Research Highlights

Observation of a novel state with broken time-reversal symmetry in multiband superconductors

With the help of muon spin relaxation measurements, physicists have observed spontaneous magnetic fields emerging inside the superconducting state of the metallic compounds Ba1−xKxFe2As2 within a specific range of K doping levels (0.7≲x≲0.85). Part of the international team was Academy Research Fellow Mikhail Silaev from the University of Jyväskylä. The presence of spontaneous magnetic fields and their spatial direction within the crystal lattice are the signatures of the novel s+is superconducting phase, breaking the time-reversal symmetry while preserving other symmetries of the crystal lattice. These experimental findings, together with thermodynamic data showing the connection between the s+is state and a change in the topology of the Fermi surface (Lifshitz transition), were published in Nature Physics.

Grinenko, V., Sarkar, R., Kihou, K. et al. "Superconductivity with broken time-reversal symmetry inside a superconducting s-wave state." Nat. Phys. (2020). JYU press release.

Anderson-Higgs Mass of Magnons in Superconductor-Ferromagnet-Superconductor Systems

AH mass generation is the cornerstone element of the Standard Theory in particle physics. It has roots in condensed matter systems, but it has not been directly observed until this work. Here I explain that in superconductor/ferromagnet/superconductor (S/F/S) systems, the magnon’s mass emerges spontaneously according to the AH mechanism below superconducting critical temperature Tc. What are the gauge field and the Goldston mode in this model? Please read about this in my paper:  

Silaev, M., "Anderson-Higgs Mass of Magnons in Superconductor-Ferromagnet-Superconductor Systems." Phys. Rev. Applied 18, L061004 (2022) 

Superconducting Triplet Rim Currents in a Spin-Textured Ferromagnetic Disk

Junctions between superconductors and ferromagnets with non-uniform magnetization are potentially useful for energy-efficient electronics. It has been predicted that by changing the magnetic, texture it is possible to effectively switch between the normal and superconducting state of such junctions. This proposal has been realized experimentally by the team of researchers from the Huygens-Kamerlingh Onnes Laboratory at Leiden University, supported theoretically by Mikhail Silaev, the leader of the Superconducting spintronics group at Jyväskylä University. It has been found and explained that superconducting correlations emerge in highly localized (sub-80 nm) channels at the rim of the ferromagnetic disk hosting the vortex-like magnetization pattern. The team demonstrated that by controlling the magnetic vortex position, it is possible to achieve the sign reversal of the Josephson current through the ferromagnet.

 R. Fermin, D. van Dinter, M. Hubert, B. Woltjes, M. Silaev, J. Aarts, and K. Lahabi, Nano Lett. 2022, 22, 6, 2209–2216. 

Photo-induced spin-triplet superconductivity

Dynamic states offer extended possibilities to control the properties of quantum matter. Here the Superconducting spintronics group led by Academy Research Fellow Mikhail Silaev from the Nanoscience Center at the University of Jyväskylä, demonstrates a class of systems that feature the dynamic spin-triplet superconducting order stimulated by light. The effect is based on the interplay of ferromagnetism, interfacial spin-orbital coupling, and the motion of Cooper pairs induced by the electromagnetic field. We hope our results will guide future experimental investigations of the dynamic superconducting orders and new generations of superconducting light-controlled electronics. 

I. V. Bobkova, A. M. Bobkov, and M. A. Silaev. "Dynamic Spin-Triplet Order Induced by Alternating Electric Fields in Superconductor-Ferromagnet-Superconductor Josephson Junctions." Phys. Rev. Lett. 127, 147701. JYU Press Release.

Superconducting spin current

Superconductors can carry not only charge current between metals but also spin currents between magnets to relatively long distances without producing excess heat. This contrasts with ordinary conductors, where such frictionless spin currents vanish within atomic distances. However, such currents can be elusive since they do not produce electric signals. In the article, the researchers describe the experimental signatures that indicate their presence, such as changes in the magnetic configuration or the modifications in the magnetic dynamical response.

R. Ojajärvi, F. S. Bergeret, M. A. Silaev, and T. T. Heikkilä, "Dynamics of Two Ferromagnetic Insulators Coupled by Superconducting Spin Current", Phys. Rev. Lett. 2022 128, 167701. JYU Press Release.