A. Gladun

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.

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.

Irreversibility line of polycrystalline Bi-2223 samples

Abstract The irreversibility line of silver-sheathed Bi-2223 tapes and of melt-processed Bi-2223 samples doped with Te has been investigated by AC susceptibility, DC magnetization and transport measurements. The temperature dependence of the irreversibility field B ∗ follows a relation B ∗ (T) ∝ (T c /T−1) 1 n with n = 0.6 which can be interpreted in the framework of a flux-creep model with an expression for the activation energy U ∝ (1− T / T c )/ B n .

Preparation and critical current density of melt-processed YBaCuO thick films and AgPd-sheathed tapes☆

Abstract Thermal analysis by differential scanning calorimetry (DSC) has shown that melting of YBa 2 Cu 3 O x is completed at 1000–1010°C in a nitrogen atmosphere. The N 2 can be exchanged by O 2 without solidification of the melt and the solidification starts in O 2 below 1000°C in the supercooled melt. Applying such a thermal treatment to YBa 2 Cu 3 O x thick films on AgPd20 ribbons a partial alignment of the c axis normal to the large plane of the films can be achieved. The textured films exhibit a strong anisotropy of the critical current density and of the irreversibility field and an improved weak link behaviour compared with that of untextured samples. A maximum J c value of 1450 A cm −2 at 77 K in zero field was measured. For AgPd-sheathed tapes produced by the ‘powder-in-tube’ method, sintering in a nitrogen atmosphere before the heat treatment in oxygen results in high values of J c , up to 5950 A cm −2 (77 K, O T).

A brief comparison of superconductivity in borocarbides and cuprates

Abstract The challenge of the present knowledge of the electronic structure and selected thermodynamic properties on the mechanism of superconductivity in rare-earth transition metal borocarbides (nitrides) is discussed with respect to somewhat unexpected similarities with as well as to clear differences from the cuprate superconductors. The effect of substitutional nonmagnetic and magnetic disorder in the rare-earth intermediate layer in between the transition metal boron networks as well as in the network itself upon various thermodynamic properties in the superconducting state of Y x R 1− x C(Ni 1− y Pt y B) 2 R= Lu or Tb is studied theoretically as well as experimentally. The suppression of the upper critical field and its positive curvature near T c can be used as highly sensitive measures of the degree of disorder.

Superconductivity in clean and disordered nonmagnetic borocarbides

Abstract The effect of weak substitutional disorder in the rare-earth intermediate layer in between the NiB-networks upon various thermodynamic properties in the superconducting state of YxLu1-xC(NiB)2 is investigated theoretically as well as experimentally. The suppression of the upper critical field and its positive curvature near Tc are shown to be a highly sensitive measure of small amounts of disorder even at the rare-earth site.

Critical current density in Ag-sheathed wires of YBaCuO

Abstract Transport critical currents in Ag-sheathed wires of YBCO have been studied. At zero field the characteristic weak-link properties were masked by the self-field of the transport current. From a scaling law with respect to temperature and diameter the self-field effects could be separated, allowing the weak-link parameters to be determined. The observed improvement of the weak-link properties with decreasing wire diameter is due to the reduction of the junction length and the enhancement of the number of junctions. For a wire of 0.25 mm diameter a maximum Josephson current density of 2.7 × 10 4 A cm −2 at 4.2 K has been obtained.

Critical current in silver sheathed Bi-2223 tapes

Silver sheathed Bi-2223 tapes have been investigated by transport and magnetization measurements in the temperature range between 4.2 K and 77 K for magnetic fields up to 12 T. The superconducting current through the tape can be explained by assuming a network of platelike grains coupled by weak links. At high temperatures the critical current is governed by flux creep of pancake vortices within the grains. At temperatures below 40 K this intragrain critical current exceeds the intergrain Josephson current leading to a weak link limitation in the tape. In this picture the differences between critical transport and magnetization currents can be explained by a reduction of the geometrical length scale of current loops. An improvement of the weak link current density is observed in multifilamentary wires and tapes with AgCu sheath material.

The influence of hydrogen charging on the glassy low temperature properties of a polycrystalline NbTi-alloy

Abstract We measured the thermal conductivity, specific heat and heat release of polycrystalline Nb37Ti63 at low temperatures. Further we charged our samples with hydrogen and investigated the influence of different charging concentrations on these thermal properties. All physical features show low temperature behaviour similar to amorphous solids. The distribution parameter P and the coupling constant γ, extracted from the experimental data, indicate two different tunneling systems in NbTi and hydrogen charged NbTi. In hydrogen charged NbTi the distribution parameter varies nearly proportional to the hydrogen concentration.

Low-temperature thermal properties of amorphous polycarbonat

The thermal conductivity λ, the specific heat c and the thermal expansion α of isotropic Polycarbonat (PC) and of PC after stretching were measured in the temperature ranges between 0.1 K and 80 K (λ, c) and between 4 K and 300 K (α). The measurements show the typical low-temperature behavior of amorphous solids. Below 20 K the results can be explained in the framework of the soft potential model. Above 20 K with increasing temperature the thermal conductivity and the thermal expansion show between 30 K and 50 K a distinct decrease in slope of the temperature dependence. PC, stretched at 438 K with a load of 2 kN, shows a high anisotropy in these two thermal properties.