D. Lipp

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.

Structure refinement of the superconducting phase YNi2B2C as a function of temperature in the range 25–300 K

Abstract X-ray powder and single crystal diffraction has been used for structure refinement of various YNi 2 B 2 C samples in the temperature range 25–300 and 180–300 K, respectively. Linear thermal expansion is strikingly anisotropic. Coefficients vary between α a =4.1×10 −7 K −1 (25 K)–1.2×10 −5 K −1 (300 K) and α c =−6.1×10 −7 K −1 (25 K)–4.5×10 −7 K −1 (300 K) while the axial ratio c / a of the unit cell decreases with increasing temperature T . Moreover, a small increase of structure parameter z B with T was found. Anisotropic mean square atomic displacements remain almost constant for C and B but increase considerably for both Ni and Y atoms. The influence of the starting composition on the net content and on structure was studied for a series of samples. Lattice parameters as derived from Rietveld refinement deviated significantly among different samples. Relations between structure geometry, superconducting transition temperature, the residual resistance ratios and composition of the samples were discussed.

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

The temperature and magnetic-field dependence of the specific heat cp(T, H) in the superconducting (sc) mixed state as well as the upper critical field Hc2(T ) have been measured for polycrystalline YxLu1−xNi2B2C and Y(Ni1−yPty)2B2C samples. The linear-in- T electronic specific-heat contribution γ(H) · T exhibits significant deviations from the usual linear-in-H law resulting in a disorder-dependent negative curvature of γ(H). The Hc2(T )d ata point to the quasi-clean limit for (Y, Lu)-substitutions and to a transition to the quasi-dirty limit for (Ni, Pt)-substitutions. The γ(H)-dependence is discussed in the unitary d-wave as well as in the quasi-clean s-wave limits. From a consideration of γ(H )d ata only,d-wave pairing cannot be ruled out.

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.

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.

Thermoelectric power of hot deformed MgB2

Abstract The thermoelectric power and the electrical resistivity have been measured for a MgB2 sample, textured by hot deformation, in the range from 100 K up to 800 K. Above 200 K, the electrical resistivity exhibits a slight negative deviation from the linear behaviour with temperature. The thermopower rises from 2.1 μV/K at 100 K to a maximum value of 6.9 μV/K at 400 K and then decreases to 5.0 μV/K at 800 K. While the resistivity can be described by a one-band Gruneisen formula, no simple explanation exists for the thermopower.

Electrical resistivity of YNi2B2C with boron deficiencies

Abstract We report on electrical resistivity measurements of polycrystalline YNi2BxC samples with different boron content (1.9⩽x⩽2.0) in the temperature range 2⩽T⩽300 K . The temperature dependence of the resistivity indicates a change of the electron density of states as well as of the phonon spectrum with boron content. A significant drop of the superconducting transition temperature Tc with rising residual resistivity ρ0 is observed.

Specific heat in the mixed state of non-magnetic borocarbides

Abstract The temperature and magnetic field dependence of the specific heat c p ( T , H ) in the superconducting mixed state has been measured for polycrystalline Y x Lu 1− x Ni 2 B 2 C and Y(Ni 1− y Pt y ) 2 B 2 C samples. The deviations from the usual linear-in- H law of the linear-in- T electronic specific heat contribution γ ( H ) T can be possibly ascribed to unconventional pairing. The γ ( H ) dependence is discussed in the unitary d-wave scenario as well as in the intermediate transition region in between dirty and clean s-wave limits. From a consideration of γ ( H ) data, unconventional pairing cannot be ruled out.