LETTERS PUBLISHED ONLINE: 13 MAY 2012 | DOI: 10.1038/NPHYS2318 Equal-spin Andreev reflection and long-range coherent transport in high-temperature superconductor/half- metallic ferromagnet junctions C. Visani 1,2 , Z. Sefrioui 3,4 , J. Tornos 3,4 , C. Leon 3,4 , J. Briatico 1,2 , M. Bibes 1,2 , A. Barthélémy 1,2 , J. Santamaría 3,4 and Javier E. Villegas 1,2 * Conventional superconductivity is incompatible with ferromag- netism, because the magnetic exchange field tends to spin- polarize electrons and breaks apart the opposite-spin singlet Cooper pairs 1 . 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 ferro- magnetic spacer 2–7 . 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 applications 9 . 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 controversial 10–12 : heterostructures that combine a half-metallic F (La 0.7 Ca 0.3 MnO 3 ) with a d-wave S (YBa 2 Cu 3 O 7 ). We found quasiparticle and electron interference effects in the conductance across the S/F interfaces that directly demon- strate the long-range propagation across La 0.7 Ca 0.3 MnO 3 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 heterostructures 12,13 . The proximity effect, usually described as the penetration or ‘leakage’ of the superconducting condensate from a S into an overlaying normal metal (N), is on a microscopic level the result of two processes. The first one is the Andreev reflection 14 , through which a normal electron incident into the S/N interface is paired with an electron inside the Fermi sea by the S energy gap, leaving a hole excitation that propagates backwards from the interface. In the conventional picture, the incident electron and the reflected hole must have opposite spins. The second process is the coherent propagation into the N material of the resulting hole/electron phase-conjugated pair 15 . The latter carries the superconducting correlations into the N, leading to a finite condensation amplitude over a certain length scale ξ N , as schematically shown in Fig. 1a. In the N, such coherent propagation is limited only by the usual dephasing mechanisms and diverges at zero temperature (T ): for diffusive systems ξ N = √ ¯ hD/2π KT and for ballistic ones ξ N = ¯ hv F /2π KT , where D is the electronic diffusion constant, K is the Boltzmann constant and v F is the Fermi velocity 15 . In clean metals, at low temperatures, ξ N can be micrometres long. If the 1 Unité Mixte de Physique CNRS/Thales, 1 Avenue A. Fresnel, 91767 Palaiseau, France, 2 Université Paris Sud 11, 91405 Orsay, France, 3 GFMC, Dpto. Física Aplicada III, Universidad Complutense de Madrid, 28040 Madrid, Spain, 4 CEI Campus Moncloa, UCM-UPM, 28040 Madrid, Spain. *e-mail: javier.villegas@thalesgroup.com. material in contact with the S is a F, the two processes above— and therefore the conventional proximity effect—are markedly suppressed 1 . On the one hand, the exchange field E ex strongly limits the length ξ F = √ ¯ hD/2E ex (ξ F = ¯ hv F /2E ex for a ballistic system) over which the phase coherence of the electron/hole pair is maintained (Fig. 1b). In weak ferromagnets, ξ F is only a few nanometres 16 . For the half-metallic F (H) La 0.7 Ca 0.3 MnO 3 (LCMO), ξ F <1 nm owing to the large E ex ∼ 3 eV (ref. 17). On the other hand, the Andreev-reflection probability is reduced owing to the spin-polarization of the conduction electrons in the F: in the extreme case of a H (100% spin-polarization), it is strictly forbidden owing to the zero density-of-states at the Fermi level within the minority-spin band, thereby hindering the penetration of superconducting correlations (Fig. 1c). However, if these could be sustained exclusively within the F majority-spin band, a long- range penetration comparable to ξ N would be expected. Such equal-spin (triplet) correlations are foreseen in the presence of a so-called ‘spin-active’ S/F interface that induces spin-flip and spin-mixing processes 18,19 . From the microscopic point of view, an unconventional equal-spin Andreev-reflection process would be required for this type of triplet correlations to propagate into a H. Our experimental approach to investigate these proximity effects consists of measuring the differential conductance of c -axis Au/YBa 2 Cu 3 O 7 /La 0.7 Ca 0.3 MnO 3 (Au/YBCO/LCMO) and Au/YBCO/LCMO/YBCO micrometre-size junctions. The oxide heterostructures were grown by sputtering deposition and a series of lithography, etching, metal and isolator deposition steps were used to fabricate the vertical junctions sketched in Fig. 2 (see Supplementary Information for details on the sample preparation). Note that, contrary to the case of ramp-based junctions in which the ab plane of the cuprate is oblique to the S/F interface 20 , in the present experiment the YBCO ab plane is parallel to it. We chose this geometry because it is exactly the one for which earlier experiments suggested long-range proximity effects across YBCO/LCMO interfaces 12,13 . Note also that, owing to the ex situ deposition of the top Au electrode, a relatively large Au/YBCO interface resistance is obtained that allows controlling—through the bias voltage V —the energy of the quasiparticles injected into the top YBCO (see Supplementary Section S3 for further details). The low-temperature (3 K) conductance versus bias for a YBCO/LCMO/YBCO trilayer junction is shown in Fig. 2b (as we show later, a similar behaviour is also found for YBCO/LCMO bilayer junctions). The conductance is the numerical derivative of the measured I (V) (inset). The background conductance shows NATURE PHYSICS | VOL 8 | JULY 2012 | www.nature.com/naturephysics 539 © 201 2 M acmillan Publishers Limited. All rights reserved.