Ferromagnetic π-phase-shifted Josephson junctions (π-junctions) give a deep impact on superconductor circuits. The key elements are three types of p-junction-based SQUIDs, which produce novel functionalities in superconductor circuits.
It is well-known that the modulation pattern of the critical current to external magnetic flux Fex is shifted by a half flux quantum (HFQ, F0/2) in a 0-π SQUID compared to that in a 0-0 SQUID (DC-SQUID). Here, a 0-π SQUID is made up of a conventional Josephson junction (0-junction) and a π-junction. The most important effect is that a circulating current is induced in a 0-π SQUID spontaneously. The current makes the phase difference of either 0- or π-junction close to the critical point. In other words, it is considered that some bias current is fed to the 0-π SQUID. Though the HFQ circuit is constructed by replacing 0-junctions in the rapid single flux quantum (RSFQ) circuit with 0-π SQUIDs, the static energy consumption is reduced drastically owing to the effect as well as the reduction in the dynamic energy consumption.
Both the second and third π-junction-based SQUID are related to a single π-junction-SQUID (π-SQUID). The difference between the two appears in the characteristics between the internal flux Fin and Fex; hysteretic and nonhysteretic characteristics can be seen in the second one referred to as a hysteretic π-SQUID and the third one as a nonhysteretic π-SQUID, respectively.
Hysteretic π-SQUIDs have two stable states corresponding to ±F0/2 without any external stimulus. The barrier height in energy potentials between the two stable states can be controlled in a range from 10-21 to 10-19 J by changing the critical current of a p-junction and the loop inductance L. This enables us to build a bias-current-free and an impulse-driven storage cells in a matrix memory. This memory can operate up to 20 GHz, which is 10 times higher than the matrix memories studied so far.
Nonhysteretic π-SQUIDs show the relationship Fin>Fex near the origin of the Fin-Fex characteristics, creating an increase self-inductance or an increased permeability. This new effect can produce increased mutual couplings or compactness of the RSFQ/HFQ circuit.
We have developed the fabrication process for π-junction-based integrated circuits. The p-junctions have a sandwich structure of Nb/PdNi/Nb or Nb/AlOx/PdNi/Nb with controlled thickness of the PdNi layer to show π-phase shifts. The critical current of Nb/PdNi/Nb junctions and that of the Nb/AlOx/PdNi/Nb can be set over 20 kA/cm2 and around several 10s A/cm2, respectively. The Nb/PdNi/Nb junctions work as a p-phase shifter in actual circuits. Our π-junctions can also be made on the 0-junction-based circuits prepared with the standard process 2 (STP2) of the CRAVITY-AIST, which increases the integration level of the HFQ or related circuits mentioned above.
Based on the π-junction-process combined with the STP2, we have demonstrated essential elements of the HFQ circuits. The bias current supplied to the circuit, that is, the static energy consumption is reduced remarkably, which is a special feature of the HFQ circuit. We have also demonstrated the several primitives of impulse-driven memories, that is, a bias-current-free storage cell driven only by impulses, a serial array of memory cells, etc. Regarding the increased self-inductances of nonhysteretic π-SQUIDs, we confirmed the effect in a DC SQUID in which a part of the loop inductance is replaced with nonhysteretic π-SQUIDs.
Acknowledgment: This work is supported by JSPS KAKENHI Grant Numbers JP 18H05211 and JP 22H05000, and JST CREST Grant Number JPMJCR20C5.