Efficiency-Tunable Field-Free Josephson Diode Effect in Nb₃Cl₈Based van der Waals Junctions

The magnetic field-free Josephson diode effect (JDE) offers a promising route toward nonreciprocal superconducting devices, yet its microscopic origin and effective control remain unclear. Here, we demonstrate a tunable field-free JDE in NbSe₂/Nb₃Cl₈/NbSe₂ van der Waals (vdW) Josephson junctions. By reducing the Nb₃Cl₈ barrier thickness from four-layer to monolayer, the diode efficiency is significantly enhanced from 1.75% to 20.88%. Moreover, the diode efficiency can be effectively modulated by an out-of-plane electric field, indicating the presence of intrinsic electric polarization in the junction. Both thickness variation and electric field tuning modify the polarization strength, thereby enabling broad control of the diode efficiency. These results uncover the crucial role of intrinsic electric polarization in realizing the field-free JDE and establish this effect as a sensitive probe of spontaneous time-reversal symmetry breaking in superconducting heterostructures.