Research interests in our research group

We are a research theory group working on various topics in superconductivity, including the vortex physics, non-equilibrium and spin-transport phenomena usual and topological superconductors. Our research is supported by the Academy of Finland research fellow program (Project No. 297439).

Projects

Long-range spin transport in high-field superconductors  

 Our recent studies of non-equilibrium spin states in superconductors revealed a new rich physics originating from the coupling of spin both to charge and heat. We have shown that superconducting wires subjected to a high magnetic field can develop a peculiar thermomagnetic effect which produces spin polarization in response to external heating. Such a thermospin effect has been observed recently by several groups using non-local measurements in superconducting Al nanowires. This effect can be considered as a superconducting analog of the famous spin Seebeck effect in magnetic materials which has triggered a new branch of “spin caloritronics”.  

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The basic mechanism of spin caloritronics effects in superconductors is illustrated in figure above. The panel (a) shows the superconducting wire subjected to the Zeeman field and temperature gradient along the wire. The panel (b) is a schematically picture of the spin-split quasiparticle spectrum at finite temperatures and zero temperatures where there are now thermally excited quasiparticle states. Bu counting the filled states (black circles) on each of the spin subbands one can see that the finite temperature develops the net spin accumulation, that is the difference between the number of spin-up and spin-down electrons. In result, since the states with different temperatures have different spin polarization, connecting them electrically through the spin-polarized electrode will result in the charge current in the detector circuit. This situation corresponds to the non-local measurement scheme shown in the Fig. panel (a) and used in many experiments to detect spin accumulation in superconductors.

 

Dynamical coupling between magnetic and superconducting orders

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We study the coupling of magnetization and supercurrent in superconductor / magnetic hybrids and magnetic superconductors. Recently, large attention has been focused on studying various ways of producing spin-triplet superconducting correlations in F/S systems. However, the mutual influence of superconducting order parameter and collective magnetization remains largely unexplored. One of the basic questions in this respect is whether one can change the order parameter distribution, e.g. the phase gradients by bringing the superconductor to interact with the magnetic texture. The reciprocal effect will allow tuning the magnetization with the superconducting current which will pave the way to the dissipation-less and phase-coherent superconducting spintronics.   The reason why these effects are difficult to study has been highlighted in our recent paper, where we have shown that the supercurrent can be coupled to the magnetization only beyond the widely-used quasiclassical theory. Therefore, this coupling is efficient only in systems with spin-filtering ferromagnetic barriers or strong ferromagnets where the exchange splitting is of the order of Fermi energy. Going beyond this limitation we demonstrate that F/S systems with spin-filtering elements can feature spontaneous charge currents and phase-shifted thermal interference currents which can be controlled by the magnetic configuration. 

 

Majorana states, spin transport and dynamics is topological superconductors and superfluids

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The keynote feature of these materials is the presence of projected Majorana states localized on boundaries, domain walls and quantized vortices. We study spin transport and dynamics in these materials to understand the experimental signatures of localized zero-energy states. For now, we have focused on the plausible topological material superfluid He-3, featuring a plethora of Andreev-Majorana states in various situations. The question of great current interest is how to observe the presence of these states exploiting their couplings to the collective order parameter degrees of freedom, such as the spin-waves and/or the motion of quantized vortices.