The van der Waals heterostructure 4Hb-TaS₂, composed of alternating layers of 1T-TaS₂ (quantum spin liquid) and 1H-TaS₂, (half of 2H-TaS₂, a BCS superconductor), exhibits several features consistent with chiral superconductivity below a critical temperature T_c ≃ 2.7 K [1]. Specifically, scanning tunneling microscopy experiments reveal peculiar properties of the vortex state, including topological edge modes [2] and the magnetic memory effect [3]. However, the symmetry of the superconducting order parameter of 4Hb-TaS₂ is poorly understood from bulk measurements.

Here, we investigate vortex penetration into 4Hb-TaS₂ for a magnetic field parallel to the layers stacking direction. Using micro-Hall-probe magnetometry [4], we focus on the field of first flux penetration and the resultant temperature dependence of the lower critical field H_c1(T ) in the entire superconducting state down to 0.002T_c. For platelet-shaped samples with the thickness d < 10 µm, we found that the out-of-plane H_c1(T ) dependence can be well described by the conventional relation derived from the BCS theory, pointing to marginal multiband effects and suggesting a nodeless superconducting order parameter of 4Hb-TaS₂. However, with increasing thickness we observed an anomalous enhancement of the penetration field deep in the super- conducting state. Specifically, samples with d ≃ 200 µm show the pronounced enhancement of H_c1 at 0.5T_c, which consists of a non-saturating T -dependence in the T = 0 limit. Our results appear to be consistent with the theoretically predicted effect of Andreev bound states on the Bean-Livingston barrier, and thus provide macroscopic evidence for a sign-changing gap function in the candidate chiral superconductor 4Hb-TaS₂.

References:

[1] A. Ribak, et al., Sci. Adv. 6, eaax9480 (2020).

[2]A. K. Nayak, et al., Nat. Phys. 17, 1413 (2021).

[3] E. Persky, et. al., Nature 607, 692 (2022).

[4] J. Juraszek, et. al, arXiv:2502.14423 (2025).

This work was supported by the Polish National Science Centre NCN (Project OPUS23 No. 2022/45/B/ST3/04117).