Charge density wave and superconductivity modulated by c-axis stacking in the TaSe2 polytypes

The layered transition metal dichalcogenide, TaSe2, exhibits rich electronic phenomena across its polymorphs, 1T, 2H, and 3R, largely driven by differences in atomic coordination and c-axis stacking. In the 1T phase, octahedral coordination and AA stacking promote strong interlayer coupling and stabilize a commensurate charge density wave (CDW) with star-of-David clusters that set in at high temperatures. The 2H phase exhibits trigonal prismatic coordination with AB stacking, and hosts both incommensurate and commensurate CDW phases and weak superconductivity at very low temperatures. The 3R phase, characterized by ABC stacking and trigonal prismatic coordination, exhibits enhanced superconductivity along with CDW order, attributed to modified interlayer hybridization and reduced CDW competition. These stacking-dependent variations in interlayer coupling are critical in tuning correlated states in the dichalcogenides.

💡 Research Summary

The authors present a comprehensive comparative study of the three polymorphs of tantalum diselenide (TaSe₂)—the 1T, 2H, and 3R phases—focusing on how the c‑axis stacking sequence controls interlayer coupling, charge‑density‑wave (CDW) order, and superconductivity. High‑quality single crystals of each polytype were grown by chemical vapor transport, their phase purity confirmed by powder X‑ray diffraction, and their crystal structures refined from high‑resolution neutron powder diffraction data collected at 3 K, 100 K, 200 K, and 300 K on the Echidna instrument at ANSTO. The refinements reveal that 1T adopts a P‑3 m1 symmetry with AA stacking and octahedral Ta coordination, 2H crystallizes in the P6₃/mmc (or Cmcm) space group with ABAB stacking and trigonal‑prismatic coordination, and 3R belongs to the non‑centrosymmetric R3m group with ABC stacking. Lattice parameters and interlayer distances increase systematically from 1T (smallest c‑axis spacing) to 3R (largest spacing), while the a‑axis is largest for 1T and comparable for 2H/3R.

Electrical transport measurements on the same crystals show distinct CDW transitions. In 1T‑TaSe₂, an incommensurate CDW appears near 600 K and a commensurate √13 × √13 “Star‑of‑David” CDW sets in at ≈473 K; the resistivity follows a high‑order polynomial rather than a simple linear T‑dependence, reflecting strong electron‑phonon coupling and partial Fermi‑surface gapping. No superconductivity is observed down to the lowest measured temperatures, indicating that the strong interlayer orbital overlap in the AA‑stacked structure stabilizes the CDW but suppresses Cooper pairing.

In the 2H polytype, an incommensurate CDW emerges at 122 K, followed by a commensurate 3 × 3 CDW at 90 K. The resistivity is linear above the CDW onset, drops sharply at the transition, and can be fitted by a polynomial below the CDW. Weak superconductivity appears only at ≈0.14 K, consistent with previous reports. The ABAB stacking reduces direct interlayer orbital overlap, allowing the system to retain metallicity even in the CDW state and to support a low‑Tc superconducting phase.

The 3R phase exhibits a 3 × 3 CDW around 114 K and, notably, a superconducting transition at ≈3.2 K (reported values up to 2.4 K). The ABC stacking further separates the layers, enhancing the two‑dimensional character of the electronic structure, diminishing interlayer hybridization, and modifying the Fermi‑surface topology. This environment weakens the CDW transition temperature while simultaneously providing favorable conditions for Cooper pairing, leading to a significantly higher Tc than in 2H. Moreover, the lack of inversion symmetry in the R3m structure introduces strong spin‑orbit coupling, opening the possibility of Ising‑type (spin‑triplet) pairing, which may explain the coexistence of CDW and superconductivity in this polytype.

By correlating structural parameters (interlayer spacing, stacking sequence, coordination geometry) with transport signatures (CDW transition temperatures, resistivity behavior, superconducting Tc), the authors demonstrate an inverse relationship between CDW stability and superconductivity: tighter interlayer coupling (1T) favors high‑temperature CDW but suppresses superconductivity, while looser coupling (3R) reduces CDW ordering temperature and enhances superconductivity. The study highlights dimensional tuning via c‑axis engineering—through pressure, chemical substitution, or layer‑number control—as a powerful route to manipulate competing electronic orders in layered transition‑metal dichalcogenides. The work thus provides a clear structural blueprint for designing TaSe₂‑based materials with targeted CDW or superconducting properties, and it underscores the broader principle that stacking‑induced changes in interlayer hybridization are a decisive factor in the emergence of correlated electronic phases.