi2B2C were studied by the de Haas — van Alphen (dHvA) effect. Several branches of dHvA frequencies were observed in the normal state for B ‖ c on single crystals of both systems. The observed dHvA frequencies and their angular dependencies are very similar indicating the same topology of the corresponding parts of the Fermi surfaces. Together with the effective cyclotron masses m c and the Fermi velocities vF, the results were discussed in the context of electronic band structure calculations. In both systems magnetoquantum oscillations were also observed in the superconducting state below the upper critical field Bc2. The additional damping of the dHvA oscillations in the vortex state strongly depends on the extremal orbit taken into consideration.