Abstract
Conventional superconductivity is incompatible with ferromagnetism, because the magnetic exchange field tends to spin-polarize electrons and breaks apart the opposite-spin singlet Cooper pairs1. Yet, the possibility of a long-range penetration of superconducting correlations into strong ferromagnets has been evinced by experiments that found Josephson coupling between superconducting electrodes separated afar by a ferromagnetic spacer2,3,4,5,6,7. This is considered a proof of the emergence at the superconductor/ferromagnetic (S/F) interfaces of equal-spin triplet pairing, which is immune to the exchange field and can therefore propagate over long distances into the F (ref. 8). This effect bears much fundamental interest and potential for spintronic applications9. However, a spectroscopic signature of the underlying microscopic mechanisms has remained elusive. Here we do show this type of evidence, notably in a S/F system for which the possible appearance of equal-spin triplet pairing is controversial10,11,12: heterostructures that combine a half-metallic F (La0.7Ca0.3MnO3) with a d-wave S (YBa2Cu3O7). We found quasiparticle and electron interference effects in the conductance across the S/F interfaces that directly demonstrate the long-range propagation across La0.7Ca0.3MnO3 of superconducting correlations, and imply the occurrence of unconventional equal-spin Andreev reflection. This allows for an understanding of the unusual proximity behaviour observed in this type of heterostructures12,13.
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Acknowledgements
J.E.V. and C.V. wish to thank J. M. Rowell, A. Goldman and S. Maekawa for fruitful discussions. We thank C. Deranlot for Au deposition, D. Deneuve for technical support for the lithography process and C. Collet for focused ion beam and scanning electron micrographs. This work was supported by French ANR grant ‘SUPERHYBRIDS-II’ and RTRA grant ‘Supraspin’, European Community’s FP7/2010 ‘Pixie’, Spanish MICINN Grants MAT 2011 27470, CSD2009-00013 (IMAGINE) and CAM S2009-MAT 1756 (PHAMA).
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J.E.V., J.S. and A.B. conceived the experiments. C.V. and J.T. grew the samples.C.V. performed the lithography processes. C.V., J.E.V. and Z.S. carried out transport experiments. C.V. analysed the data. C.V. and J.E.V. wrote the paper. All of the authors contributed to the discussion leading to the understanding of the data and contributed to the preparation of the manuscript.
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Visani, C., Sefrioui, Z., Tornos, J. et al. Equal-spin Andreev reflection and long-range coherent transport in high-temperature superconductor/half-metallic ferromagnet junctions. Nature Phys 8, 539–543 (2012). https://doi.org/10.1038/nphys2318
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DOI: https://doi.org/10.1038/nphys2318
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