S.-L. Drechsler

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.

Zn doping of YBa2Cu3O7 in melt textured materials: peak effect and high trapped fields

Abstract The previously introduced modified melt crystallization process (MMCP) has been applied to prepare single grain YBCO bulk material with Zn partially substituting for Cu. Hole doping was controlled by an appropriate oxidizing treatment of the as-grown bulk. A field induced pinning was indicated by a well pronounced peak of the critical current density jc in the jc vs. H relationship for the maximal oxidized (overdoped) material containing Zn, whereas pure overdoped YBCO shows no peak effect. The peak effect for Zn-doped YBCO appearing for T=77 K at a relatively high field of about 3 T can be attributed to pair breaking by locally induced magnetic moments due to in plane Zn for Cu substitution. Besides high quality of the bulk YBCO, the peak effect is the reason for the trapped field as large as 1.12 T at 77 K in the cylinder of only 25 mm in diameter.

Heat and charge transport properties of MgB2

Abstract A polycrystalline sample of the MgB 2 superconductor was investigated by measurements of the electrical resistivity, the thermopower and the thermal conductivity in the temperature range between 1.8 and 300 K in zero magnetic field. The electrical resistivity shows a superconducting transition at T c =38.7 K and, similarly to borocarbides, a T 2.4 behaviour up to 200 K. The electron diffusion thermopower and its band-structure-derived value indicate the dominant hole character of the charge carriers. The total thermopower can be explained by the diffusion term renormalized by a significant electron–phonon interaction and a phonon drag term. In the thermal conductivity, for decreasing temperature, a significant decrease below T c is observed resulting in a T 3 behaviour below 7 K. The reduced Lorenz number exhibits values smaller than 1 and a characteristic minimum which resembles the behaviour of non-magnetic borocarbides.

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.

Specific heat and upper critical fields in KFe2As2 single crystals

We report low-temperature specific heat measurements for high-quality single crystalline KFe2As2 (T_c about 3.5 K). The investigated zero-field specific heat data yields an unusually large nominal Sommerfeld coefficient gamma_n of 94(3) mJ/mol K^2 which is however significantly affected by extrinsic contributions as evidenced by a sizable residual linear specific heat and various theoretical considerations including also an analysis of Kadowaki-Woods relations. Then KFe2As2 should be classified as a weak to intermediately strong coupling superconductor with a total electron-boson coupling constant lambda_tot near 1 (including a calculated weak electron-phonon coupling constant of lambda_el-ph =0.17. From specific heat and ac susceptibility studies in external magnetic fields the magnetic phase diagram has been constructed. We confirm the high anisotropy of the upper critical fields B_c2(T) ranging from a factor of 5 near T_c to a slightly reduced value approaching T=0 for fields B || ab

Evidence for strong electron-phonon coupling in MgCNi3

and || c and show that their ratio Gamma slightly exceeds the mass anisotropy of 4.35 derived from our full-relativistic LDA-band structure calculations. Its slight reduction when approaching T=0 is not a consequence of Pauli-limiting as in less perfect samples but point likely to a multiband effect. We also report irreversibility field data obtained from ac susceptibility measurements. The double-maximum in the T-dependence of its imaginary part for fields B || c indicates a peak-effect in the T-dependence of critical currents.

Realization of a large J(2) quasi-2D spin-half Heisenberg system: Li(2)VOSiO(4).

The title compound is investigated by specific heat measurements in the normal and superconducting states supplemented by upper critical field transport, susceptibility, and magnetization measurements. From a detailed analysis including also full potential electronic structure calculations for the Fermi surface sheets, Fermi velocities, and partial densities of states the presence of both strong electron-phonon interactions and considerable pair breaking has been revealed. The specific heat and the upper critical field data can be described to a first approximation by an effective single-band model close to the clean limit derived from a strongly coupled predominant hole subsystem with small Fermi velocities. However, in order to account also for Hall-conductivity and thermopower data in the literature, an effective general two-band model is proposed. This two-band model provides a flexible enough frame to describe consistently all available data within a scenario of phonon mediated

Specific heat and disorder in the mixed state of non-magnetic borocarbides

s

Evidence for CuO conducting band splitting in the nodal direction of Bi2Sr2CaCu2O8+d

-wave superconductivity somewhat suppressed by a sizable electron-paramagnon or electron-electron Coulomb interaction. For quantitative details the relevance of soft phonons and of a Van Hove\char21{}type singularity in the electronic density of states near the Fermi energy is suggested.