Krivoruchko V.N.,Donetsk Physics and Technology Institute NAS of the Ukraine |
Tarenkov V.Y.,Donetsk Physics and Technology Institute NAS of the Ukraine
Physical Review B - Condensed Matter and Materials Physics | Year: 2012
The interplay of superconductivity and magnetism was studied in a composite prepared from ferromagnetic half-metallic La 0.67Sr 0.33MnO 3 (LSMO) nanoparticles and the s-wave superconductor MgB 2. A few principal effects have been found. With the onset of MgB 2 superconductivity, a spectacular drop of the sample resistance was detected and complete superconductivity was observed at temperatures up to 20 K. The basic nanocomposite characteristics (critical temperature, current-voltage dependence, percolation threshold, etc.) are strongly affected by the half-metallic LSMO and, most probably, cannot be quantitatively explained within the framework of a conventional percolation scenario. Point contact (PC) spectroscopy was used to measure directly the superconducting energy coupling. For small voltage, an excess current and doubling of the PC normal-state conductance were detected. Conductance peaks corresponding to three energy gaps are clearly observed. Two of these gaps we identified as enhanced Δ π and Δ σ gaps originating from the MgB 2; the third gap Δ tr is more than three times larger than the largest MgB 2 gap. The temperature behavior of Δ tr does not follow the BCS dependence. The experimental results have a natural and qualitative explanation within the phase-coherency scenario of proximity-induced superconductivity. Specifically, at low temperature, a p-wave spin-triplet condensate with pairing energy Δ tr is essentially sustained in LSMO but is incapable of displaying a long-range superconducting response because of a phase-disordered state. The proximity coupling to MgB 2 restores the long-range phase coherency of the superconducting state, which, in turn, enhances the superconducting state of the MgB 2. © 2012 American Physical Society.