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Magnetic, electric-field and light induced control of spin-polarized supercurrents: fundamentals for an offbeat electronics

Periodic Reporting for period 4 - SUSPINTRONICS (Magnetic, electric-field and light induced control of spin-polarized supercurrents: fundamentals for an offbeat electronics)

Reporting period: 2020-04-01 to 2021-09-30

Superconductors are materials in which electrical currents flow without energy loss. Ferroics have a different remarkable property: memory. Among them we find ferromagnets, which present a spontaneous magnetic moment that can be reversed by magnetic field pulses; and ferroelectrics, which present instead a dipolar moment switchable by electric field pulses. All of those properties are of great technological relevance, and hold potential for novel applications. However, those based on superconductors are generally disconnected from those based on ferroics. This is because superconductivity and ferroic order are usually antagonistic –very rare natural materials show both characteristics simultaneously.

This project was devoted to the study of hybrids in which superconducting and ferroics are deliberately put in intimate contact. This provided a platform to study, understand and control the competing interactions between them. With two main goals. On the one hand, searching the conditions under which the coexistence of superconductivity and magnetism can be promoted; for instance, to create and manipulate dissipation-less currents that carry a net spin, or to couple changes in the magnetic moment with changes in the superconducting state. All of that is somewhat equivalent to endowing superconductivity with magnetic memory. The second goal was to transfer into superconductors the ferroic’s sensitivity to certain stimuli (for instance, electric fields or light).

To reach those goals, the research project encompassed a series of electrical transport experiments in oxide-based superconducting/ferroic heterostructures, junctions, and circuits. The work has included the growth, structural and functional characterization of novel materials combinations, as well as the development of approaches for the fabrication junctions and circuits based on them.

The obtained results are very interesting at various levels.

First of all, from the scientific point of view. Regarding the interaction between superconductivity and magnetism, we have demonstrated of extremely long-range Josephson effect across half-metallic ferromagnets, which is a central result within the action because it shows that spin information can travel over long distances (micrometric) protected by quantum coherence at relatively high temperatures. We have also unveiled the effects of the interaction between supercurrents and non-trivial/topological spin textures, and shown via imaging that screening supercurrents can be used to manipulate the spin texture of ferromagnet in heterostructures, yielding unconventional magnetic ground states that can be exploited to store information. We have also shown for the first time spin-pumping into d-wave superconductors and found that the presence of quasiparticles enhances spin transport in the superconducting phase. Concerning the control of supercurrents by electric-field and illumination effects: we have shown that they can be used able to realize field-effect switchable Josephson junction. We have also demonstrated gain tunnel electro-resistance in high-temperature superconducting junctions. An important facet of this demonstration is that we found that the studied devices show memristive behavior which is enhanced in the superconducting phase. This is a major result, which has opened the door to further studies in which the redox reaction leading to the tunnel resistance switching can be controlled by visible light.

Second, from the educational point of view: this action has allowed training young researchers (2 Ph.D. 7 postdocs, 3 engineers) in state-of-the-art technologies. Those team members have either pursued a scientific career or joined the industry in Europe.

Third, the action has developed the basis for novel technologies, which has led to patents and ultimately to the transfer of research results towards actual applications of superconductivity and spintronics via the close ties of the PI's lab with industrials

Because of all of the above, we believe that the action has contributed to enhancing European competitiveness in areas related to communication and quantum computing technologies, among others.
After optimizing the growth of different types of oxide heterostructures combining superconductors and ferroelectrics, on one side, and superconductors, ferromagnets, half metals, and normal metals on the other, we have undertaken different work lines which have allowed obtaining the following key results. We have:

a) developed vortex pinning in cuprate/half-metal nanostructures (Nano Letters 2015).
b) exploited ferroelectric-field effects to modulate Josephson coupling (Phys. Rev. Applied 2017)
c) determined the factors that determine the strength of the coupling between ferroelectric polarization and superconducting state (Phys. Rev. Materials 2018)
d) demonstrated the control of the nickelates' ground state via light illumination (PNAS 2018)
e) unveiled novel resonant Josephson effects in ferromagnetic Josephson junctions (PRB 2019)
f) found giant quasiparticle tunneling electroresistance effects in vertical junctions based on cuprates (Nature Communications 2020)
g) demonstrated the propagation of d-eave superconducting correlations in graphene (PRL 2020)
h) demonstrated the magnetic frustration of electron correlations in vanadium oxide (Phys. Rev. B 2020)
i) unveiled a coupling between the superconducting state and topological magnetic textures (Physical Review Applied 2020)
j) we have shown spin-pumping into d-wave superconductors (Phys. Rev. B 2021)
k) discovered of giant spin hall effects at iridate/manganite heterostructures (Nature Communications 2021)
m) demonstrated of long-range Josephson effect in cuprate/manganite junctions (Nature Materials 2022)

In addition, the action has resulted in five invention patents four of them already granted: EP2945160, EP3143694. EP3 459 125 B1 & EP3255723)
We consider that, while of the above publications constitute significant progress with respect to the state of the art, the following-ones constitute a major advance: m), f), g), j) and k).
Planar reconfigurable Josephson weak-link defined by ferroelectric field effect
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