Normal-state resistivity of superconducting Bi1.95Sr1.65La0.4CuO6+δ single crystals

The anisotropy of the normal-state resistivity of a number of superconducting Bi_1.95Sr_1.65La_0.4CuO_6+δ single crystals has been measured by using both a six- and a four-terminal technique. We find that the ab-plane resistivity _ab in general increases almost linearly with temperature (d_ab/dT> 0). the temperature variation of the c-axis resistivity _c, however, is strongly sample dependent. Both metallic-like (d_c/dT> 0) and mixed temperature dependences of _c, including d_c/dT < 0 are observed. The mixed _c(T) data can be well described by fitting to _c(T) = A + BT + C/T, indicating that the temperature dependence of _c(T) is the result of a competition between metallic and non-metallic terms. The temperature T_min at which _c reaches a minimum increases with increasing d_ab/dT, suggesting that the metallic term of _c arises from misaligned Cu---O planes. This is confirmed by an electron-microscope (HRTEM) analysis of the samples. The anisotropy ratio _c/_ab is of the order of 10³ but increases monotonically with decreasing temperature, indicating differences in the scattering processes parallel and perpendicular to the Cu---O plane.     

NMR studies of U0.2Y0.8Pd3 and YPd3 at low temperatures

We report results of ⁸⁹Y-NMR measurements on U_0.2Y_0.8Pd₃ as well as YPd₃, performed at frequencies between 9 and 16MHz and at temperatures between 0.25 and 2 K. In this temperature range the average Knight shift is +0.5%, temperature-independent and the same for both materials. Also the NMR linewidth for both compounds was found to be temperature and field independent but one order of magnitude larger for U_0.2Y_0.8Pd₃ than for YPd₃. For U_0.2Y_0.8Pd₃ our results indicate a distribution of internal static fields at the Y sites and a small temperature-dependent enhancement of the spin-lattice relaxation rate T ₁ ^?1 with respect to YPd₃. The NMR spectra are consistent with the presence of very small frozen U moments, but the temperature dependence of the spin-lattice relaxation rate indicates a more complicated situation. In particular (T₁T)^?1 shows an anomalous temperature dependence.     

Structure and low temperature properties of SrB6

A detailed study of the room temperature structure of high quality single crystalline SrB6 has been made. Minute changes in interatomic distances may drastically affect the electronic spectrum of this compound. Electronic transport properties both at zero and non-zero frequencies above ⁴He temperatures indicate that SrBP₆ is close to be a semiconductor. However, at very low temperatures this compound enters a metallic state with a low concentration of itinerant charge carriers. Possible correlation effects are indicated by anomalous temperature dependences of the electrical resistivity and the specific heat below 1 K.     

Muon Knight shift and μ+ site in the monopnictide CeAs

The transverse field μSR-signal from μ⁺ implanted in a CeAs single crystal shows a splitting into two components, displaying a positive and a negative Knight shift, respectively which both scale reasonably well with the magnetic bulk susceptibility above 10 K. The orientation dependence of the frequency shifts, measured at 15 K, is fully accounted for by the demagnetization field, implying an isotropic Knight shift, consistent with the μ⁺ residing at the only interstitial site of high (cubic) symmetry, i.e., at the position (1/4,1/4,1/4). The occurrence of a split signal remains unexplained. This revised version was published online in July 2006 with corrections to the Cover Date.     

Fractal basin boundaries

Basin boundaries for dynamical systems can be either smooth or fractal. This paper investigates fractal basin boundaries. One practical consequence of such boundaries is that they can lead to great difficulty in predicting to which attractor a system eventually goes. The structure of fractal basin boundaries can be classified as being either locally connected or locally disconnected. Examples and discussion of both types of structures are given, and it appears that fractal basin boundaries should be common in typical dynamical systems. Lyapunov numbers and the dimension for the measure generated by inverse orbits are also discussed.     

In-plane London penetration depths near the critical temperature of Tl2Ba2Ca n−1Cu n O2n+4 and (Bi,Pb)2Sr2Ca n−1 Cu n O2n+4 (n=2,3)

From isothermalM(H) curves nearT _c , measured on polycrystalline Tl₂Ba₂Ca _n–1Cu _n O_2n +4 and (Bi, Pb)₂Sr₂Ca _n–1Cu _n O_2n+4 (n=2,3) samples, we deduce the in-plane penetration depths _ab as functions of temperature. An estimate according to the BCS weak-coupling clean-limit fit, which produces the data nearT _c very well, yields _ab(0)=3100 Å, 2320 Å 2210 Å, and 1960 Å for Bi₂Sr₂CaCu₂O₈, Bi_1.6Pb_0.4Sr₂Ca₂Cu₃O₁₀, Tl₂Ba₂CaCu₂O₈, and Tl₂Ba₂Ca₂Cu₃O₁₀, respectively. A comparison between strong-coupling and weak coupling fitting curves clearly favours the weak-coupling temperature dependence of _ab nearT _c .