Competing Charge Density Wave States Above Room Temperature in 2D van der Waals Ta(Co,Ni)₂Te₂ Nanoflakes

Charge density waves (CDWs) are emergent phenomena in quantum materials, characterized by periodic modulations in charge ordering. Recently, novel functionalities of CDW phases in van der Waals (vdW) materials, such as 1T-Ta(S,Se)₂, have opened new prospects for engineering and exploring CDW-driven symmetry-breaking quantum phenomena near room temperature. However, there is a scarcity of two-dimensional (2D) materials that exhibit high-temperature CDW transitions and can be easily exfoliated to fabricate nanodevices. Here we report on TaCo₂Te₂ (space group Pmna), a 2D van der Waals Peierls-distorted metal that exhibits a one-dimensional commensurate CDW (1D-C-CDW) phase at room temperature and a reversible CDW transition (T_c) near 573 K. In-situ heating transmission electron microscopy (TEM) reveals irreversible hidden CDW ordering above T_c in exfoliated nanoflakes and focused ion beam (FIB) fabricated lamellae (<100 nm) of TaCo₂Te₂, manifesting as ordered incommensurate (O-IC) CDW superlattice peaks. Interestingly, we also observed O-IC CDW ordering in TaNi₂Te₂ (space group Pmna) nanoflakes, an undistorted version of TaCo₂Te₂, with a multi-q vector CDW ordering, in contrast to the 1D O-IC CDW ordering in TaCo₂Te₂. This irreversible O-IC CDW ordering is stable only in Ta(Co,Ni)₂Te₂ nanoflakes, not in the bulk crystal. The presence of internal residual strain and soft phonon modes facilitates the nucleation of competing charge orderings in Ta(Co,Ni)₂Te₂ nanoflakes at high temperatures and offers thermomechanical manipulation as a route to engineering CDW states at and above room temperature.