Realization of Multiple Charge-Density Waves in NbTe₂ at the Monolayer Limit.

Abstract

Layered transition-metal dichalcogenides down to the monolayer (ML) limit provide a fertile platform for exploring charge-density waves (CDWs). Here, we experimentally unveil the richness of the CDW phases in ML-NbTe₂ for the first time. Not only the theoretically predicted 4 × 4 and 4 × 1 phases but also two unexpected <math xmlns="http://www.w3.org/1998/Math/MathML"><msqrt><mn>28</mn></msqrt><mi>×</mi><msqrt><mn>28</mn></msqrt></math> and <math xmlns="http://www.w3.org/1998/Math/MathML"><msqrt><mn>19</mn></msqrt><mi>×</mi><msqrt><mn>19</mn></msqrt></math> phases are realized. For such a complex CDW system, we establish an exhaustive growth phase diagram via systematic efforts in the material synthesis and scanning tunneling microscope characterization. Moreover, the energetically stable phase is the larger-scale order (<math xmlns="http://www.w3.org/1998/Math/MathML"><msqrt><mn>19</mn></msqrt><mi>×</mi><msqrt><mn>19</mn></msqrt></math>), which is surprisingly in contradiction to the prior prediction (4 × 4). These findings are confirmed using two different kinetic pathways: i.e., direct growth at proper growth temperatures (<i>T</i>) and low-<i>T</i> growth followed by high-<i>T</i> annealing. Our results provide a comprehensive diagram of the "zoo" of CDW orders in ML-NbTe₂.