Degenerate monolayer Ising superconductors via chiral-achiral molecule intercalation

Engineering unconventional superconductors is a central challenge in condensed matter physics. Molecule-intercalated TaS2 superlattices have recently been reported to host such states, yet their origin remains debated, underscoring the urgent need for controlled, device-integrated studies. Here, we report that nanometer-thick TaS2 and NbSe2 intercalated with chiral and achiral organic cations instead exhibit robust monolayer-like Ising superconductivity, with no evidence of unconventional pairing. Using high-quality superlattices integrated into devices, we disentangle the roles of interlayer coupling and charge transfer in shaping their superconducting behavior. In TaS2, intercalation induces interlayer decoupling regardless of molecular size or symmetry, yielding monolayer-like Ising superconductivity. NbSe2 instead retains quasi-three-dimensional transport, with a gradual Ising enhancement and near-monolayer behavior only at the largest interlayer spacing. Transport remains reciprocal across all superlattices, consistent with preserved inversion symmetry and incompatible with parity-breaking superconductivity and noncentrosymmetric monolayers. We attribute the behavior to electronically detached monolayers with opposite spin-split bands, coupled through thermal and tunneling processes, which overall preserve inversion symmetry. These findings establish molecular intercalation compounds as a robust, device-ready, platform for engineering advanced superconducting superlattices.

💡 Research Summary

In this work the authors develop a scalable, device‑compatible galvanic intercalation technique to insert organic cations into nanometer‑thick flakes of the transition‑metal dichalcogenides 2H‑TaS₂ and 2H‑NbSe₂. By placing a low‑reduction‑potential metal electrode (In for TaS₂, Mg for NbSe₂) in contact with a flake that is immersed in an electrolyte containing the target molecules, a spontaneous redox reaction transfers one electron per intercalated ion into the host lattice while expanding the van‑der‑Waals gap. A series of intercalants is employed, including linear alkyl‑ammonium cations of different lengths (TEA, CTA, etc.) and two enantiomeric proline‑derived chiral cations (L‑Pr, D‑Pr). X‑ray diffraction and high‑resolution STEM confirm the formation of well‑ordered organic/ inorganic superlattices with an increased interlayer spacing d_int of 1.0–1.5 nm, independent of molecular symmetry.

Transport measurements reveal a striking dichotomy between the two host materials. In TaS₂, all intercalants produce a robust two‑dimensional (2D) Ising‑type superconducting state. The critical temperature T_c is ~2.8 K for achiral cations, matching the monolayer limit, and rises to ~4.7 K for the chiral D‑Pr species—about 35 % higher than the monolayer value. Hall data show suppression of the charge‑carrier sign change associated with the charge‑density‑wave (CDW) transition at ~70 K and a reduction of the sheet carrier density by roughly 10 %. The authors argue that the apparent carrier depletion reflects not only electron doping but also a reduction in the number of layers that effectively conduct, because the expanded spacing electronically isolates individual layers.

NbSe₂ behaves differently. Here the critical temperature decreases monotonically with increasing intercalant size, reaching only the monolayer benchmark (≤ 3.1 K) for the largest molecules. Hall measurements indicate only modest carrier depletion (≈ 10¹³ cm⁻²) and a preserved three‑dimensional transport pathway, implying that the intercalated molecules do not fully decouple the layers. Consequently, NbSe₂ retains a quasi‑3D character, and only the largest intercalants produce a near‑monolayer response.

The authors probe Ising superconductivity by measuring the angular dependence of the upper critical field H_c2(θ). For TaS₂ superlattices the in‑plane critical field at θ = 0° exceeds the Pauli limit by a factor of ~7 (H_c2‖(0) ≈ 32 T), and the data fit a 2D Ising model with a sharp cusp at θ = 0°, confirming that each electronically isolated layer experiences strong spin‑orbit locking. The same cusp is observed in NbSe₂, but the extracted H_c2‖(0) values are much smaller (up to ~3 T) and increase only gradually with interlayer spacing, consistent with incomplete decoupling.

A crucial test for parity‑breaking superconductivity is non‑reciprocal transport (different resistance for opposite current directions). All devices, regardless of chiral or achiral intercalants, display fully reciprocal I‑V characteristics, indicating that the overall superlattice retains inversion symmetry. The authors therefore propose a picture in which each detached layer possesses opposite spin‑split bands (spin‑layer locking) but the stack as a whole remains inversion‑symmetric, allowing only conventional Ising pairing.

In summary, the study demonstrates that (i) galvanic intercalation provides a controllable route to tune interlayer spacing and carrier density in TMDs, (ii) TaS₂ readily becomes a true monolayer‑like Ising superconductor regardless of molecular size or chirality, (iii) NbSe₂ requires large intercalants and significant charge transfer to approach the 2D limit, and (iv) the presence of chiral molecules does not induce unconventional, parity‑breaking superconductivity. These findings establish molecular‑intercalated TMD superlattices as a robust, scalable platform for engineering advanced superconducting devices, including potential applications in topological quantum circuits and spin‑tronic superconductors.