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arXiv:1003.3083 · Physical Review Letters

Josephson π State in a Ferromagnetic Insulator

Shiro Kawabata, Yasuhiro Asano, Yukio Tanaka, Alexander A. Golubov, Satoshi Kashiwaya

Published Mar 19, 2010• 52 citations
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Abstract

We predict anomalous atomic-scale $0\mathrm{\text{\ensuremath{-}}}\ensuremath{\pi}$ transitions in a Josephson junction with a ferromagnetic-insulator (FI) barrier. The ground state of such junction alternates between 0 and $\ensuremath{\pi}$ states when thickness of FI is increasing by a single atomic layer. We find that the mechanism of the $0\mathrm{\text{\ensuremath{-}}}\ensuremath{\pi}$ transition can be attributed to thickness-dependent phase shifts between the wave numbers of electrons and holes in FI. Based on these results, we show that a stable $\ensuremath{\pi}$ state can be realized in junctions based on high-${T}_{c}$ superconductors with a ${\mathrm{La}}_{2}{\mathrm{BaCuO}}_{5}$ barrier.

DOI
10.1103/physrevlett.104.117002

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We report a hybrid fabrication process for superconductor large-scale integrated circuits (ICs) with ferromagnetic π-shifted Josephson junctions (π junctions) and conventional 0 junctions. π junctions have a Nb/PdNi/Nb sandwich structure and are formed on the reliable Nb four-layer structure used for Nb/AlOx/Nb 0-junction-based ICs. The additional process for making π junctions causes little damage to the 0 junctions. The π phase shift for the superconducting macroscopic wave function is confirmed by forming 0-0-π SQUIDs that have two 0 junctions and one π junction in a superconducting loop. The π junction serves as the π phase shifter because the critical currents of the π junctions are much larger than those of the 0 junctions. The modulation patterns of the 0-0-π SQUIDs show a clear shift by the magnetic field corresponding to the half flux quantum (HFQ). The critical currents of the 0-0-π SQUIDs without fields, which are referred to as the nominal critical currents, are reduced depending on the product of the loop inductance and the critical current of the 0 junction. The nominal critical current reaches 1/5 of the critical current of the single 0 junction. The small values of the nominal critical currents contribute to the reduction of the power consumption in the HFQ circuit directly.
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