Signatures of time-reversal-symmetry breaking in multiband 2H-TaS2 revealed by zero-field Josephson nonreciprocity

Abstract

Superconductors that spontaneously break time-reversal symmetry host complex order parameters and are widely regarded as a hallmark of unconventional superconductivity. Whether such symmetry breaking can also arise in superconductors with nominally isotropic spin-singlet pairing remains an open question. Here we report a zero-field Josephson diode effect in noncentrosymmetric 2H-TaS2/2H-NbSe2 van der Waals junctions. The diode efficiency shows no systematic correlation with supercurrent amplitude, TaS2 thickness, or normal-state resistance, arguing against simple extrinsic, purely interfacial, or transparency-driven mechanisms. Time-reversal-symmetric scenarios are further tested using symmetry-controlled and molecule-intercalated control devices, in which the nonreciprocal response is absent or strongly reduced. Normal-state Hall transport in TaS2 exhibits a nonlinear response consistent with multiband correlated electronic states. Within a Josephson framework, our modelling shows that interband scattering acts as a phase-locking mechanism generating an intrinsic anomalous phase difference and a nonsinusoidal asymmetric current-phase relation, leading to finite zero-field rectification. Together, zero-field Josephson nonreciprocity and nonlinear Hall transport provide complementary evidence for a multiband superconducting phase structure in 2H-TaS2, consistent with intrinsic time-reversal-symmetry breaking.