Research

Research and collaboration

Nonequilibrium superconductivity with spin-splitting fields: spin transport and thermoelectric effects

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 magnetic field can develop a peculiar thermospin effect which produces spin polarization in response to external heating or the injection of quasiparticles from the adjacent voltage-biased normal electrode. The spin polarization and spin currents generated in such a way survive in the superconductor at length scales much larger than the spin relaxation length in the normal state. Detection of spin polarization can be done with the help of ferromagnetic electrodes which convert it to electric signals. Basic mechanisms of spin transport (spin Seebeck) and thermoelectric effects in spin-split superconductors are illustrated in figure on the left. These phenomena are discussed in detail in our review paper Colloquium: Nonequilibrium effects in superconductors with a spin-splitting field, Bergeret, Silaev, Virtanen, Heikkilä, Rev. Mod. Phys 90, 041001 (2018). In addition to developing the unified theory of transport effects in such systems we suggested several new effects, such as the  Supercurrent-induced charge-spin conversion in spin-split superconductors, Aikebaier, Silaev, Heikkilä, Phys. Rev. B 98, 024516 (2018) and Very large flux-flow spin Hall effect in type-II superconductors, A. Vargunin, M.A. Silaev arXiv:1803.03442.

  1. Long-range spin accumulation from heat injection in mesoscopic superconductors with Zeeman splitting, Silaev, Virtanen, Bergeret, Heikkilä, Phys. Rev. Lett. 114 (16), 167002 (2015).         
  2.  Spin Hanle effect in mesoscopic superconductors, Silaev, Virtanen, Heikkilä, Bergeret, Phys. Rev. B, 91, 024506 (2015).                                                                                                                                         
  3. Colloquium: Nonequilibrium effects in superconductors with a spin-splitting field, Bergeret, Silaev, Virtanen,  Heikkilä, Rev. Mod. Phys 90 (4), 041001 (2018).                                                                       
  4. θ0- thermal Josephson junction, Silaev, Phys. Rev. B 96, 064519 (2017).                                         
  5. Supercurrent Induced Charge-Spin Conversion in Spin-Split Superconductors, Aikebaier, Silaev, Heikkilä, Phys. Rev. B 98, 024516 (2018).                                                                                                    
  6. Flux flow spin Hall effect in type-II superconductors with spin-splitting field, Vargunin, Silaev, arXiv:1803.03442.

Electromagnetic properties of multiband superconductors

fig7S-FieldStructureShort.bmp

In recent paper Polarization of the spontaneous magnetic field and magnetic fluctuations in s+is anisotropic multiband superconductorsV. L. Vadimov and M. A. Silaev, Phys. Rev. B 98, 104504 (2018) we show that multiband superconductors with broken time-reversal symmetry can produce spontaneous currents and magnetic fields in response to the local variations of pairing constants. Considering the iron pnictide superconductor Ba1-xKxFe2As2 as an example we demonstrate that in s+is the local defects produce fields which are polarized mainly in ab crystal plane like shown in the figure above. The same is true for the random magnetic fields which are produced by the and the order parameter fluctuations near the critical point. Our findings have been recently confirmed with the help of muon spin rotation experiments in Ba1-xKxFe2As2 which revealed the presence of spontaneous fields polarized in the ab plane [Emerging superconductivity with broken time reversal symmetry inside a superconducting s-wave state, Grinenko et al. arXiv:1809.03610]. These time-reversal symmetry breaking superconductors are expected also to feature peculiar thermomagnetic and thermoelectric effects discussed in our older papers in collaboration with prof.  Egor Babaev and dr. Julien Garaud (KTH, Stockholm). 

Publications:

  1.  Polarization of the spontaneous magnetic field and magnetic fluctuations in s+is anisotropic multiband superconductors, Vadimov, Silaev, Phys. Rev. B 98, 104504 (2018).
  2.  Emerging superconductivity with broken time reversal symmetry inside a superconducting s-wave state, Grinenko et al. arXiv:1809.03610.                                                                                                 
  3. Change of the vortex core structure in two-band superconductors at impurity-scattering-driven s±/s++ crossover, Garaud, Silaev, Babaev,  Phys. Rev. B 96, 140503 (2017).                               
  4. Phase diagram of dirty two-band superconductors and observability of impurity-induced s+is state, Silaev, Garaud, Babaev, Phys. Rev. B 95, 024517 (2017).                                                                           
  5. Non-London electrodynamics in a multiband London model: anisotropy-induced non-localities and multiple complex magnetic field penetration lengths, Silaev, Winyard, Babaev,                                           
  6. Thermoelectric Signatures of Time-Reversal Symmetry Breaking States in Multiband Superconductors, Garaud, Silaev, Babaev, Phys. Rev. Lett. 116, 097002 (2016).

fig7S-FieldStructureShort.bmp