Realization of multiple charge density waves in NbTe2 at the monolayer limit

TL;DR

This study reveals multiple charge density wave phases in monolayer NbTe2, including unexpected complex patterns, and establishes a detailed phase diagram, challenging prior theoretical predictions and expanding the understanding of 2D quantum materials.

Contribution

It experimentally demonstrates the coexistence of multiple CDW phases in monolayer NbTe2 and maps their stability, providing new insights into 2D charge order phenomena.

Findings

Identification of four distinct CDW phases in monolayer NbTe2.

The stable phase is the sqrt(19)*sqrt(19) order, contrary to prior predictions.

Successful synthesis and characterization of complex CDW patterns in 2D NbTe2.

Abstract

Abstract: Layered transition-metal dichalcogenides (TMDCs) down to the monolayer (ML) limit provide a fertile platform for exploring charge-density waves (CDWs). Though bulk NbTe2 is known to harbor a single axis 3*1 CDW coexisting with non-trivial quantum properties, the scenario in the ML limit is still experimentally unknown. In this study, we unveil the richness of the CDW phases in ML NbTe2, where not only the theoretically predicted 4*4 and 4*1 phases, but also two unexpected sqrt(28)*sqrt(28) and sqrt(19)*sqrt(19) phases, can be 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, we report that the energetically stable phase is the larger scale order (sqrt(19)*sqrt(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 1T-NbTe2 for the first time and offer a new material platform for studying novel quantum phases in the 2D limit.