Our group collaborated with Shaoming Dong's team to achieve superconducting magnetic heterostructures in a two-dimensional material system. On November 12th, the relevant research findings were published online in the journal Nature Communications (DOI: 10.1038/s41467-021-26946-w) under the title “Van der Waals ferromagnetic Josephson junctions”. The corresponding authors of the paper are Shaoming Dong, academician of the Chinese Academy of Sciences Shanghai Institute of Silicate Research, and Faxian Xiu, professor of the Department of Physics of Fudan University. The first author is doctoral student Linfeng Ai of our group, a, and the co first authors are researcher Jinshan Yang and Dr. Enze Zhang .
The study of superconducting ferromagnetic interface characteristics has long been of interest in the field of condensed matter materials. Due to the opposition of order parameters, unconventional properties such as spin triplet pairing and topological superconducting states can be observed in the heterojunctions formed by them. When two superconductors combine through magnetic materials to form a Josephson junction, the spin orientation of the transport Cooper pair is modulated by magnetic exchange, which also causes a change in the phase of the superconducting ground state. Based on this principle, magnetic field controlled converters (switches) or phase adjustable Josephson structures can be prepared, which can be used as new non-dissipative devices in superconducting quantum computing. Two dimensional layered materials, with their easy dissociation and thickness dependent structural characteristics, provide an excellent experimental platform for studying interface effects based on van der Waals interactions and making it easier to prepare low scattering and low defect devices.
To study the characteristics of the superconducting ferromagnetic interface in this system, we prepared a ferromagnetic Josephson junction composed of superconductor niobium selenide (NbSe2) and intrinsic magnetic material chromium germanium tellurium (Cr2Ge2Te6) using two-dimensional transfer stacking technology, and used hexagonal boron nitride (hBN) as a cover packaging to prevent sample oxidation and greatly improve material stability. Through transport measurements, the Fraunhofer pattern of superconducting current oscillation with magnetic field and the Fiske step reflecting the high-order electromagnetic mode of the junction resonant cavity were successfully observed, demonstrating the excellent interface characteristics of the device junction. Meanwhile, by changing the scanning direction, the research team observed that the superconducting current shifted in the same direction as the magnetization curve of the intermediate layer, and the measured ferromagnetic hysteresis also depends on the magnetization process of the tunneling layer. More interestingly, this phenomenon only exists when the device is in a superconducting state and disappears in a normal state, manifested as the result of the combined effect of superconductivity ferromagnetism.
Figure. (a-b) Fraunhofer patterns and signs of 0- π Josephson junctions in van der Waals ferromagnetic Josephson junctions. (c) Non-trivial Ground State Phase in Chromium Germanium Tellurium Superconducting Quantum Interference Devices
In addition to studying the modulation of superconducting tunneling current by magnetic exchange interactions, how the ground state phase of Josephson junctions is affected by the magnetic layer is also a widely concerned issue in quantum computing applications. In some devices with thicker intermediate layers, we observed an anomalous magnetic field dependence of superconducting current, where the Fraunhofer pattern shifted from the classical zero field critical current to a minimum, indicating the presence of regions with ground state phases of 0 and π simultaneously in the junction region, resulting in mutual cancellation of the opposing superconducting currents. This may be caused by multi domain structures with different magnetization directions in the tunneling layer. Furthermore, we prepared a hybrid superconducting quantum interference device based on chromium germanium tellurium, and observed that it has a non-trivial ground state phase between 0 and π, showing signs of 0-π transition in temperature dependence, thereby proving the existence of π phase coupling caused by magnetic exchange in the Josephson junction.
Our group has achieved high-quality magnetic Josephson junction devices in van der Waals materials, which is of great significance for a deeper understanding of the proximity effect and spin state at the superconducting ferromagnetic interface; The variation of superconducting current modulated by magnetic tunneling layer helps to accurately characterize the characteristics of two-dimensional magnetic insulators (such as chromium triiodide CrI3, chromium trichloride CrCl3, etc.); At the same time, the ferromagnetic Josephson junction with a layered structure and adjustable ground state phase also provides an experimental basis for the future preparation of non-dissipative spin excited or arbitrary phase superconducting quantum devices.
Paper link: https://www.nature.com/articles/s41467-021-26946-w
