Photo-induced spin-triplet superconductivity
In view of the possible applications, it is highly desirable to integrate the dynamic quantum states into the existing quantum architectures which are based on the Josephson transistors – the weak links between the superconducting electrodes with controllable amplitude of the maximal dissipationless current it can sustain.
The international research group including researchers from Ģֱ in Finland and Moscow Institute for Physics and Technology in Russia have predicted a new type of dynamic superconducting order. In the paper published in Physical Review Letters (I. V. Bobkova, A. M. Bobkov, and M. A. Silaev Phys. Rev. Lett. 127, 147701 (2021)) they propose several applications of this fundamental effect in Josephson junction-based electronics, such as Josephson transistor which can be switched by light, and photo-magnetic SQUID devices. This finding can facilitate the development of novel superconducting photo-electronic devices.
“Our work was motivated by the fundamental question of what happens when the electromagnetic irradiation is applied to the system consisting of the coupled ferromagnet and superconductor”, explains Mikhail Silaev, the senior researcher from JYU who was among the team which conducted the study.
The characteristic feature of superconductivity is that it can support the non-dissipative electrical current mediated by the electrons which are bound pairwise to form the so-called Cooper pairs. In most of the superconducting materials, Cooper pairs consist of the electrons with the opposite spin. Such spin-singlet Cooper pairs cannot penetrate far into the ferromagnetic electrode where most of the electrons have only one preferred spin state. But what the researchers have found is that light converts some of the Cooper pairs into the spin-triplet state consisting of electrons with parallel spins. Such states go well with the magnetic ordering and in result there appears a photo-induced spin-triplet superconductivity deep in the ferromagnetic electrode, at distances of the order of several microns from the superconducting interface (see the left panel of the figure). The peculiar feature of this induced order is that the wave function of spin-triplet Cooper pairs oscillates and has zero time average. But nevertheless, it can maintain the dissipationless direct current. This feature is shown to enable the Josephson photo-active transistor which is switched into the superconducting state by light.
“For decades there has been an intensive interest in the construction of superconducting transistors based on the Josephson junctions with controllable switching between superconducting and resistive states. Such devices are expected to pave the way for energy-saving superconducting computers and can be used in leading-edge quantum information architectures. Main efforts have been focused on the Josephson currents controlled by electrostatic gates. Such mechanisms can have certain limitations by the switching frequency. Here we suggest a qualitatively different approach by using dynamic spin-triplet superconducting states driven by the electromagnetic signal. This mechanism can help to achieve switching rates in the terahertz and even the visible light frequency domains. Besides that, the reported dynamic spin-triplet superconducting state extends our knowledge about the non-equilibrium states of quantum matter which have become quite a fashionable topic recently”, explains Mikhail Silaev.
I. V. Bobkova, A. M. Bobkov, and M. A. Silaev, Phys. Rev. Lett. 127, 147701 (2021)