Investigation of the superconducting energy gap in the compound LuNi₂B₂C by the method of point contact spectroscopy: two-gap approximation

It is shown that the two-gap approximation is applicable for describing the dV/dI(V) spectra of LuNi₂B₂C–Ag point contacts in a wide interval of temperatures. The values and the temperature dependences of the large and the small gaps in the ab plane and in the c direction were estimated using the generalized BTK model [A. Plecenik, M. Grajacar, S. Benacka P. Seidel, A. Pfuch, Phys. Rev. B49, 10016 (1994)]] and the equations of [S.I. Beloborodko, Fiz. Nizk. Temp. 29, 868 (2003) [Low Temp. Phys. 29, 650 (2003)]. In the BCS extrapolation the critical temperature of the small gap is 10 K in the ab plane and 14.5 K in the c direction. The absolute values of the gaps are ∆₀ab = 2.16 meV and ∆₀с = 1.94 meV. For the large gaps the critical temperature Tc coincides with the bulk, Tc = 16.8 K, and their absolute values are very close, being about 3 meV in both orientations. In the c direction the contributions to the conductivity from the small and the large gaps remain practically identical up to 10–11 K. In the ab plane the contribution from the small gap is much smaller and decreases rapidly as a temperature rises

Observation of anisotropic effect of antiferromagnetic ordering on the superconducting gap in ErNi₂B₂C

The point-contact spectra of the Andreev reflection dV / dI curves of the superconducting rare-earth nickel borocarbide ErNi₂B₂C (Tc≈11K) have been analyzed in the «one-gap» and «two-gap» approximations using the generalized Blonder–Tinkham–Klapwijk model and the Beloborod'ko model allowing for the pair-breaking effect of magnetic impurities. Experimental and calculated curves have been compared not only in shape, but in magnitude as well, which provide more reliable data for determining the temperature dependence of the energy gap (or superconducting order parameter) Δ(T). The anisotropic effect of antiferromagnetic ordering at TN≈6K on the superconducting gap/order parameter has been determined: as the temperature is lowered, Δ decreases by∼25% in the c-direction and only by∼4% in the ab-plane. It is found that the pair-breaking parameter increases in the vicinity of the magnetic transitions, the increase being more pronounced in the c-direction. The efficiency of the models was tested for providing Δ(T) data for ErNi₂B₂C from Andreev reflection spectra

Hysteresis and stepwise structure in MR curves of granular superconducting ruthenocuprates RuSr2_2(Gd1.5_{1.5}Ce0.5)_{0.5})Cu2_2O10−δ_{10-\delta}}

Granular superconductivity effects in a polycrystalline sample of ruthenocuprate RuSr$_2$(Gd$_{1.5}$Ce$_{0.5})$Cu$_2$O$_{10-\delta}$ are studied. The main attention has been devoted to manifestation of these effects in current and magnetic-field dependences of resistive transition to superconducting state. It is found that current dependences of differential resistance taken at different temperatures intersect strictly at two definite values of current demonstrating crossing point effect. This phenomenon has been explained taking into account inhomogeneous state of intergrain medium which can be considered as a two-component system. The particular attention has been given to magnetoresistance (MR) hysteresis in mixed state of this inhomogeneous system and to influence of applied current and temperature on this phenomenon. Two types of hysteresis (clockwise and anticlockwise) have been found with transition from clockwise to anticlockwise hysteresis with increasing temperature. Stepwise structure in MR hysteretic curves has been observed in low-field range. Possible reasons of the change in hysteresis behavior with increasing temperature and appearance of the stepwise structure in MR curves are discussed taking into consideration inhomogeneous state of the granular superconductor studied.Comment: 31 pages, 13 figure

Determination of superconducting anisotropy from magnetization data on random powders as applied to LuNi2_2B2_2C, YNi2_2B2_2C and MgB2_2

The recently discovered intermetallic superconductor MgB2 appears to have a highly anisotopic upper critical field with Hc2(max)/Hc2(min} = \gamma > 5. In order to determine the temperature dependence of both Hc2(max) and Hc2(min) we propose a method of extracting the superconducting anisotropy from the magnetization M(H,T) of randomly oriented powder samples. The method is based on two features in dM/dT the onset of diamagnetism at Tc(max), that is commonly associated with Hc2, and a kink in dM/dT at a lower temperature Tc(min). Results for LuNi2B2C and YNi2B2C powders are in agreement with anisotropic Hc2 obtained from magneto-transport measurements on single crystals. Using this method on four different types of MgB2 powder samples we are able to determine Hc2(max)(T) and Hc2(min)(T) with \gamma \approx 6

Specific Heat Study of the Magnetic Superconductor HoNi2B2C

The complex magnetic transitions and superconductivity of HoNi2B2C were studied via the dependence of the heat capacity on temperature and in-plane field angle. We provide an extended, comprehensive magnetic phase diagram for B // [100] and B // [110] based on the thermodynamic measurements. Three magnetic transitions and the superconducting transition were clearly observed. The 5.2 K transition (T_{N}) shows a hysteresis with temperature, indicating the first order nature of the transition at B=0 T. The 6 K transition (T_{M}), namely the onset of the long-range ordering, displays a dramatic in-plane anisotropy: T_{M} increases with increasing magnetic field for B // [100] while it decreases with increasing field for B // [110]. The anomalous anisotropy in T_{M} indicates that the transition is related to the a-axis spiral structure. The 5.5 K transition (T^{*}) shows similar behavior to the 5.2 K transition, i.e., a small in-plane anisotropy and scaling with Ising model. This last transition is ascribed to the change from a^{*} dominant phase to c^{*} dominant phase.Comment: 9 pages, 11 figure

Neutron diffraction study of anomalous high-field magnetic phases in TmNi₂B₂C

We present a B,T phase diagram of the magnetic superconductor TmNi2B2C obtained by neutron scatterin