Two-dimensional (2D) materials are an ideal platform to artificially engineer heterostructures with new functionalities due to the weak van der Waals bonding between layers. Monolayers hosting symmetry-broken phases, such as superconductivity, magnetism, ferroelectricity, charge density waves (CDWs), or multiferroicity, represent the most interesting building blocks to design novel phases of matter.
One of the main challenges in the task of engineering novel functional materials with broken-symmetry monolayers is to overcome the restrictions imposed by the reduced dimensionality, which may prevent the formation of these phases, and the competition between ordered phases due to the subtle interplay of different interactions. For instance, CDW phases have been reported to destroy or promote 2D ferromagnetism. VSe2 is a paradigmatic example of this, as it has been shown, both experimentally and theoretically, that the CDW order quenches the emergence of itinerant ferromagnetism.
VSe2 adopts a layered, quasi 2-D structure. In contrast to the strong covalent bonding within each VSe2 layer is the weak van der Waals coupling between planes, which well explains the anisotropy in physical properties and also makes ionic or molecular intercalations in the van der Waals gap feasible.
In its bulk form, VSe2develops a commensurate CDW phase below 110 K. This CDW phase opens pseudogaps at the Fermi level, impeding the emergence of ferromagnetism. Experiments and calculations have shown that the CDW transition is most likely driven by the collapse of a low-energy acoustic mode and that the electron−phonon coupling is the origin of the instability. But there is a problem, VSe2 belongs to the series of layered transition metal dichalcogenides that develops a 3D-CDW as a function of temperature: unrelated experiments have reported distinct CDW orders with different transition temperatures. These experimental contradictions point to the presence of different competing CDW orders, which can lead to different low-temperature phases depending on the substrate.
This competition among different CDW orders is also apparent when the harmonic phonons of the VSe2 monolayer within density functional theory are calculated. In the presence of competing orders, only calculations considering anharmonicity can disentangle what is the CDW order and the transition temperature, as it has already been shown in different transition metal dichalcogenides.
Now, a team of researchers presents 1 a theoretical analysis of the CDW orders arising in monolayer VSe2 using nonperturbative anharmonic phonon calculations based on the stochastic self-consistent harmonic approximation. This formalism has been crucial to understand and characterize the CDWs in several transition metal dichalcogenides as it overcomes the limitations of the harmonic analysis, allowing to determine the dependence of the CDW order as a function of temperature.
The effect of strain and external van der Waals interactions, since these two parameters are intrinsic to any experiment, are analysed. The analysis reveals that monolayer VSe2 develops two independent charge density wave orders that compete as a function of strain. Variations of only 1.5% in the lattice parameter are enough to stabilize one order or the other. Transition temperatures of circa 220 K have been found for both CDW orders, in very good agreement with experiments.
These results solve previous experimental contradictions, highlighting the high tunability and substrate dependency of the CDW orders of monolayer VSe2.
Author: César Tomé López is a science writer and the editor of Mapping Ignorance
Disclaimer: Parts of this article may have been copied verbatim or almost verbatim from the referenced research paper/s.