Two superconducting thin films systems with potential integration of different quantum functionalities

Abstract Quantum computation based on superconducting circuits utilizes superconducting qubits with Josephson junctions (JJs). Engineering high-coherence qubits requires materials optimization. In this work, we present two superconducting thin film systems, grown on silicon (Si), and one obtained from the other via annealing. Cobalt (Co) thin films grown on Si were found to be superconducting (Banu et al 2020 Europhys. Lett. 131 47001). These films also happen to be a self-organized hybrid superconductor/ferromagnet/superconductor (S/F/S) structure. Here we present our results on the superconductivity of a hybrid Co film followed by the superconductivity of a CoSi 2 film, which was prepared by annealing the Co film. The hybrid Co film revealed superconducting transition temperature T c = 5 K and anisotropy in the upper critical field between the in-plane and out-of-plane directions. The anisotropy is of the order of ratio of lateral dimensions to thickness of the superconducting Co grains, suggesting a quasi-2D nature of superconductivity. On the other hand, CoSi 2 film showed a T c = 900 mK. In the resistivity vs temperature curve, we observe a peak near T c . Magnetic field scan as a function of T shows a monotonic increase in intensity of this peak with temperature. The origin of the peak has been explained in terms of a parallel resistive model for the particular measurement configuration. Although our CoSi 2 film contains grain boundaries, we observed a perpendicular critical field of 15 mT and a critical current density of 3.8×10 7 Am −2 , comparable to epitaxial CoSi 2 films. The S/F/S hybrids are important for superconducting π -qubits (Yamashita et al 2005 Phys. Rev. Lett. 95 097001). For conventional qubits, with S/I(insulator)/S JJs, Al is the commonly used superconducting material. CoSi 2 , with its <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mn>1</mml:mn> <mml:mrow> <mml:mo>/</mml:mo> </mml:mrow> <mml:mi>f</mml:mi> </mml:mrow> </mml:math> noise two–to–three orders of magnitude smaller compared to Al (Chiu et al 2017 ACS Nano 11 516), is a promising material for high-coherence qubits. Localized annealing by laser pulses, could additionally provide the capability of integrating the functionalities of both S/F/S and S/I/S junctions on the same substrate.