Dynamic spin-triplet order induced by alternating electric fields in superconductor-ferromagnet-superconductor Josephson junctions



IV Bobkova, AM Bobkov, MA Silaev


Dynamic states offer extended possibilities to control the properties of quantum matter. Recent efforts are focused on studying the ordered states which appear exclusively under the time-dependent drives. Here, we demonstrate a class of systems with dynamic spin-triplet superconducting order stimulated by the alternating electric field. The effect is based on the interplay of ferromagnetism, interfacial spin-orbital coupling, and the condensate motion driven by the field, which converts hidden static p -wave order produced by the joint action of the ferromagnetism and the spin-orbital coupling into dynamic s-wave equal-spin-triplet correlations. We demonstrate that the critical current of Josephson junctions hosting these states is proportional to the electromagnetic power supplied by external irradiation or the ac current source. Based on these unusual properties, we propose the scheme of a Josephson transistor which can be switched by the ac voltage and demonstrates an even-numbered sequence of Shapiro steps. Combining the photoactive Josephson junctions with recently discovered Josephson phase batteries, we find photomagnetic SQUID devices that can generate spontaneous magnetic fields while exposed to irradiation.



Thanks for the excellent and engaging presentation, Mike. I have a question though. Without EM radiation SC/FM heterostructure have been studied for a while and the leading scenario involves odd-frequency superconductivity (see, e.g., https://journals.aps.org/rmp/abstract/10.1103/RevModPhys.77.1321 by Efetov et al), which in a sense a spin-triplet s-wave superconductor (with the Fermi statistics satisfied due to the odd-frequency structure of the Gorkov Green function). This type of order is long ranged and is insensitive to disorder. See for example, this PennState experiment https://www.nature.com/articles/nphys1621

Yet, you seem to mention that in your system equilibrium superconductivity is fragile. Why? 

What do you assume the equilibrium state to be? Have you considered driving on top of the odd-frequency state?


Thanks a lot for watching my presentation and for the question!
Indeed, robust spin-triplet Cooper pairs can exist in equilibrium.
But to generate them, magnetic inhomogeneity is needed, e.g., see Rev. Mod. Phys. (2005) by Bergeret, Volkov, Efetov 
or some interfacial spin-flip scattering, see  https://www.nature.com/articles/nphys831 .
Alternatively, one can use interfacial spin-orbit coupling, as suggested by 
However, in configuration with Rashba spin-orbit coupling and the in-plane magnetization, it occurs that no
robust spin-triplet Cooper pairs are produced. 
In this case, adding the orbital motion of condensate induced by an electromagnetic field is the only robust spin-triplet Cooper pairs source. This is the key finding of our paper. 

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