Tuesday, August 26, 2008

4:00 pm - 5:00 pm

Supercurrents through a ferromagnet. Josephson pi-junctions as superconducting phase inverters (Seminar) Speaking: Valery V. Ryazanov Abstract: Novel superconducting weak links, a specific type of Josephson junctions, called ‘\pi-junctions’ were realized in the last decade. The origin of the \pi-state in a Superconductor - Ferromagnet - Superconductor (SFS) junction is an oscillating and sign-reversing superconducting order parameter induced in the ferromagnet close to the SF-interface. It is a remarkable consequence of superconductivity and ferromagnetism coexistence resulted in nonzero Cooper pair momentum in a ferromagnet [1]. Due to the superconducting wave function spatial oscillations different signs of the order parameter can occur at the two banks of the Josephson SFS junction when the ferromagnetic layer thickness is of the order of half a period. The pi-state corresponds to a sign change of the supercurrent as compared with conventional “0-state” (i.e. to the inverse Josephson current-phase relation I_s= -I_c sin \phi) and a negative junction coupling energy [2,3]. The SFS junctions show the reentrant temperature and F-layer thickness dependence of the critical current with vanishing amplitude at 0-\pi-transition points [4]. The crossover to the \pi-state also manifests itself in the half-period shift of the magnetic field dependence of the critical current in SFS arrays and interferometers [5]. The shift is associated with the appearance of spontaneous supercurrents and magnetic flux in the arrays even in case of zero applied field, with the ground state degenerated with respect to the two possible current flow directions [6]. Recently fabricated SFS \pi-junctions (Nb-Cu/Ni-Nb) have high critical current density up to 104 A/cm2 and are suitable for possible applications as new elements of digital and quantum logics called superconducting phase inverters [7-9]. These junctions are based on a classical niobium thin film technology so they can be incorporated directly into existing architectures of superconducting electronics. Location: ENPH 501 Sponsor: I. Lyuksyutov

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