A. Kreyssig

Superconductivity and disorder in YxLu1−xNi2B2C

Abstract The superconducting transition temperature T c , the temperature dependence of the upper critical field H c2 ( T ), and the lattice structure of polycrystalline Y x Lu 1− x Ni 2 B 2 C samples have been studied by susceptibility, resistivity, and X-ray measurements, respectively. All samples exhibit the same LuNi 2 B 2 C-type structure and the lattice parameters depend linearly on the yttrium content. The residual resistivity ratio (RRR) decreases steeply first for slight deviations from both pure stoichiometric limiting contents and then saturates at a low level in a broad plateau in between 0.1 x T c ( x ) and H c2 ( x ) at fixed T change nearly parabolically. The H c2 ( T )-curves show a positive curvature for all samples. This dependence can be described within a broad temperature region 0.3 T c T ≤0.95 T c by a simple and convenient empirical expression H c2 ( T )∝(1− T / T c ) α . The parameter α >1 describing the positive curvature of H c2 ( T ) characterizes the sample quality and decreases with increasing disorder in the Y x Lu 1− x Ni 2 B 2 C samples. However, the case of complete disorder (dirty limit) where α =1 is not reached yet in the investigated samples. The nonmonotonous dependence of T c and H c2 upon x is attributed to partial disorder within the Y–Lu basis layers. Together with the RRR-saturation they reflect the complex multiband character of the materials under consideration.

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.

Magnetic structure and superconductivity in (HoxY1−x)Ni2B2C

The superconducting transition and the magnetic structure of polycrystalline (HoxY1−x)Ni2B2C have been investigated for x⩾0.75 by resistivity measurements and neutron diffraction, respectively. For HoNi2B2C, the well known antiferromagnetic structure with c- and a-axis components existing between 4.5 and 6 K has been confirmed. Whereas for all investigated samples the component having the a-axis modulated structure exists in a narrow temperature range only, the intensity connected to the spiral magnetic state along the c-axis increases for x=0.85 and x=0.75 monotonically as the temperature decreases to the lowest measuring temperature of 1.6 K. Our results strongly suggest that the incommensurate modulated structure along the a-axis is responsible for pair breaking in the investigated (HoxY1−x)Ni2B2C samples. On the other hand, the magnetic spiral structure with c-axis wave vector was found to coexist with superconductivity. A linear suppression of the superconducting transition temperature with increasin...

Evidence of tetragonal to orthorhombic distortion of HoNi2B2C in the magnetically ordered state

Using high resolution neutron scattering on a powder sample of HoNi2u200a11B2C magnetoelastic effects are observed in this compound for the first time. At low temperatures the tetragonal lattice is distorted along the [110] direction, in which the long-range ordered Ho moments are aligned. The length of the unit cell in [110] direction is shortened by about 0.19% compared to its length in [110] direction. This lattice distortion is considered to be related to the existence of the commensurate c-axis modulated antiferromagnetic structure.

Frustration in R2PdSi3 (R = Tb,Er) compounds: spin-glass or magnetic short range order? Neutron diffraction studies

Measurements of macroscopic magnetic properties of the isostructural compounds Tb2PdSi3 and Er2PdSi3 show an additional phase transition below the ordering temperature. The behaviours of the two compounds are similar and are interpreted as spin-glass-like phase transitions in the literature. In this contribution, we present detailed neutron diffraction studies of the mechanism of the spin-glass-like phase transition on a microscopic scale. We find a fundamental difference between the two compounds in their magnetic structures and, therefore, the spin-glass-like phase transitions. In the Tb2PdSi3 compound an additional antiferromagnetic short range ordered phase is found within the long range ordered phase. The appearance of the short range order is linked to the spin-glass-like transition. In contrast, the Er2PdSi3 compound shows only long range order. However, the antiferromagnetic order undergoes a modification within the ordered state. The temperature of the transition matches the spin-glass-like transition.

Breakdown of de Gennes scaling in HoxLu1-xNi2B2C

Abstract The temperature dependence of magnetic ordering and the superconducting transition have been studied for polycrystalline Ho x Lu 1− x Ni 2 B 2 C compounds by susceptibility and resistivity measurements as well as neutron diffraction. For Ho concentrations in the range 0⩽ x ⩽0.7, the superconducting transition temperature, T c , decreases from 16.5xa0K at x =0 linearly with increasing x and, consequently, with increasing effective de Gennes factor. This is in accordance with a generalized Abrikosov–Gorkov theory. At x =0.7, T c reaches 8.5xa0K i.e. the value of HoNi 2 B 2 C. Pair breaking by the Ho magnetic moments is stronger in Ho x Lu 1− x Ni 2 B 2 C compared to Ho x Y 1− x Ni 2 B 2 C. This is attributed to the difference between the lattice constants of the Ho–Y and the Ho–Lu systems resulting in different electronic structure parameters. A complete breakdown of the scaling behaviour of T c occurs for x >0.7, where T c becomes independent of the effective de Gennes factor. In this range of x re-entrant behaviour and the presence of an incommensurate a -axis, modulated antiferromagnetic structures have been observed below a characteristic temperature T m . Commensurate and incommensurate c -axis modulated antiferromagnetic structures that are observed both in the Ho–Y and the Ho–Lu systems coexist with superconductivity.

Influence of magnetic order on superconducting properties of HoxY1−xNi2B2C

Abstract We investigated the magnetic structure of Ho x Y 1− x Ni 2 B 2 C for x = 1, 0.85 and 0.75 by means of neutron diffraction as a function of temperature between 1.5 and 10 K in zero external field. We observed several magnetic phases, one with commensurate wave vector (0 0 1) and two with incommensurate wave vectors (0 0 0.915) and (0.585 0 0). Only the temperature dependence of the a -axis modulated structure coincides with the reentrant behavior found for the investigated samples by resistance measurements. This strongly suggests that the a -axis structure is responsible for pair breaking, while both the magnetic structures with c -axis wave vectors do not destroy superconductivity.

Direct observation of the superconducting gap in phonon spectra.

We show that the superconducting energy gap 2Delta can be directly observed in phonon spectra, as predicted by recent theories. In addition, since each phonon probes the gap on only a small part of the Fermi surface, the gap anisotropy can be studied in detail. Our neutron scattering investigation of the anisotropic conventional superconductor YNi2B2C demonstrates this new application of phonon spectroscopy.

Suppression of superconductivity by nonmagnetic impurities, structural properties and magnetic ordering in HoxLa1−xNi2B2C

Abstract Superconducting and magnetic properties as well as the lattice structure of polycrystalline Ho x La 1− x Ni 2 B 2 C samples have been studied by susceptibility, X-ray, and neutron diffraction measurements, respectively. A miscibility gap has been found between x =0.4 and x =0.7 if the samples are prepared by standard arc-melting. Samples in this concentration range have been successfully prepared by melt-spinning. All samples exhibit the LuNi 2 B 2 C-type structure and the lattice parameters depend linearly on the La content. Already a low lanthanum concentration of about 10% leads to a rapid depression of T c from 8.5 K to 2 K. Furthermore this concentration leads to a complete depression of the a -axis modulated incommensurate antiferromagnetic structure. Two other magnetic structures, one with commensurate (0 0 1) and one with incommensurate wave vector (0 0 0.915) were found to be less sensitive to the La concentration. The neutron-diffraction peak intensities connected to these structures decrease with increasing La content and disappear at x =0.75 (commensurate structure) and x =0.55 (incommensurate structure). A comparison with Ho x R 1− x Ni 2 B 2 C compounds (R=Y, Lu) shows that these two magnetic structures are influenced in different way by the size of the nonmagnetic rare-earth ion R. No size effect was observed for the magnetic ordering temperature of the commensurate antiferromagnetic structure, whereas the incommensurate spiral structure along c -axis was found to be sensitive to the difference in the size of the R and the Ho ions.

Field and temperature dependence of magnetic order in HoNi211B2C

Abstract We investigated the magnetic structures of HoNi2 11 B 2 C by means of powder neutron diffraction as function of temperature (2–30xa0K) and magnetic field (0–7xa0T). The results are compared to the magnetic phase diagram as deduced from other experiments and theoretical models.