The Josephson effect results from the coupling of two superconductors through a non-superconducting material (insulator, ordinary metal, graphene…), allowing the passage of an electric supercurrent without resistance, and therefore, without energy dissipation. It is also characterized by magnetic flux quantization and quantum interference phenomena, which manifest themselves by a modulation of the supercurrent under magnetic field and resonances under electromagnetic radiation. These unique properties are the cornerstone of superconducting electronics. For years, researchers have pursued Josephson coupling through ferromagnets, because they add an interesting property: their conduction electrons are spin-polarized and constitute an information vector, exploited in another field, that of spintronics.

Indeed, the ultimate goal has been to develop a “superconducting spintronics” in which the information carried by the spin would be protected by quantum coherence and processed via the Josephson effect. In this quest, the generation of “triplet” superconductivity has played a major role, as it allows to overcome the natural antagonism between ferromagnetism and conventional superconductivity. However, the experimental realizations have been limited to very low temperature superconductors (a few K), and the Josephson effects at very long distance (beyond 10 nanometers), in particular the interference phenomena linked to quantum coherence, have remained elusive until now. These problems have been overcome by our recent experiments, which demonstrate the Josephson effect at high temperature (tens of K) and over very long distances (micrometers) through a ferromagnetic half-metal in which the spin polarization is almost complete.

For this, we have fabricated heterostructures based on the high-temperature superconducting oxides YBa2Cu3O7 and the ferromagnetic half-metal La0.7Sr0.3MnO3, using sputtering and lithography techniques. They obtained planar junctions allowing to study the coupling between two YBa2Cu3O7 electrodes, separated by a channel of La0.7Sr0.3MnO3 of micrometric size. Using magneto-transport measurements, they demonstrated the circulation of a supercurrent through the La0.7Sr0.3MnO3, its modulation by the application of the magnetic field, and resonance phenomena produced by the absorption of electromagnetic radiation (microwaves) which demonstrate the macroscopic phase coherence, and also present unconventional characteristics expected in the framework of “triplet” type superconductivity.

Figure 1 (a) Transmission electron microscopy image showing a cross section of the YBCO/LSMO interface. (b) Resonance pattern observed in the differential resistance of the junctions as a function of injected current and microwave radiation power.

This work is the result of a collaboration between the SUPRA team from the CNRS/Thales Joint Physics Unit, the Complutense University of Madrid (Spain), the Laboratory of Physics and Material Studies (CNRS and ESPCI, Paris) and the Laboratoire Ondes et Matière d’Aquitanie (CNRS and University of Bordeaux). They were carried out in the framework of the ERC project “SUSPINTRONICS” and the ANR project “SUPERTRONICS”.

Extremely long range, high-temperature Josephson coupling across a half metallic ferromagnet
D. Sanchez-Manzano, S. Mesoraca, F. Cuellar, M. Cabero, V. Rouco, G. Orfila, X. Palermo, A. Balan, L. Marcano, A. Sander, M. Rocci, J. Garcia-Barriocanal, F. Gallego, J. Tornos, A. Rivera, F. Mompean, M. Garcia-Hernandez, J. M. Gonzalez-Calbet, C. Leon, S. Valencia, C. Feuillet-Palma, N. Bergeal, A.I. Buzdin, J. Lesueur, Javier E. Villegas* and J. Santamaria
Nature Materials, (2021) https://doi.org/10.1038/s41563-021-01162-5