Combining materials that have different properties can give rise to new quantum physical effects which are not possible in individual materials. Such effects can be both of fundamental interest and have potential technological implications.
In materials, there exists particles besides the well-known electrons, protons, and neutrons that form atoms. For instance, in magnetic materials there exists a particle known as a magnon. The magnon describes fluctuations in the magnetization of the material and can be used to transport a quantum property known as spin, just like electrons can be used to transport charge. There is currently intense research focus on identifying material combinations that can enhance spin-transport as an alternative to transport of charge. The interplay between antiferromagnetic and superconducting materials, such as how magnons and electrons in superconductors can combine to transport spin with low energy loss, is far less explored. Another material type with interesting properties regarding spin is topological insulators. These states are robust to imperfections in the material and are protected against certain types of external influences the material might be exposed to. We will determine if a novel type of superconductivity can arise when combining topological materials are coupled to magnons in antiferromagnets. Using antiferromagnets rather than ferromagnets to achieve such effects is advantageous for several reasons. The absence of a net magnetization in antiferromagnets makes it robust toward external magnetic fields might disrupt spin signals. Using antiferromagnets has the additional effect that the time dynamics of antiferromagnets is much faster. Finally, antiferromagnets have no stray magnetic field which can disturb other magnetic elements in close proximity. A key impact of the proposed research will be to determine to what extent ferromagnetic elements in superconducting devices can be replaced with antiferromagnetic layers.
The main research question is which quantum physical effects can occur in hybrid structures involving antiferromagnets, topological insulators and/or superconductors, both in and out of equilibrium. In equilibrium, the main emphasis is placed on discovering new superconducting stated mediated by squeezed magnons and how the magnon squeezing can be strongly enhanced. Out of equilibrium, the goal is to develop a general kinetic theory for transport phenomena of spin, heat, and charge.
The project objectives described above have not been addressed previously in the literature. The successful demonstration of large thermoelectric effects and coupled magnonic and Cooper pair spin supercurrents would have a large impact on the field of superconducting spintronics and with high likelihood spawn experimental activity. Using antiferromagnets rather than ferromagnets is advantageous for several reasons. One is the absence of a net magnetic moment in antiferromagnet, which makes is much more robust toward external magnetic field perturbations that might disrupt spin signals. Also, the dynamics of antiferromagnets is much faster than ferromagnets. Finally, antiferromagnets have no stray field which can disturb other magnetic elements in close proximity, which is important for potential applications.
Moreover, the phenomenon of squeezing magnons, which we will utilize heavily, is uniquely a prominent feature of quantum antiferromagnets, unlike in ferromagnets. Namely, a spin-1 object may be viewed as a coherent superposition of odd-numbered states of spin-1 magnons, giving rise to strong quantum fluctuations. For the above reasons, it is clear that demonstration of strong-coupling superconductivity via squeezed magnons as well as new non-equilibrium phenomena governing the transport of charge, spin, and heat in antiferromagnet/superconductor structures is likely to have a considerable impact and elicit interest from theorists and experimentalists alike.