Superconducting and magnetic properties of a new EuAsFeO0.85F0.15 superconductor

Polycrystalline samples of a new superconducting EuAsFeO0.85F0.15 compound with critical temperature Tc=11K were prepared by solid state synthesis. Its electric and magnetic properties have been investigated in magnetic fields from 0.1 to 140000 Oe. Critical magnetic fields Hc1, and Hc2 were measured and hence the magnetic penetration depths and the coherence length have been estimated. The temperature dependence Hc2 (T) exhibits clear hyperbolic - type behavior starting with the lowest fields. The data derived were used to estimate probable high Tc and Hc2 in compounds doped with rare-earths having small atomic radii.Comment: 12 pages, 10 figures, 13 reference

Point-contact spectroscopy of the nickel borocarbide superconductor YNi2B2C in the normal and superconducting state

Point-contact (PC) spectroscopy measurements of YNi2B2C single crystals in the normal and superconducting (SC) state (T_c=15.4K) for the main crystallographic directions are reported. The PC study reveals the electron-phonon interaction (EPI) spectral function with dominant phonon maximum around 12 meV and further weak structures (hump or kink) at higher energy at about 50 meV. No "soft" modes below 12 meV are resolved in the normal state. The PC EPI spectra are qualitatively similar for the different directions. Contrary, directional study of the SC gap results in \Delta_[100]=1.5 meV for the a direction and \Delta_[001]=2.3 meV along the c axis; however the critical temperature T_c in PC in all cases is near to that in the bulk sample. The value 2\Delta_[001]/kT_c=3.6 is close to the BCS value of 3.52, and the temperature dependence \Delta_[001](T) is BCS-like, while the for small gap \Delta_[100](T) is below BCS behavior at T>T_c/2 similarly as in the two-gap superconductor MgB2. It is supposed that the directional variation \Delta can be attributed to a multiband nature of the SC state in YNi2B2C.Comment: 9 pages, 10 figures, to be published in a special issue of J. Low Temp. Phys. in honour of Prof. H. von Loehneyse

Dissipative Electron Transport through Andreev Interferometers

We consider the conductance of an Andreev interferometer, i.e., a hybrid structure where a dissipative current flows through a mesoscopic normal (N) sample in contact with two superconducting (S) "mirrors". Giant conductance oscillations are predicted if the superconducting phase difference $\phi$ is varied. Conductance maxima appear when $\phi$ is on odd multiple of $\pi$ due to a bunching at the Fermi energy of quasiparticle energy levels formed by Andreev reflections at the N-S boundaries. For a ballistic normal sample the oscillation amplitude is giant and proportional to the number of open transverse modes. We estimate using both analytical and numerical methods how scattering and mode mixing --- which tend to lift the level degeneracy at the Fermi energy --- effect the giant oscillations. These are shown to survive in a diffusive sample at temperatures much smaller than the Thouless temperature provided there are potential barriers between the sample and the normal electron reservoirs. Our results are in good agreement with previous work on conductance oscillations of diffusive samples, which we propose can be understood in terms of a Feynman path integral description of quasiparticle trajectories.Comment: 24 pages, revtex, 12 figures in eps forma

Reversible transitions in high - T

The influence of electric fields and currents has been investigated in the high-Tc superconductors YBaCuO and BiSrCaCuO using a point-contact geometry with Ag as the counterelectrode, which reveal switching transitions between states of a different resistance. The origin of this effect in point contacts is associated with electromigration of the oxygen, driven by the electric field as well as by the current-induced "electron wind". The switching effect preserves its basic features at elevated temperatures up to room temperature and in high magnetic fields up to 10 T

Direct evidence for the occurrence of superconductivity in the magnetic compound YFe4Al8

For the first time we present direct evidence for superconductivity in the ternary magnetic compound YFe4Al8 with the ThMn12 type structure, found via point-contact (PC) experiments on contacts between a silver needle and single- crystal YFe4Al8, which reveal a distinct Andreev-reflection current. The spectra measured prove the existence of a normal- superconducting interface and exhibit a triangular-like shape in the vicinity of zero bias voltage, implying an unconventional type of superconductivity. The derived dependences of the order parameter versus temperature Delta(T) and magnetic field Delta(H) are presented. Delta(T) follows BCS theory, whereas Delta(H) does not satisfy any theoretical predictions. In some cases there exists noticeable superconductivity enhancement by a weak magnetic field. The data obtained imply a very inhomogeneous distribution of superconductivity over the sample volume in spite of its single-crystal structure. We assume that the reason is associated with inherent magnetic inhomogeneities of this material. The highest values for the critical temperature T-c, upper critical magnetic field H-c2, and ratio 2Delta(0)/kT(c) are 7.4 K, 5 T, and 7.2, respectively. (C) 2002 American Institute of Physics

Two superconducting states of HoNi

Andreev-reflection spectra of superconducting-normal contacts with $\rm HoNi_2B_2C$ show a continuous increase of the superconducting order parameter at the antiferromagnetic phase transition $T_{\rm N} = 5\;$ K without re-entrant behaviour below the superconducting critical temperature $T_{\rm c} = 9\;$ K. A change is found in the superconducting ground state at $T_{\rm c}^* = 6.5\;$ K (zero magnetic field), and the magnetic-field–temperature phase diagram corresponding to the two superconducting states is reconstructed