Driving the field-free Josephson diode effect using Kagome Mott insulator barriers

Josephson junctions (JJs), devices consisting of two superconductors separated by a barrier, are of great technological importance, being a cornerstone of quantum information processing. Classical understanding of superconductor-insulator-superconductor JJs is that conventional insulator’s properties, other than magnetism, do not significantly influence the junction’s behavior. However, recent work on quantum material (QM) JJs - using Mott insulator Nb3Br8 - resulted in magnetic field-free non-reciprocal superconductivity, termed the Josephson diode effect (JDE), implying the QM’s intrinsic properties can modulate superconductivity in non-trivial ways. To date, the underlying mechanism and dependence of the JDE on correlation strength (U/t) has not been elucidated. Here we fabricate QMJJs using correlated Kagome insulators with varying U/t, Nb3X8 (X=Cl, Br, I), observing a decreasing trend of the field-free JDE with Nb3Cl8 reaching ~48% efficiency, Nb3Br8 ~6%, and Nb3I8 having no discernible JDE, matching the trend of decreasing U/t from Cl to I and suggesting correlation in insulators drives the field-free JDE.

💡 Research Summary

In this work the authors investigate how the intrinsic electronic correlations of a barrier material can generate a magnetic‑field‑free Josephson diode effect (JDE) in vertical superconductor‑insulator‑superconductor (SIS) junctions. They focus on the family of kagome‑lattice Mott insulators Nb₃X₈ (X = Cl, Br, I), whose correlation strength (U/t) systematically decreases from Cl to I due to increasing Nb–Nb trimer bond lengths and inter‑layer spacing. Using high‑quality NbSe₂ flakes as the superconducting electrodes and encapsulating the heterostructures with h‑BN, they fabricate four‑probe vertical Josephson junctions with Nb₃X₈ barriers of each composition. The devices are measured from 260 mK up to 5 K and in in‑plane magnetic fields up to ±1.5 T.

Key experimental observations are:

1. Field‑free JDE in Nb₃Cl₈ junctions – The I‑V characteristics show a pronounced asymmetry between the positive and negative critical currents (Ic⁺ ≫ |Ic⁻|). The diode efficiency η = (Ic⁺ − |Ic⁻|)/(Ic⁺ + |Ic⁻|) reaches ≈48 % at 260 mK with zero magnetic field. The effect appears below ≈1.6 K, well below the NbSe₂ critical temperature.

2. Anomalous temperature dependence – The Ic(T) curves deviate strongly from the Ambegaokar‑Baratoff prediction. Fitting yields an unusually small superconducting gap (Δ ≈ 75 µeV, only ~5 % of the NbSe₂ bulk gap) and a normal‑state resistance consistent with the measured value. A distinct “kink” in the negative‑bias critical current further signals unconventional tunnelling dynamics.

3. Magnetic‑field response – Ic⁺ displays a Fraunhofer‑like oscillation with field, confirming conventional Josephson coupling, whereas Ic⁻ is heavily suppressed and shows only faint, asymmetric modulations. The diode efficiency remains sizable up to ≈300 mT (≈50 %) and vanishes near 1.2 T, indicating that the non‑reciprocity is robust against modest fields but ultimately tied to the superconducting state.

4. Correlation‑strength dependence – Devices with Nb₃Br₈ barriers also exhibit a field‑free JDE, but the efficiency drops dramatically to ≈6 % and the Fraunhofer pattern is clearer, reflecting weaker asymmetry. In contrast, Nb₃I₈ junctions show virtually no difference between Ic⁺ and Ic⁻ (η ≈ 0 %), demonstrating that when electronic correlations are weak the diode effect disappears.

The authors interpret these findings in terms of symmetry breaking induced by strong electronic correlations in the breathing kagome lattice. Although the barrier is non‑magnetic, the Mott‑insulating state can generate an internal exchange field or orbital texture that lifts both inversion and time‑reversal symmetry locally, leading to direction‑dependent Cooper‑pair tunnelling. The pronounced asymmetry between forward and reverse critical currents suggests distinct tunnelling channels (e.g., phase‑slip dominated for one polarity, quasiparticle‑assisted for the opposite). This mechanism is fundamentally different from previously reported magnetic‑field‑dependent JDEs that rely on ferromagnetic barriers or external fields.

The work establishes a clear experimental correlation between the barrier’s U/t ratio and the magnitude of the field‑free JDE, providing the first systematic demonstration that a strongly correlated, non‑magnetic insulator can act as an active element in superconducting diodes. It opens a new design paradigm for superconducting electronics, where the electronic correlation strength of the barrier can be tuned (by chemical substitution, strain, or gating) to engineer non‑reciprocal superconducting transport without magnetic materials. Future directions include quantitative theoretical modeling (e.g., DMFT‑based multi‑band Hubbard calculations) to link U/t to the effective internal symmetry‑breaking field, and exploration of other kagome or correlated van‑der‑Waals insulators to broaden the material toolbox for dissipationless rectifiers, gyrators, circulators, and superconducting logic elements.