Robust superconductivity coexisting with ferromagnetism display unusual behaviour
Dr Jagadeesh S Moodera, Massachusetts Institute of Technology, USA
Proximity coupling across superconductor-ferromagnet bilayers can give rise to the triplet component of the superconducting condensate. Superconductivity and ferromagnetism have been reported to coexist in a Ni/Bi bilayer system. Thus spin polarised triplet supercurrent in ferromagnetic-superconducting Josephson junctions can be expected. The Moodera group has investigated the complex and rich behaviour of this phenomenon in bilayers of Ni/Ga and Ni/Bi systems, including Josephson and quasiparticle tunnelling. Ni/Ga as well as Ni/Bi bilayer systems show unusual superconductivity, with high Tc that can co-exist with ferromagnetism. Tunnelling spectroscopy studies at low temperatures show the presence of three superconducting energy gaps in the bilayers, attributable to surface, interface and bulk states within the bilayers. Ni layers, ranging from 0.8 to 6nm thick, confirmed to be ferromagnetic by the magnetisation studies while spin polarised tunnelling studies revealed that the tunnelling electrons coming from the Ni surface were spin polarised and simultaneously displayed superconducting gap, supporting the co-existence of SC and FM. The interplay of SC and FM with the presence of spin polarised carriers in such bilayer system could be a strong case for triplet pairing. In addition, the observed Josephson current could be spin polarised. Interestingly, the superconductivity in the Ni/Bi bilayer is expected to be topological. The occurrence of zero bias conductance may reflect odd-frequency symmetry in the superconducting condensate, supporting the presence of a non-zero component associated with triplet pair superconductivity insensitive to disorder. Jagadeesh Moodera will present this ongoing work leaving it open for discussion. This work has been done in collaboration with Madison Sutula, Sebastian Bergeret, Jia Song, Valeria Lauter and Niladri Banerjee.
Transport properties of topological superconducting hybrid structures
Dr Cecilia Holmqvist, Linnaeus University, Sweden
Dirac materials with strong spin-orbit interaction have been shown to generate large surface spin accumulations in response to applied currents. Such materials have, in addition, been demonstrated to exert spin-orbit torques on adjacent ferromagnetic structures. This magnetoelectric effect in these materials is strong due to the efficient spin-momentum locking. In heterostructures consisting of superconductors and three-dimensional superconductors, this spin-momentum locking leads to an induced unconventional superconductivity that may be useful for superconducting spintronics. Here, Cecilia Holmqvist investigates theoretically the quantum transport properties of a ballistic junction consisting of two topological superconductors coupled over a quantum dot that is coupled to a ferromagnet. The spin-orbit torques acting on the ferromagnet are examined and are shown to depend strongly on the magnetisation direction relative to the current direction.
Magnetic moment manipulation by a superconducting current in Josephson junctions
Professor Alexander Buzdin, University of Bordeaux, France
Recently several mechanisms realising the direct coupling between magnetic moment and Josephson current in S/F/S junctions have been proposed. In such junctions, the ac Josephson effect may generate a magnetic precession providing then a feedback to the current. Magnetic dynamics results in several anomalies of current-phase relations (second harmonic, dissipative current) which are strongly enhanced near the ferromagnetic resonance frequency. The simulations of magnetic moment dynamics show that by applying an electric current pulse, it may be possible to realise the full magnetisation reversal which is quite important for the elaboration of superconducting spintronic devices with low dissipation.
Superconducting field-effect transistors go metal
Professor Francesco Giazotto, National Enterprise for Nanoscience and Nanotechnology, Italy
In their original formulation of superconductivity, the London brothers predicted the exponential suppression of an electrostatic field inside a superconductor over the London penetration depth. Despite a few experiments indicating hints of perturbation induced by electrostatic fields, no clue has been provided so far on the possibility to manipulate conventional superconductors via field-effect. In this talk, Francesco Giazotto will show the evidence of full field-effect control of the supercurrent in all-metallic transistors made of different BCS superconducting films. At a low temperature, the field-effect transistors (FETs) show a monotonic decay of the critical current under increasing electrostatic field up to total quenching for gate voltage values as large as ±40V in titanium-based devices. A similar behaviour, though less pronounced, was observed in aluminum FETs. In addition, Francesco will report on the realisation of Ti-based Dayem bridge Josephson field-effect transistors. The latter show full suppression of IC for gate voltages as low as ±8V. Finally, Francesco will show the behaviour of mesoscopic superconductor-normal metal-superconductor Josephson field-effect transistors that will reveal the impact of electrostatic fields even on proximity metals thereby suggesting that the field effect is universal. Possible electronic and circuital schemes based on this all-metallic technology will be discussed.