Hall-effect in LuNi_2B_2C and YNi_2B_2C borocarbides: a comparative study

The Hall effect in LuNi_2B_2C and YNi_2B_2C borocarbides has been investigated in normal and superconducting mixed states. The Hall resistivity rho_{xy} for both compounds is negative in the normal as well as in the mixed state and has no sign reversal below T_c typical for high-T_c superconductors. In the mixed state the behavior of both systems is quite similar. The scaling relation rho_{xy}\sim\rho_{xx}^\beta (\rho_{xx} is the longitudinal resistivity) was found with \beta=2.0 and 2.1 for annealed Lu- and Y-based compounds, respectively. The scaling exponent \beta decreases with increasing degree of disorder and can be varied by annealing. This is attributed to a variation of the strength of flux pinning. In the normal state weakly temperature dependent Hall coefficients were observed for both compounds. A distinct nonlinearity in the \rho_{xy} dependence on field H was found for LuNi_2B_2C in the normal state below 40K, accompanied by a large magnetoresistance (MR) reaching +90% for H=160kOe at T=20K. At the same time for YNi_2B_2C only linear \rho_{xy}(H) dependences were observed in the normal state with an approximately three times lower MR value. This difference in the normal state behavior of the very similar Lu- and Y-based borocarbides seems to be connected with the difference in the topology of the Fermi surface of these compounds.Comment: 11 RevTeX pages, 8 embedded EPS figures, submitted to Phys. Rev.

Microstructure and superconducting properties of hot isostatically pressed MgB2

Bulk samples of MgB2 have been formed by hot isostatic pressing (HIPping) of commercial powder at 100MPa and 950=B0C. The resulting material is 100% dense with a sharp superconducting transition at 37.5K. Microstructural studies have indicated the presence of small amounts of second phases within the material, namely MgO and B rich compositions, probably MgB4. Magnetisation measurements performed at 20K have revealed values of Jc=1.3 x 106A/cm2 at zero field, and 9.3 x 105A/cm2 at 1T. Magneto optical (MO) studies have shown direct evidence for the superconducting homogeneity and strong intergranular current flow in the material.Comment: 3 pages, 6 figures, text updated, new references included and discussed. Submitted to Superconductor Science and Technolog

Heat and Charge Transport Properties of MgB2

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 1.8K and 300K in zero magnetic field. The electrical resistivity shows a superconducting transition at T_c=38.7K and, similarly to borocarbides, a T^2.4 behaviour up to 200K. The electron diffusion thermopower and its bandstructure-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 7K. The reduced Lorenz number exhibits values smaller than 1 and a characteristic minimum which resembles the behaviour of non-magnetic borocarbides.Comment: 7 pages, 5 figures; added references and minor changes; accepted for publication in Physica

Anisotropy parameters of superconducting MgB2_2

Data on macroscopic superconducting anisotropy of MgB$_2$ are reviewed. The data are described within a weak coupling two-gaps anisotropic s-wave model of superconductivity. The calculated ratio of the upper critical fields $\gamma_H=H_{c2,ab}/H_{c2,c}$ increases with decreasing temperature in agreement with available data, whereas the calculated ratio of London penetration depths $\gamma_{\lambda}=\lambda_c/\lambda_{ab}$ decreases to reach $\approx 1.1$ at T=0. Possible macroscopic consequences of $\gamma_{\lambda}\ne\gamma_H$ are discussed.Comment: accepted to Physica C, special MgB2 issu

Magnetic anomalies in the spin chain system, Sr3_3Cu1−x_{1-x}Znx_xIrO6_6

We report the results of ac and dc magnetization (M) and heat-capacity (C) measurements on the solid solution, Sr$_3$Cu$_{1-x}$Zn$_x$IrO$_6$. While the Zn end member is known to form in a rhombohedral pseudo one-dimensional K$_4$CdCl$_6$ structure with an antiferromagnetic ordering temperature of (T$_N$ =) 19 K, the Cu end member has been reported to form in a monoclinically distorted form with a Curie temperature of (T$_C$ =) 19 K. The magnetism of the Zn compound is found to be robust to synthetic conditions and is broadly consistent with the behavior known in the literature. However, we find a lower magnetic ordering temperature (T$_o$) for our Cu compound (~ 13 K), thereby suggesting that T$_o$ is sensitive to synthetic conditions. The Cu sample appears to be in a spin-glass-like state at low temperatures, judged by a frequency dependence of ac magnetic susceptibility and a broadening of the C anomaly at the onset of magnetic ordering, in sharp contrast to earlier proposals. Small applications of magnetic field, however, drive this system to ferromagnetism as inferred from the M data. Small substitutions for Cu/Zn (x = 0.75 or 0.25) significantly depress magnetic ordering; in other words, T$_o$ varies non-monotonically with x (T$_o$ ~ 6, 3 and 4 K for x = 0.25, 0.5, and 0.67 respectively). The plot of inverse susceptibility versus temperature is non-linear in the paramagnetic state as if correlations within (or among) the magnetic chains continuously vary with temperature. The results establishComment: 7 pages, 7 figures, Revte

Upper critical field and irreversibility line in superconducting MgB_2

The upper critical field Hc2(T) of sintered pellets of the recently discovered MgB_2 superconductor was investigated by transport, ac susceptibility and dc magnetization measurements in magnetic fields up to 16 T covering a temperature range between Tc ~ 39 K and T = 3 K ~ 0.1Tc. The temperature dependence of the upper critical field, Hc2(T), shows a positive curvature near Tc similar to that found for the borocarbides YNi_2B_2C and LuNi_2B_2C indicating that MgB_2 is in the clean limit. The irreversibility line was consistently determined from dc magnetization measurements and from the imaginary component of ac susceptibility. The irreversibility field was found to increase up to 8.5 T at 10 K.Comment: 8 pages with 5 figures, submitted to Solid State Communication