π phase shifter based on NbN-based ferromagnetic Josephson junction on a silicon substrate

Abstract

In the field of superconducting electronics, a π phase shifter based on a ferromagnetic Josephson junction is expected to provide various advantages to classical and quantum superconducting devices. Here we report niobium nitride (NbN)-based ferromagnetic π junctions on a silicon (Si) substrate with a titanium nitride (TiN) buffer layer, which have applications to flux-bias-free flux quantum bits (qubits) and classical digital logic elements. We fabricated and characterized NbN/aluminum nitride (AlN)/NbN Josephson junctions, NbN/copper nickel (CuNi)/NbN ferromagnetic Josephson junctions, and superconducting quantum interference devices (SQUIDs) consisting of these junctions on the Si substrate. The fabricated NbN/AlN/NbN junctions showed a high junction quality suitable for qubit applications. Furthermore, the magnetic field dependence of the SQUID’s critical current indicated that the NbN/CuNi/NbN junction worked as a π phase shifter on the Si substrate.

Magnetic Josephson Junctions: New Phenomena and Physics with Diluted Alloy, Conventional Ferromagnet, and Multilayer Barriers

“…The observed half-flux-quantum shift results from the interference of the 0-and π-junctions in the loop, and represents direct evidence of the π-state. The half-fluxquantum shift has also been reported for a three-junction loop with the π-junction [17][18]. In addition, the spontaneous magnetic flux, which was induced by the π shift, in the superconducting loop with a single π-junction was measured using a micro-Hall sensor [19].…”

“…Although the theoretical concept of the π-state in the ferromagnetic Josephson junctions have been proposed by Buzdin in 1982 [4], a number of experimental reports have been presented since 2000. The key for successful demonstration of ferromagnetic Josephson junctions was to adopt "weak" ferromagnetic alloys for the barrier of the junction [8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23]. The surviving length of the order parameters penetrating into the ferromagnetic barrier is characterized by the coherence length in the ferromagnet, .…”

“…With regard to the superconducting material of the ferromagnetic Josephson junctions, Nb which is often used for various superconducting devices such as SFQ logic circuits, has been adopted. Recently, with a view toward the application of the above devices, NbN-based junctions, which enable higher temperature operation of the logic circuit and/or an improved coherence time in the qubits, have been reported [14,18]. Also for the NbN-based ferromagnetic Josephson junctions, the 0-π transitions in the temperature and ferromagnetic barrier thickness dependences and half-fluxquantum shift have been observed.…”

Weak Spin-Flip Scattering in Pd89Ni11 Interlayer of NbN-Based Ferromagnetic Josephson Junctions

“…For NbN-based ferromagnetic Josephson junctions, the ferromagnetic CuNi interlayer has been studied, and a π-phase shift in NbN/CuNi/NbN junctions has been demonstrated 20,21 .…”

“…First, the NbN-based SIS (NbN/AlN/NbN) junctions were fabricated. Then, the SFS junction was constructed to form the SQUID structure 21 . Although the fabrication process of the SFS junction was nearly the same as that for the single junction, the junction part was defined by the lift-off technique to avoid damage to the underlying structure, including the SIS junctions.…”