The physical properties of a hybrid structure can be very different from those of its individual constituents (“A” and “B”), and from their simple addition. In hybrids, proximity effects, confinement, and the interplay between competing orders often yield novel physical properties, nonexistent in nature. Our main focus is on understanding the physical mechanisms involved in that. This is the crucial step towards tailoring the hybrids’ properties to create new functionalities, the ultimate goal of this research.
In our studies, “A” is a superconductor and “B” can be a ferroelectric, a ferromagnet, or a material with “exotic” electronic properties (topological insulators, graphene, etc). In some of these hybrids, superconducting properties such as long-range phase-coherent charge transport are transferred into “B”. In other realizations, ferroic properties (hysteresis, remanence) or their sensitivity to external stimuli (light, electric and magnetic field history) are imprinted into the superconductor (“A”). Examples of this are given below.
The magneto-resistance of a superconducting thin film in parallel magnetic fields is typically weak and non-hysteretic (left graph). However, if a judiciously chosen ferromagnet is deposited on top of the superconductor, a large, hysteretic magneto-resistance is obtained (right graph). The hybrid’s behavior is unusual for a superconductor (hysteresis, remanence) and also for a ferromagnet (zero resistance, 100% resistance variation). These effects, which are due to the magnetostatic coupling between the superconductor and the ferromagnet [1], illustrate the philosophy of this research line.
Superconducting/ferromagnet hybrids offer other interesting opportunities. Conventional superconductivity is incompatible with ferromagnetism, because the magnetic exchange field tends to spin-polarize electrons and breaks apart the opposite-spin singlet Cooper pairs. For this reason, the penetration of a superconducting current into a half-metal (a strong ferromagnet in which 100% of the conduction electrons are spin-polarized) is generally forbidden. However, our experiments (conductance spectra) in high-temperature superconductor/half-metal junctions (left graph) suggest the occurrence of an unconventional mechanism, the equal-spin Andreev reflection (right sketch), which can lift that restriction [2]. The equal-spin Andreev reflection allows the transfer of the superconducting correlations into the half-metal, so that its electrons can carry both a supercurrent and a net spin. This opens the door to high-temperature superconducting spintronics.
Another example is given by ferroelectric/superconductor hybrids. If a ferroelectric layer is deposited on a sufficiently thin oxide superconductor, the superconducting critical temperature can be strongly modulated by switching the ferroelectric polarization via voltage pulses (left graph) [3]. Thus, the superconducting-to-normal transition is controlled by the electric-field history, a possibility absent in bare superconductors. That effect is due to the interface charge accumulation induced by the ferroelectric, which “dopes” the superconductor in a nonvolatile, reversible way. Interestingly, this effect can be produced at the nanoscale [3] by “writing” ferroelectric domains using an AFM. This allows one to locally weaken/enhance superconductivity, and thereby to “imprint” the ferroelectric domain structure in the superconductor. This can be used to manipulate flux quanta [3].
The key ingredients the above research are the use of high quality oxide heterostructures [4] and of advanced lithography techniques for oxides [5–7].
[1] C. Visani, P. J. Metaxas, A. Collaudin, B. Calvet, R. Bernard, J. Briatico, C. Deranlot, K. Bouzehouane, and J. E. Villegas, Phys. Rev. B 84, 054539 (2011).
[2] C. Visani, Z. Sefrioui, J. Tornos, C. Leon, J. Briatico, M. Bibes, A. Barthélémy, J. Santamaría, and J. E. Villegas, Nat. Phys. 8, 539 (2012).
[3] A. Crassous, R. Bernard, S. Fusil, K. Bouzehouane, D. Le Bourdais, S. Enouz-Vedrenne, J. Briatico, M. Bibes, A. Barthélémy, and J. E. Villegas, Phys. Rev. Lett. 107, 247002 (2011).
[4] M. Bibes, J. E. Villegas, and A. Barthélémy, Adv. Phys. 60, 5 (2011).
[5] J. E. Villegas, I. Swiecicki, R. Bernard, A. Crassous, J. Briatico, T. Wolf, N. Bergeal, J. Lesueur, C. Ulysse, G. Faini, X. Hallet, and L. Piraux, Nanotechnology 22, 75302 (2011).
[6] I. Swiecicki, C. Ulysse, T. Wolf, R. Bernard, N. Bergeal, J. Briatico, G. Faini, J. Lesueur, and J. E. Villegas, Phys. Rev. B 85, 224502 (2012).
[7] J. Trastoy, V. Rouco, C. Ulysse, R. Bernard, A. Palau, T. Puig, G. Faini, J. Lesueur, J. Briatico, and J. E. Villegas, New J. Phys. 15, 103022 (2013).