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

TL;DR

This work addresses how c-axis stacking in $TaSe_{2}$ polymorphs controls interlayer coupling and the competition between CDW order and superconductivity. The authors synthesize and characterize 1T, 2H, and 3R polytypes via CVT growth and neutron diffraction, linking structural parameters to CDW transitions and $T_c$. They demonstrate an anti-correlation between CDW transition temperature $T_{CDW}$ and interlayer spacing, with 1T showing high $T_{CDW}$ and no superconductivity and 3R showing elevated $T_c$ and coexisting CDW, attributed to increased two-dimensionality and altered interlayer hybridization. The potential for Ising-like pairing in non-centrosymmetric 3R and the broader implication that stacking manipulation can tune intertwined orders in layered dichalcogenides are highlighted.

Abstract

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.

Figures (5)

• Figure 1: The room temperature crystal structures of the three polytypes above their CDW transition. (a) 1T - TaSe$_{2}$ with the P$\bar{3}m1$ symmetry with AA stacking; (b) The 2H - TaSe$_{2}$ with the P6$_{3}$/mmc symmetry and AB type stacking where the upper layer is rotated by 60${^0}$. (c) The 3R - TaSe$_{2}$ with ABC type stacking with each layer displaced by 1/3 along the diagonal direction resulting in the R3m symmetry. (d), (e) and (f) precision images obtained from single crystal diffraction measurement for 1T, 2H and 3R - TaSe$_{2}$ phases in the [HK0] plane at room temperature.
• Figure 2: (a) A plot of the CDW transition temperatures for the 1T, 2H and 3R phases. The data for the 1T phase was obtained from Ref. samnakay2014. The right axis shows the interlayer distance for each phase at 3 K obtained from the neutron data refinement. (b) The $\sqrt{13} \times \sqrt{13}$ CDW modulation for 1T phase. The CCDW "Star-of-David" pattern is shown in red. (c) and d) $3 \times 3$ CDW modulation for the 2H and 3R phases, respectively. The unit cells above and below the CCDW transitions are shown with green solid and blue dashed lines, respectively.
• Figure 3: Plots of the temperature dependence of the resistivity for (a) 1T - TaSe$_2$ (b) 2H - TaSe$_2$ and (c) 3R - TaSe$_2$ single crystals. The inset shows the resistivity for 3R near the superconducting transition temperature. The fitting results for the different regions are shown in Table II.
• Figure 4: The neutron diffraction data collected at Echidna at 3 K. In (a), the neutron data for the 1T - TaSe$_{2}$ sample is compared to the model calculated using the P$\bar{3}m1$ space group. Additional phases are present in small amounts. In (b), the 2H - TaSe$_{2}$ diffraction data are compared to the model calculated using Cmcm space group. In (c), the 3R - TaSe$_2$ diffraction data is compared to the model calculated using the R3m space group. The impurities are listed in Table I.
• Figure 5: A plot of the temperature dependence of (a) the inter-layer separation, (b) the a-lattice parameter and (c) the c-lattice parameter as obtained from the refinement of the neutron diffraction data. The error bars are smaller than the size of the data points.