Multiple Quantum States Induced in 1T-TaSe₂ by Controlling the Stacking Order of Charge Density Waves

Van der Waals (vdW) materials afford unprecedented opportunities for control of electronic properties by utilizing the stacking degree of freedom. An intriguing frontier, largely unexplored, is the stacking of charge density wave (CDW) phases that is a broken-symmetry state with periodically modulated charge density and the atomic lattice. Employing density functional theory, it is uncovered that the stacking order can play a significant role in the quantum phase transitions of layered 1T-TaSe₂ with a striking 2D CDW order. By controlling the vertical stacking order of CDWs, bulk 1T-TaSe₂ can host various electronic phases including quasi-1D and 3D metals and band insulators. Particularly, the ground-state stacking configuration shows 3D metallicity due to the enhanced intralayer and interlayer electron hopping, and the second lowest energy configuration shows band insulating behavior via interlayer dimerization, implying potential metal-insulator transition. In ultrathin-layer 1T-TaSe₂, not only the stacking order but also the thickness dictate the electronic properties. While the monolayer is a Mott insulator, the bilayer (trilayer) is a band insulator (metal). More interestingly, the four-layer emerges as an insulator or a semimetal dependent on its stacking order. The wide-tunable electronic properties of 1T-TaSe₂ CDW compound will open a new pathway for designing novel quantum devices.