Tunneling Into the Magnetic Superconductor

Single-particle tunneling experiments have been carried out on ErRh(,4)B(,4)-Lu(,x)O(,y)-Sb junctions in the temperature range from 70 K to 0.5 K, permitting the density of states of the magnetic superconductor ErRh(,4)B(,4) to be probed in the paramagnetic, superconducting, and ferromagnetic states. The ErRh(,4)B(,4) films used in these studies were dc sputtered and characterized using X-ray diffraction, Auger spectroscopy and Electron microscopy. The upper and lower superconducting transition temperatures of the films were 8.1 K and 1.0 K respectively. Evidence of a first order phase transition at the lower transition, T(,c2), was seen in the resistive transition, as well as in the temperature dependence of the energy gap peak in the density of states. The zero-bias tunneling conductance exhibited a minimum around 1.6 K and rapidly increased with decreasing temperature until it reached the normal -state value at T(,c2). This minimum at 1.6 K may be signature of a magnetic transition. The temperature dependence of resistive transition, the zero-bias conductance in low fields, and pair tunneling data taken earlier indicate the possible relevance of a picture involving a percolating mixture of superconducting and normal-magnetic grains. The data supply a qualitative picture of the behavior of the density of states. As T(,c2) is approached, there is no clear broadening of the gap peak, although there is some filling in of the sub-gap conductance. From the data, ErRh(,4)B(,4) appears to have a ratio of 2(DELTA)/k(,B)T(,c) = 3.8 which is larger than the BCS value of 3.52. There is no indication of gapless superconductivity. Pair-breaking due to spin-flip scattering, leakage current, as well as the presence of non-superconducting minority phases may be responsible for the rounding of the gap in the density of states and the non-zero conductance at zero-bias in the superconducting states. The percolating grain picture may also play a role in increasing the sub-gap conductance. Another feature of the single-particle tunneling data was the appearance of a series of up to 4 sharp resonant features, prominent in the derivative of the conductance (dG/dV) near T(,c2) in both the superconducting and normal phases. These peaks may be related to the coexistence of ferromagnetic order and superconductivity near but above T(,c2).