Realization of the field-free Josephson Diode (2103.15809v2)

Abstract: The superconducting analog to the semiconducting diode, the Josephson diode, has long been sought, with multiple avenues to realization proposed by theorists. Exhibiting magnetic-field free, single directional superconductivity with Josephson coupling of the supercurrent across a tunnel barrier, it would serve as the building-block for next-generation superconducting circuit technology. Here we realized the field-free Josephson diode using an inversion symmetry breaking heterostructure of $\mathrm{NbSe_2/Nb_3Br_8/NbSe_2}$. We demonstrate, for the first time without magnetic field, the junction can be superconducting in one direction while normal in the opposite direction. Based on that, half-wave rectification of a square-wave excitation was achieved with low switching current density ($~2.2\times 10^2 \mathrm{A/cm^2}$), high rectification ratio ($~10^4$) and high robustness (at least $10^4$ cycles). We also demonstrate symmetric $\Delta I_\mathrm{c}$ (the difference between positive and negative critical currents) behavior with field and the expected Fraunhofer current phase relation of a Josephson junction. This realization raises fundamental questions about the Josephson effect through an insulator when breaking symmetry, and opens the door to ultralow power, high speed, superconducting circuits for logic and signal modulation.

• The paper demonstrates a field-free Josephson diode built from a NbSe₂/Nb₃Br₈/NbSe₂ heterostructure with a rectification ratio of around 10⁴ and ultralow switching current density.
• The study employs synthesis and detailed electrical characterization, confirming nonreciprocal superconductivity through asymmetric tunneling and a Fraunhofer pattern in the Josephson coupling.
• The findings pave the way for energy-efficient superconducting circuits and quantum devices like SQUIDs and superconducting qubits, showing significant practical and theoretical implications.

Realization of the Field-Free Josephson Diode

The paper "Realization of the Field-Free Josephson Diode" presents a significant advancement in the field of superconducting electronics through the experimental realization of a field-free Josephson diode (JD). The research primarily focuses on synthesizing and characterizing a van der Waals heterostructure composed of NbSe₂/Nb₃Br₈/NbSe₂, demonstrating nonreciprocal superconductivity without an applied magnetic field, a property that has not been previously observed in bulk superconductors or conventional Josephson junctions (JJs).

The Josephson diode described in this paper demonstrates field-free nonreciprocal current-voltage characteristics. Notably, the junction is capable of being superconductive with a positive current and resistive with a negative current, thereby permitting half-wave rectification at zero magnetic field. This asymmetric behavior is validated by a significant rectification ratio (~10⁴), ultralow switching current density (2.2×10² A/cm²), and substantial durability over 10,000 cycles. The paper emphasizes that these properties are intrinsic, ruling out the Joule heating effect as a cause for the observed behavior.

From a theoretical perspective, the authors propose the diode's nonreciprocal behavior results from asymmetric tunneling of the supercurrent across the tunnel barrier, coupled with inversion symmetry breaking in the junction. This is supported by the observed symmetric ΔIʙ behavior, contrasting with previously reported antisymmetric responses dependent on magnetic field direction. The paper further corroborates the presence of a Josephson effect through its characteristic single-slit Fraunhofer pattern, confirming the junction's Josephson coupling.

Mechanistically, the researchers suggest that the nonreciprocal effect may originate from the obstructed atomic insulator phase of Nb₃Br₈, combined with the asymmetric interfaces between the NbSe₂ electrodes and the Nb₃Br₈ barrier. This suggests potential polarization-induced dipole formation, which aligns with existing theoretical frameworks predicting modulation of tunneling probabilities in such systems.

This work's implications are manifold, both practically and theoretically. It opens avenues for creating superconducting devices that function without an external magnetic field, potentially leading to more energy-efficient superconducting circuit technology. The realization of a field-free JD could significantly impact technologies such as superconducting quantum interference devices (SQUIDs), superconducting qubits, and rapid single flux quantum devices. Moreover, this paper paves the way for exploring quantum materials characterized by unique properties like topological states and noncollinear magnetic states, potentially leading to novel Josephson phenomena.

Future research directions could explore optimization of this Josephson diode architecture by altering the barrier materials or junction geometry. Additionally, integrating other materials with intrinsic ferroelectricity, magnetoelectricity, or topological features with superconducting electrodes may yield further emergent properties and applications. This paper thus establishes a foundation for not only advancing Josephson diode technology but also leveraging the unique properties of quantum materials in superconducting electronics.