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 28 × 28 and 19 × 19 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 ( 19 × 19 ), 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 (T) and low-T growth followed by high-T annealing. Our results provide a comprehensive diagram of the “zoo” of CDW orders in ML-NbTe2.

Accelerated Data-Driven Discovery and Screening of Two-Dimensional Magnets Using Graph Neural Networks

“…However, they may also indicate that the crystal is on the verge of structural transitions or distortions, such as the formation of phases hosting a charge density wave (CDW) . For example, MoS 2 undergoes a transition from 1T to 1T′ through a 2 × 1 distorted structure. , Furthermore, many transition metal dichalcogenide monolayers undergo a transition to a CDW, including NbSe 2 , TaS 2 , TaSe 2 , TiSe 2 , VS 2 , and NbTe 2 . Moreover, a transition to a CDW has been theoretically predicted for the CrTe 2 monolayer and was shown to stabilize the imaginary phonon modes through crystal by a transition to a 3 × 3 supercell .…”

“…106,107 Furthermore, many transition metal dichalcogenide monolayers undergo a transition to a CDW, including NbSe 2 , 108 TaS 2 , 109 TaSe 2 , 110 TiSe 2 , 111 VS 2 , 112 and NbTe 2 . 113 Moreover, a transition to a CDW has been theoretically predicted for the CrTe 2 monolayer and was shown to stabilize the imaginary phonon modes through crystal by a transition to a × 3 3 supercell. 114 In fact, because these transitions typically require the construction of supercells capable of accommodating the onset of the new phase, one of the approaches to stabilizing such materials is to extend the atoms along the direction of instability.…”

van der Waals Engineering of Charge Density Waves in One-Dimensional Nb6Te6 Nanowires

“…On the basis of our previous work, monolayer NbTe 2 thin films can be prepared within the growth temperature range of 250 < T sub < 450 °C. 37 1D structures with widths ranging from subnanometer to a few nanometers appear at T sub > 450 °C (Figure 1a) and become the dominant phase at T sub = 510 °C (Figure 1b). As T sub increases to 550 °C (Figure 1c), the reaction between Nb and Te atoms becomes incomplete, leaving amorphous clusters on the surface.…”

“…Figure a–e shows the controllable morphology evolution from 2D flakes to 1D wires with various growth temperatures ( T sub ) and deposition times. On the basis of our previous work, monolayer NbTe 2 thin films can be prepared within the growth temperature range of 250 < T sub < 450 °C . 1D structures with widths ranging from subnanometer to a few nanometers appear at T sub > 450 °C (Figure a) and become the dominant phase at T sub = 510 °C (Figure b).…”

Charge density waves in two-dimensional transition metal dichalcogenides

“…These questions have been answered by investigating the ML 1T-TaTe 2 films grown on BLG substrate using MBE [242,243] 28 CDW orders is not fully understood yet because the conventional pictures of CDW transition, such as Peierls instability or momentum-dependent strong electron-phonon coupling, are not clearly connected in these systems [242,244]. Nonetheless, the emergence of new types of CDW orders in the ML TMDCs strongly indicates that the stronger interlayer coupling in the Te system plays a significant role in creating a dramatic transformation of quantum orders.…”