Hybrid superconductor/ferromagnet Josepson structures and bottlenecks of ultrafast superconducting electronics
Prof. Dr. Valery V. Ryazanov, Institute of Solid State Physics, Russian Academy of Sciences, Chernogolovka, Moscow
- 07 Dec 2012 11:00
Superconducting single-flux-quantum (SFQ) digital circuits offer unique features that include not only high-speed operation (> 100 GHz) with low power (< 1 μW/gate at 100 GHz), but also the use of superconductive transmission lines that transmit pico-second SFQ pulses over centimeters at the speed of light with negligible loss, dispersion, and crosstalk. Apart from “cryophobia” the current SFQ digital circuits have two main disadvantages: too large master cell and the lack of dense, fast, energy efficient memory electrically compatible with the traditional SFQ circuits. The main goal of the talk is to show that recent fundamental physics research in superconductor-ferromagnet thin-film structures (SF-structures) created a new opportunity to solve this long-standing problem.
Superconductivity and ferromagnetism, two deeply antagonistic electronic properties, can coexist in form of Josephson SFS junctions (SFS JJ). The most impressive feature of the SFS JJ is the ability to be in a Josephson state with the phase difference inversion or π-state. The physical origin of the superconducting phase inversion is a spatial variation of the superconducting order parameter in the ferromagnet arising as a response of the Cooper pair to an energy difference between the two electron spin directions. The inclusion of a π-junction into a superconducting loop results in superconducting phase inversion and generates spontaneous flux (phase shift). This feature makes the SFS JJs valuable phase-shifting elements for utilization in superconducting circuits. Recently, a Toggle Flip-Flop (TFF) with the embedded SFS π-junction was successfully demonstrated . The use of SFS π-junctions can lead to a smaller circuit area and larger operation margins. The SFS JJs are also suitable for integration with Josephson qubits. A quantum Josephson circuit, a π-biased phase qubit, has been recently demonstrated too.
It was also shown that the critical current of an SFS JJ can be changed significantly by remagnetization of the F-interlayer. A Josephson magnetic memory element proposed in Ref. requires ferromagnet layer with an in-plane magnetic anisotropy and small coercive field. Specifically, an application of small external magnetic field changed the magnetization of the ferromagnetic layer that in turn changes the junction critical current Ic, allowing the realization of two distinct states with high and low Ic, corresponding to logical “0” and “1” states, respectively. It was demonstrated too in Ref. that by inserting an additional isolation tunnel layer (I) in the SFS JJ (i.e., fabricating a superconductor-insulator-ferromagnet-superconductor (SIFS) structure), one should be able to increase significantly the junction characteristic voltage Vc to achieve high switching frequency. The superconducting-ferromagnetic Josephson junctions are electrically compatible with traditional superconductor-insulator-superconductor (SIS) Josephson junctions used for digital energy-efficient single flux quantum (eSFQ/ERSFQ) circuits. Both SFS and SIS Josephson junction circuits have similar fabrication process and can be integrated on a single chip. As a result, a combination of SFS and SIS JJs can be used to form addressable memory cells, energy-efficient memory periphery circuits and programmable logic elements.
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