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Study shows that monolayer tungsten ditelluride is an excitonic insulator

Study shows that monolayer tungsten ditelluride is an excitonic insulator
Cartoon illustration of the excitonic isolator formed in the individual atomic layer of tungsten ditelluride. The excitons are charge-neutral composite particles like the hydrogen atoms, hidden in the two-dimensional electrical insulator. Credit: Wu Lab, Princeton University.

Tungsten ditelluride (WTe2) is a transition metal dichalcogenide with several advantageous properties and characteristics, making it an ideal material for a wide range of electronic applications. Previous studies have determined that 2D WTe2 crystals arranged in a single layer form the first monolayer topological insulator that exhibits topological properties that survive up to very high temperatures (~ 100 K).

Over the last few years, physicists have been able to understand the origin of the material's topology quite well. Nevertheless, the reasons why WTe2 monolayers behave like an insulator (i.e. electrons cannot move freely in the material) remain blurred. Theoretical predictions and calculations suggest that the material should in principle be a semi-metal, where electrons and holes coexist and move freely.

Researchers at Princeton University have recently conducted a study examining the electronic properties of monolayer WTe2, hoping to better understand the reasons why it acts as an insulator. Their paper, published in Natural physics, provides strong evidence that the material is an excitonic isolator arising from the spontaneous formation of electron-hole-bound states known as 'excitons'.

"The original purpose of our work was to understand the quantum properties of the very new 2D material monolayer WTe2", Sanfeng Wu, one of the researchers who conducted the study, told" Over the years, different ways of explaining the origin of the isolator state were inconsistently discussed in the literature. Our work carried out a systematic study to solve this puzzle and found strong evidence that this 2D insulator is an excitonic insulator, a long-in-demand quantum state of electronic matter in solids. "

The existence of excitonic insulators was first predicted in the 1960s. At that time, physicists suggested that in semiconductors or semiconductors at small intervals, electrons and holes can sometimes be combined to form composite particles (ie, excitons). This process would again lead to a highly insulating phase which would differ significantly from ordinary electrical insulators.

"Excitons are charge-neutral particles, like hydrogen atoms," Wu explained. The concept of excitons is not new in semiconductor physics, for example excitons play key roles in optical excitations and emissions of semiconductors. However, the optically excited excitons in a semiconductor are very short-lived as they must decay, for example by emitting light, within nanoseconds. In contrast, the excitons in an excitonic insulator do not emit light and do not decay. "

In excitonic insulators, excitons are hidden in the insulator state, making them difficult to detect experimentally. As a result, it has so far proved incredibly challenging to detect the existence of excitonic isolator states.

To show that WTe2 monolayer is an excitonic insulator, Wu and his colleagues first tried to rule out all other known possible explanations for its insulating behavior. This included the possibility of a disturbance-induced isolation phase and a trivial insulator with a band gap similar to typical semiconductors.

"This is a very important step, but typically very difficult to do for 3D candidate materials," Wu said. "We examined the role of the disorders by comparing samples with different levels of impurity and found that cleaner samples host stronger isolating states, revealing that the isolating state is an inherent property of the monolayer in the pure boundary, rather than induced by interference. . "

In their experiments, the researchers also ruled out the possibility that monolayer WTe2 is a tape insulator. To do this, they examined a 2D WTe2 crystal using electron tunnel spectroscopy, a famous and powerful technique for distinguishing correlated isolation states from trivial band insulators.

"We concluded that the insulating state of the monolayer is evolving due to inherent electronic correlations," Wu said. "By combining this with the fact that the state occurs exactly at charge neutrality, which means that the number of electrons and holes is exactly equal, it became obvious that the monolayer insulator is an excitonic insulator."

Interestingly, Wu and his colleagues also found that the monolayer WTe2 sample, they examined, exhibited unusual transport behaviors consistent with those that would be expected in an excitonic isolator. Subsequently, they developed a theoretical model that considers electron-hole correlations, which further supports the formation of an excitonic insulator phase.

"We have put together two remarkable results that could have broad implications," Wu said. "First, our study adds a significant new aspect to the understanding of a 2D topological material that also shows many other unusual quantum properties. This finding revises our understanding of quantum physics, where topology and electron correlations are both important. end up leading to new discoveries, especially in this new class of materials. "

The recent study conducted by this team of researchers shows that monolayer WTe2 is a very promising 2D excitonic insulator candidate. In the future, it may inform further studies examining monolayer WTe2 or other materials with similar structures, to investigate the possibility of uncovering more excitonic insulation materials.

"Our work provides valuable opportunities to experimentally tackle the 6-year-old problem of excitonic insulators," Wu said. "Our results already inspire new ideas to directly detect the hidden excitons using approaches that are impossible for previous candidate materials."

The results collected by Wu and his colleagues open up new fascinating possibilities for the development of new experimental techniques for detecting neutral quantum phases hidden in insulators. This can improve the current understanding of electrical insulators, and more importantly, lead to the discovery of new types of electrical insulators beyond standard.

"Our work identifies monolayer WTe2 as a unique and unprecedented platform for future studies of not only excitonic insulating state but also other possible new quantum phases such as excitonic superconductivity, especially since monolayer WTe2 can be tuned electrostatically from the excitonic isolator state to a superconductor state, "Yanyu Jia, a graduate student and lead author of the paper, told" Revealing the underlying relationships between the two phases will be interesting and certainly deepen our understanding of quantum phenomena in materials. "

In their next studies, Wu, Jia, and their colleagues will try to devise alternative experimental procedures that will allow them to detect excitons in the baseline state directly and even more definitively. In addition, they would like to conduct further research focusing on any new quantum phases that could characterize excitonic insulators.

"A key factor here is that we are not dealing with single excitons; instead, the excit density here is ~ 1012 cm-2", as Wu added." As with many atoms together, we can have different phases of matter, we expect these many excitons to form interesting new electronic phases of different kinds. So there should be a rich quantum world hidden in such electrical insulators, and we hope to be able to reveal them. "

Excitonic insulators: Experimental observation of a new class of materials

More information: Yanyu Jia et al., Evidence for a monolayer excitonic insulator, Natural physics (2021). DOI: 10.1038 / s41567-021-01422-w

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Citation: Study shows that monolayer tungsten ditelluride is an excitonic insulator (2022, 19 January) retrieved 19 January 2022 from

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