We present electronic structure calculations together with resistivity, susceptibility, and specific heat measurements for TaB2 to search for the recently contradictorily reported superconductivity and to study related normal state properties. We ascribe the absence of superconductivity down to 1.5 K for our TaB2 samples to the generally weak electron phonon coupling derived from comparison of the calculated and measured specific heat constants. For the E2g and the B1g Γ point phonons we derive from the calculated deformation potentials very small electron phonon couplings for these modes, opposite to the strong coupling of the E2g mode in MgB2, probably responsible for its high Tc. In comparison to MgB2, we discuss the origin of the quite different features in the density of states and of the Fermi surfaces. The differences are mainly due to the strong hybridization between Ta 5d and B 2p states outside the hexagonal basis plane.74. 25-q,71.20.-b

We study the impact of a weak bond disorder on the spinon heat transport in the S = 1/2 antiferromagnetic (AFM) Heisenberg chain material Sr1−xCaxCuO2. We observe a drastic suppression in the magnetic heat conductivity κmag even at tiny disorder levels (i.e., Ca-doping levels), in stark contrast to previous findings for κmag of S = 1/2 two-dimensional square lattice and two-leg spinladder systems, where a similar bond disorder has no effect on κmag. Hence, our results underpin the exceptional role of integrability of the S = 1/2 AFM Heisenberg chain model and suggest that the bond disorder effectively destroys the ballistic nature of its heat transport. We further show that the suppression of κmag is captured by an effective spinon-impurity scattering length, which exhibits the same doping dependence as the long-distance exponential decay length of the spin-spin correlation as determined by density-matrix renormalization group calculations.

No abstract

Abstract. We report 139 La, 57 Fe and 75 As nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) measurements on powders of the new LaO 1−x F x FeAs superconductor for x = 0 and 0.1 at temperatures up to 480 K, and compare our measured NQR spectra with local density approximation (LDA) calculations. For all three nuclei in the x = 0.1 material, it is found that the local Knight shift increases monotonically with an increase in temperature, and scales with the macroscopic susceptibility, suggesting a single magnetic degree of freedom. Surprisingly, the spin lattice relaxation rates for all nuclei also scale with one another, despite the fact that the form factors for each site sample different regions of q-space. This result suggests a lack of any q-space structure in the dynamical spin susceptibility that might be expected in the presence of antiferromagnetic correlations. Rather, our results are more compatible with simple quasi-particle scattering. Furthermore, we find that the 7

␤-FeSi 2 crystals were grown from high-purity starting materials by chemical vapor transport. The crystals were in situ n-type doped adding Co to the source material. Electrical properties have been investigated by temperature-dependent resistivity and Hall effect measurements. Above 100 K, we observed conventional conduction band transport. The Co doping was found to create a shallow donor level at E c -0.053 eV. Hall mobilities up to 50 cm 2 /Vs were observed. At lower temperatures defect band conduction dominates the electrical transport and negative magnetoresistance is observed. The main contribution to the Hall voltage observed at low temperatures arises from the anomalous Hall effect.